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Humber River

State of the Watershed Report –

Fluvial Geomorphology

2008

Cold Creek east of Kingsview Dr. in Bolton

Humber_Fluvial_Geomorphology_FINAL_062508.doc

EXECUTIVE SUMMARY

• Fluvial geomorphology is the study of the processes responsible for the shape and form of watercourses. These processes determine and constantly change the form of river and stream channels and determine the stability of channel form.

• Recent studies suggest that current stormwater management approaches based on detention may not be sufficient to manage impacts to watercourses from increases in runoff and flow volume in urban areas.

• The total length of river and streams in the Humber River watershed is approximately 2032 kilometres, with the majority (78%) being small tributaries.

• The Main Humber subwatershed is characterized by an irregular drainage network that is the result of the hummocky topography and variable geology in this area, as well as an extensive number of groundwater discharge locations.

• The drainage network of the West Humber River subwatershed, as well as Rainbow Creek, is much more regular than in the Main Humber, as the relatively resistant till material and uniform slope create watercourses that are oriented in the same northwest-southeast direction.

• The East Humber River subwatershed has a typical branched drainage network that can be attributed to less extensive groundwater discharge, more regular topography and more resistant and impermeable surface soils than in the Main Humber.

• Many of the tributaries of Black Creek have been eliminated as a result of past agricultural or urban development practices. Channels that remain are much larger and wider than those of similar drainage area in the East, Main and West Humber. South of Wilson Avenue, the main branch of the creek has been channelized, realigned or hardened, leaving little of the original alignment and character.

• The Lower Humber River channel is very large and becomes a dominant feature in the landscape. Many reaches were realigned, channelized or straightened in the past to facilitate development, which increased the overall slope resulting in downcutting of the channel. A series of grade control structures have been constructed between Bloor Street and Lawrence Avenue to prevent further bed degradation and erosion.

• Stream channels in the Black Creek subwatershed and drainage areas of tributaries to the Lower Humber (i.e., Albion Creek, Berry Creek, Emery Creek, Humber Creek and Silver Creek) in the City of Toronto are unstable and adjusting to a changed pattern of stream flow, largely due to increased surface run-off from impervious surfaces associated with urban developments and absence of stormwater management controls.

• Stream channels in the West Humber subwatershed have been highly modified in the past through transition from forested conditions to agricultural land uses accompanied by agricultural drain construction, culvert and bridge construction, and channel re-alignment.

• Natural riparian vegetation was observed within 60% of all riparian areas in the watershed, indicating that a significant portion of Humber River stream banks lack the protection that natural riparian vegetation can provide.

Humber River State of the Watershed Repor t – Fluv ial Geomorphology

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TABLE OF CONTENTS 1.0 INTRODUCTION............................................................................................................ 1

2.0 UNDERSTANDING FLUVIAL GEOMORPHOLOGY...................................................... 2

3.0 MEASURING FLUVIAL GEOMORPHOLOGY ............................................................... 5

4.0 EXISTING CONDITIONS IN THE HUMBER WATERSHED............................................... 7

4.1 Regional Watershed Monitoring Program Sites............................................................. 7

4.2 City of Toronto Study Sites........................................................................................... 18

4.3 Subwatershed and Drainage Study Sites .................................................................... 19

7.0 REFERENCES............................................................................................................. 25

LIST OF FIGURES Figure 1: Humber River Watershed Stream Order and Fluvial Geomorphology Monitoring

Stations..................................................................................................................................... 8 Figure 2: Humber River Watershed Topography....................................................................... 10 Figure 3: Cold Creek at Castlederg Sideroad (RWMP site GHU-32) ........................................ 13 Figure 4: Main Humber River at Kirby Road (RWMP site GHU-21)........................................... 13 Figure 5: West Humber River (main branch) at Mayfield Road (RWMP site GHU-17) ............. 14 Figure 6: Lower West Humber River at Islington Avenue (RWMP site GHU-3)......................... 14 Figure 7: East Humber River west of Bathurst Street (RWMP site GHU-22)............................. 15 Figure 8: East Humber River at Langstaff Road (RWMP site GHU-11) ..................................... 15 Figure 9: Black Creek between Steeles Avenue and Finch Avenue (RWMP site GHU-4)........ 16 Figure 10: Black Creek south of Wilson Avenue......................................................................... 16 Figure 11: Lower Humber River at Scarlett Road (RWMP site GHU-2)...................................... 17 Figure 12: Humber marshes........................................................................................................ 17

LIST OF TABLES

Table 1: Humber River Watershed Stream Length by Order ...................................................... 7 Table 2: Morphologic Characteristics of the Humber River and Tributaries ............................. 11


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1.0 INTRODUCTION

In 1997, the Humber Watershed Task Force released the Humber River Watershed Strategy, Legacy: A Strategy For A Healthy Humber (MTRCA, 1997), which provided thirty objectives for a healthy, sustainable watershed, and a set of actions necessary to achieve them. It also provided an overview of the state of the Humber River watershed at that time. Since the release of the watershed strategy, a significant amount of new information has become available through monitoring, special studies and the experiences of watershed partners. In 2004, the Toronto and Region Conservation Authority (TRCA), in partnership with watershed municipalities and the Humber Watershed Alliance initiated a study to develop an integrated watershed management plan for the Humber River. This study was initiated to fulfill the watershed planning requirements of the Oak Ridges Moraine Conservation Plan, 2002, and to update the strategies and recommendations of Legacy, in light of new information, a stronger scientific foundation and better understanding of the effects of human actions on natural ecosystems. The watershed plan is intended to inform and guide municipalities, provincial and federal governments, TRCA, non-governmental organizations and private landowners regarding management actions needed to maintain and improve watershed health. This State of the Watershed Report summarizes available information on current conditions obtained through the Regional Watershed Monitoring Program and more detailed studies in selected areas. This information establishes baseline conditions for a number of reaches, which will be used to evaluate changes and assess trends in the future. This report also describes emerging trends, drawing on evidence from adjacent urbanizing watersheds, and identifies potential watershed management issues and opportunities in the Humber pertaining to fluvial geomorphology. Indicators of watershed health and associated targets are established and used to rate current conditions. Ratings for a full suite of indicators of watershed health are summarized in, Listen to Your River: A Report Card on the Health of the Humber River Watershed (TRCA, 2007). This State of the Watershed report also provides an overview of current management strategies and introduces some innovative approaches to address existing and potential future issues, which will be considered for inclusion in the Humber River Watershed Plan. It begins with an overview of factors that influence watershed conditions and the indicators being used to track current conditions and evaluate watershed health.


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2.0 UNDERSTANDING FLUVIAL GEOMORPHOLOGY

Fluvial geomorphology is a study of the processes responsible for the shape and form, or morphology, of watercourses. In simple terms, fluvial geomorphology describes the processes whereby sediment (e.g., silt, sand, gravel) and water are transported from the headwaters of a watershed to its mouth. These processes determine and constantly change the form of river and stream channels and determine the stability of channel morphology. Fluvial geomorphology studies identify and quantify these processes, which are dependent on climate, land use, topography, geology, vegetation and other natural and anthropogenic influences. Protecting, managing and restoring the shape and form of watercourses requires a thorough understanding of fluvial geomorphology and the effects of urbanization on geomorphic processes. Urbanization affects the movement of water and sediment in a watershed and therefore affects geomorphic processes. Aquatic species which have adapted to the natural shape and form of watercourses are impacted. Human lives and property are also put at risk if adverse impacts to channel form occur. To avoid these results, the potential impacts of urbanization should be addressed through planning and design of new developments and application of best management practices, as experience has shown that it is extremely difficult to repair urban watercourses after damage has occurred. The Humber River watershed is rapidly urbanizing and the river system is reacting to this change. There are reaches where the stream channel is no longer stable and is adjusting to a changed pattern of stream flow, largely due to increased surface runoff from impervious surfaces associated with urban developments. Traditional engineering practices employed to deal with excessive flows exacerbate these conditions. Through enhanced understanding of natural systems, efforts are being made to restore natural function to impacted watercourses. A watercourse, by its very nature, is a dynamic system responding to a constant change in flow and sediment supply. The amount of flow in a natural watercourse is determined primarily by climate and geology. Climate controls the amount of water delivered to the surface of the watercourse and how and when it arrives. Geology exerts a fundamental control on what happens to the water once it arrives at the ground surface. Through its effect on infiltration, geology establishes the volume and proportion of groundwater and surface water available to flow through a drainage basin. Geology also determines the volume and properties of sediment supplied to the channel, and the strength and erodibility of the surficial material through which the watercourse flows. A complex underlying geology and topography can result in considerable variation in channel character, as well as sensitivity to potential impacts, within the same drainage system. Watercourses respond to changes in flow and sediment supply with frequent adjustments in channel position and shape accomplished through erosion and deposition. This self-regulating ability is an inherent characteristic of natural watercourses that allows channel morphology to remain relatively constant. The state in which flow and sediment supply are balanced to achieve this stable channel form is referred to as “dynamic equilibrium”. In a condition of dynamic equilibrium, channel morphology is stable but not static, since it changes gradually as sediment is deposited and re-mobilized throughout the watercourse. For example, in many natural watercourses the outsides of channel bends tend to erode, but there is corresponding


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deposition of material on the insides of bends. This gives the channel the appearance of ‘migrating’ across the floodplain or in a downstream direction. This kind of erosion and deposition is natural and is essential to maintaining the balance between flow and sediment supply in the system. Dynamic equilibrium is also critical for riparian and aquatic biota, which are adapted to the habitat provided by this constantly evolving but stable condition. Geomorphic Processes and the Human Landscape Over periods of centuries or even decades, human activities can affect geomorphic processes on a scale that transcends natural impacts with an effect likened to a major global climate change (Knighton, 1998). Deforestation reduces evapo-transpiration and infiltration and increases runoff and sediment supply to watercourses. Farming involves tile drainage and watercourse re-direction through ditches, which reduces stream length and alters flow and habitat potential. Urban development typically results in the extensive compression and paving of land surfaces, which significantly reduces infiltration and dramatically increases runoff to watercourses unless extensive mitigation is applied. When changes in flow regime and sediment supply from land clearing and urbanization exceed the thresholds for self-regulation in affected watercourses, the dynamic equilibrium will be upset, causing the channel to become unstable. In such circumstances the watercourse adjusts with physical changes that occur much more rapidly than the controlled adjustments of the natural dynamic equilibrium. These changes are rapid, extensive and often catastrophic and may include severe bank erosion, a lowering of the bed level of the stream, or major changes to the path of the channel itself. Such changes can result in destruction of aquatic and riparian habitat, damage to infrastructure and property, and risks to public safety. Research into the effects of urbanization on watercourses has indicated that the critical threshold, at which channel destabilization begins, typically corresponds to a total drainage basin imperviousness of three to five percent (Hammer, 1972; Booth, 1990). Significant enlargement of the channel cross-section begins once the drainage basin reaches five to ten percent imperviousness. It is estimated that the channel will continue to enlarge, in response to urbanization, for a period of 35 to 65 years after the end of development in the watershed. Once adjustment of the channel to urbanization is complete, the cross-sectional area may be up to 6 times greater than that of the channel prior to disturbance (e.g., Hammer, 1972). This enlargement can occur by erosion of the channel banks and incision of the channel bed, the degree of each being determined by their relative resistance to erosion. In addition to cross-section enlargement, urban watercourses also experience adjustment of their plan form as the channel attempts to evolve a new meander pattern that is compatible with the new hydrologic and sediment regime. This adjustment process is thought to take an order of magnitude longer than cross-section change, resulting in a total period of instability as a result of urbanization that may be measured in centuries. It is theorized that urban watercourses will eventually achieve a new form of dynamic equilibrium through these adjustments. Even if this should occur, experience suggests that the ultimate form of an urban watercourse will bear little resemblance to a natural river or stream and will not possess the stability or structure required to support diverse aquatic ecosystems (Booth and Jackson, 1997; Fuerstenberg, 1997). In addition to the effects of land use change, human induced change can also include activities that result in direct modification to watercourse channels themselves. Agricultural practices


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can sometimes result in the realignment and channelization of watercourses resulting in loss of natural channel forms and habitats. Tillage immediately adjacent to watercourses causes channel instability as bank vegetation that would normally control erosion rates is lost. In the past, channels were realigned and straightened to facilitate development, changing aquatic habitat and intensifying channel instability as the resulting artificial channel forms lacked natural adjustment mechanisms. Furthermore, historic approaches to flood control have emphasized the rapid removal of water from the landscape, generally via the realignment, enlargement, and hardening of river and stream networks. The resultant increase in flow velocities and reduction in flow attenuation from the disconnection of channelized watercourses from their floodplain has amplified the increase in flows caused by urban land uses and exacerbated the resultant erosion. Historically, the management of channel instability and increased erosion in impacted urban watercourses has been addressed using engineered erosion protection. This has involved a variety of modifications to river and stream channels including hardening of bed and/or banks with concrete, riprap, gabion baskets or armour stone as well as the installation of weirs and other grade control measures. However, in many cases such works have failed because they are undermined or circumvented by the watercourse channel as it adjusts either to maintain its natural evolutionary path or to respond to continued urbanization. Such works also affect aquatic and riparian habitat within and adjacent to the watercourse. Hardening of the channel increases velocities and decreases natural attenuation of flows, exaggerating the urban land use impacts on physical channel form described above. As a result, these conventional engineering approaches have typically resulted in a cycle of failure of the installed protection and ongoing channel degradation, leading to regular repairs on existing works and to the need for constructing new protection works elsewhere. In recognition of the negative outcomes of past erosion management approaches, current practices include consideration of geomorphic and ecological processes, as well as potential impacts on upstream and downstream areas when designing and constructing erosion protection works. In some cases, large sections of watercourse are reconstructed in an attempt to restore equilibrium conditions through a practice referred to as “natural channel design”. However, the complexity of geomorphic processes in urban systems and the constraints created by infrastructure and private property make it difficult to truly recreate natural channels, and the performance of such projects in restoring natural physical and ecological function of watercourses is still unknown. Further, conventional erosion protection works continue to be constructed for sites or areas immediately at risk where there is insufficient time, space or funding to examine more comprehensive solutions. Over the past two decades, development has increasingly incorporated stormwater management measures to attempt to mitigate the unbalance between the urban hydrologic regime and the natural channel form. By far the most popular and widely-used approach is the design of end-of-pipe stormwater ponds or wetlands to detain the excess runoff from urban developments and release it slowly at a rate that is considered to be safe to the stability of the receiving watercourse. The design of such facilities is predicated on the assumption that flows in the watercourse below the level required to initiate sediment transport of the median substrate particle size will not result in erosion. Currently, there is increasing evidence that these facilities may not be protecting receiving watercourses downstream of new developments (Booth and Jackson, 1997). It is speculated that this may be due an


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oversimplification of complex mechanisms of erosion and sediment transport in current design practices in that the release of flows at low rates may not be sufficient to mitigate their impacts (Aquafor Beech Limited, 2007). In addition, there is evidence that these facilities may not perform to detain flows in real-world conditions even to the degree for which they are designed (Bengtsson and Westerstrom, 1992). Such results suggest that stormwater management approaches based on detention may not be sufficient to manage the watercourse impacts from increases in runoff and flow volume in urban areas.

3.0 MEASURING FLUVIAL GEOMORPHOLOGY

Measurement of fluvial geomorphology and geomorphic process involves both examination of channel morphology and investigation of the flow regime and sediment supply that drive geomorphic processes in the watershed. The combined information resulting from these measurements allows the geomorphic condition of a watershed to be determined, and repeated measurements over a period of time allow for the impacts of land use change and urbanization on geomorphic processes to be evaluated. Characterization of conditions in a large area with respect to fluvial geomorphology is made difficult by limitations in the ability to collect information. As there are thousands of kilometers of defined rivers and streams in the Humber River watershed, it is not practical or economically possible to maintain current data describing the condition of each segment of watercourse within the system. However, the Toronto Region Conservation Authority (TRCA) has initiated long-term geomorphic monitoring as part of its Regional Watershed Monitoring Program (RWMP), focusing on a limited number of sites that were selected to be representative of the broader range of conditions within each watershed. In the Humber River watershed 35 geomorphic monitoring stations have been established, as shown in Figure 1. Monitoring was initiated on all sites in 2001, using standard fluvial geomorphology documentation and measurement techniques to characterize channel form , dimensions and critical discharge (i.e. erosion threshold) values (Parish Geomorphic Limited, 2002 and 2003). Reference points were set up to allow TRCA to monitor changes of some parameters over the course of repeated measurements, which are planned on a three-year cycle. In addition to geomorphic measurements, upon initiation of the geomorphic monitoring, all sites were subject to rapid assessments which provide a qualitative measure of watercourse condition, as well as basic historic air photo analysis to provide some measure of the types of change that have occurred over the past several decades. In addition to the assessment and monitoring work completed for the 35 RWMP sites, detailed reach-based assessments of channel, stream bank and substrate characteristics, and critical discharge values (i.e. erosion threshold) have also been completed for portions of the stream network within West Humber River subwatersheds in support of urban growth planning initiatives in the City of Brampton (Aquafor Beech Limited et al., 1997), within the City of Toronto portions of the Lower Humber, West Humber and Black Creek subwatersheds in support of the Wet Weather Flow Management Master Plan (XCG Consultants Limited, 2003), and Centreville Creek and Cold Creek subwatersheds in support of subwatershed planning studies (Parish Geomorphic Ltd, 2002; Aquafor Beech Limited and Parish Geomorphic Limited, 2004).


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Measuring channel morphology involves an examination of the complex three-dimensional geometry of a watercourse. Typically, channel morphology is defined in three different planes: plan form (the form of the channel when viewed from above); longitudinal profile (the elevation and gradient of the bed in a lengthwise direction); and cross-section (the size and shape of the channel in cross-profile). Measurements in these planes are taken using topographic survey equipment and then transferred into two-dimensional representations that can be interpreted and compared with subsequent surveys. The rate of erosion and morphological change is also monitored using erosion pins that are driven into the channel banks. Another fundamental aspect of channel morphology is the bed material, or substrate, which is an important factor in forming the channel geometry. The composition of the bed material may also provide insight into the watershed sediment supply. Bed material is characterized by sampling the substrate and analyzing the particle size distribution of the sampled material. Flow regime can be measured directly by gauging flow in a watercourse at discrete locations. However, as the installation and calibration of stream gauges is time consuming and expensive, the flow regime is often predicted using computer hydrologic models. This method also allows the effects of land use changes on flow regime to be predicted by modifying the input parameters of the model. Empirical stream flow data from gauges or modelled flow data can be related to geomorphic conditions using various indicators relate the effect, or potential effect, of changes in flow to sediment transport and erosion. One type of indicator used is an erosion index, which is an indicator of the length of time that flow in the creek exceeds a rate at which erosion is assumed to occur (i.e. critical discharge), and the magnitude of flow during that time. In theoretical terms, an erosion index can be used comparatively to examine the change in erosion potential as a result of different flow conditions. However, the results of such analyses must be used with caution as complex erosive processes cannot be described through the designation of a simple erosion threshold, and therefore the amount of erosion or channel instability that will actually occur may not relate directly to the calculated erosion index. The degree of disturbance to river and stream channels and corridors within a watershed can provide a measure of their resilience and ability to self-regulate. Where river and stream channels have been disturbed through artificial confinement, straightening or hardening, they are unable or less able to modify their form in a controlled manner in response to changes in flow regime or sediment supply and as a result often erode catastrophically or transfer erosion problems to downstream areas. Further, where river valleys and stream corridors are constricted by infrastructure or development, the watercourse channel may not be able to migrate naturally across its floodplain and is typically hardened or altered to prevent this from occurring. While watercourses may still be impacted by flow changes from urbanization, where channels and corridors remain undisturbed the resultant changes are more contained and controlled, and present less of a threat to life, infrastructure and property when they occur.


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4.0 EXISTING CONDITIONS IN THE HUMBER WATERSHED

4.1 Regional Watershed Monitoring Program Sites

The total length of river and streams in the Humber River watershed is approximately 2032 kilometres, with the majority (78%) being small tributary streams , classified as either first, second, or third order. Stream order is a measure of the degree of stream branching within a watershed; a first-order stream is an unbranched tributary, a second-order stream is a tributary formed by the connection of two or more first-order streams, a third-order stream is a tributary formed by two or more second-order streams and so on. The principal stream order of a watershed is the largest stream order present within it, and therefore the Humber River watershed is a seventh order system due to the order of the main branch of the river. This is typical of many larger watersheds in southern Ontario that drain to the north shore of Lake Ontario. Table 1 and Figure 1 illustrate the channel length associated with each stream order in the Humber River watershed, and in the primary subwatersheds.

Table 1 Humber River Watershed Stream Length by Order

Stream

Order

Total

(km)

Main

Humber

West

Humber

East

Humber

Lower

Humber

Black

Creek

1 736 375 182 148 18 13

2 471 238 106 107 9 11

3 368 176 91 67 3 31

4 189 99 61 29 0 0

5 124 68 29 27 0 0

6 78 32 12 34 0 0

7 66 35 0 0 31 0

Total 2032 1023 481 412 61 55

Bifurcation ratio is a measure of the degree of branching of tributaries within a watershed and is defined as the average ratio of stream length of a particular order to the length of streams of the next greatest order. The average bifurcation ratio for the Humber River watershed is 1.5, which is typical of the soils and topography of many southern Ontario watersheds. The drainage density of the Humber River watershed, which is the ratio of the total length of river and stream channels to the overall watershed area, is 1.4. This is a relatively low value, which is typically indicative of a watershed that naturally experiences relatively low runoff and greater infiltration of fallen precipitation. While this may be accurate in case of the upper portions of the Main and East Humber subwatersheds, a higher drainage density would be expected for portions of the watershed located on the till soils of the South Slope and Peel Plain. This may be partly explained by the elimination of many first and second order streams as a result of agricultural practices as well as historic urban development practices.


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Like most fluvial systems in southern Ontario, the Humber River and its tributaries are ‘semi-alluvial’ in nature (Ashmore and Church, 2001). A purely alluvial watercourse is one in which the form of the channel and surrounding floodplain are entirely the consequence of the transport and deposition of sediment, or alluvium, supplied to the watercourse channel from the surrounding landscape. In southern Ontario, streams are relatively young in geologic terms and most were created at or after the end of the Wisconsin glaciation around 10,000 BC. Because of the relatively short time since their creation, and the erosion resistant till surface soils left by glaciers, there has not been enough erosion and supply of alluvium into stream corridors to create an exclusive control on channel form. As a result, in addition to the characteristics of alluvium transported from upstream, the morphology of stream channels is at least partially controlled by the properties of the glacial deposits through which they flow. In the case of the Humber River watershed, this controlling material includes glacial till, glacial lake and pond deposits and glacial outwash material. Further, the large and well-defined valleys of the main Humber River and its major tributaries were likely formed by glacial outwash rather than the current rivers and streams that reside in them, and therefore the development and evolution of these watercourses is often controlled by the much larger outwash channel. While the morphology of semi-alluvial streams can superficially appear similar to alluvial streams, their channels often consist of a relatively thin layer of alluvium on top of a non-erodible or highly resistant base material such as till or bedrock. For this reason, these steams may not develop morphological features such as pools and riffles with the same regularity or predictable form (Foster, 1999). This is important as the majority of research in theoretical and applied fluvial geomorphology has been conducted on alluvial streams, and, therefore, conventional models, theories and methods must be applied with caution to semi-alluvial situations. In addition to glacial influences, the variable topography (Figure 2), geology and land use of the Humber River watershed result in highly diverse morphology in the river and its tributaries, both between the headwaters and the lake as well as between from the eastern to the western watershed boundaries. The highest elevation areas occur above the Niagara Escarpment, in the extreme northwestern portion of the watershed (blue areas on Figure 2), with the steepest slopes occurring on the escarpment while much of the remainder of the watershed is characterized by gentle slopes. Table 2 summarizes some of the morphologic characteristics of river and stream channels measured at Regional Watershed Monitoring Program (RWMP) stations throughout the watershed (refer to Figure 1 for locations).


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Table 2 Morphologic Characteristics of Humber River and Tributary Channels

Area ID#

Drainage

Area

(km2)

Avg.

Width

(m) 1

Avg.

Depth

(m)

Slope

(%)

Median

Substrate

(cm)

Critical

Discharge

(m3/s)

Average

Bank

Height (m)

GHU-35 30.4 8.12 0.59 0.78 0.77 0.4 1.1

GHU-34 42.3 9.22 0.64 0.43 0.11 0.072 1

GHU-33 172.3 14.88 0.83 0.07 1.4 5.54 1.6

GHU-32 7.5 2.37 0.42 0.36 fines 0.62 0.9

GHU-31 8.3 3.2 0.28 0.25 fines 0.78 1.1

GHU-29 197.2 15.3 0.77 0.13 0.9 2.34 1.9

GHU-28 60.9 7.97 0.78 0.09 0.041 1.4 1.6

GHU-27 279.0 14.56 0.8 0.29 1.9 4.49 1.7

GHU-26 6.4 3.99 0.32 0.98 0.021 0.02 1.9

GHU-21 293.3 14.79 0.79 0.14 0.55 1.46 2.1

GHU-13 2.9 6.78 0.3 0.49 fines 0.18 0.9

GHU-12 4.0 5.09 0.4 0.92 0.078 0.069 2.1

GHU-6 14.9 4.07 0.6 0.36 0.005 0.65 1.3

Main Humber

GHU-7 43.3 9.26 0.62 0.46 2.2 1.36 2.1

GHU-19 8.9 6.05 0.29 0.71 1.9 0.43 1

GHU-18 27.9 7.38 0.42 1.03 1.9 0.88 0.9

GHU-17 28.7 5.48 0.48 0.34 0.65 0.58 1.4

GHU-16 5.9 3.96 0.25 0.59 silt 0.98 0.7

GHU-15 20.9 6.3 0.34 0.57 0.06 0.063 1

GHU-14 47.7 5.99 0.42 0.48 0.56 0.19 0.9

GHU-10 54.1 11.86 0.59 0.14 0.57 4.74 1.3

GHU-9 74.8 6.91 0.4 0.61 2.8 1.68 1.2

GHU-8 11.0 5.22 0.32 0.74 0.04 0.03 1

West Humber

GHU-3 201.8 20.21 0.63 0.58 3.3 5.74 2.3

GHU-30 5.1 5.78 0.24 0.60 fines 0.1 0.8

GHU-25 129.9 8.68 0.7 0.18 0.97 1.57 2.3

GHU-24 93.3 7.71 0.79 0.17 0.24 0.21 1.7

GHU-23 72.3 10.51 0.67 0.54 0.49 0.44 1.8

GHU-22 21.8 2.79 0.48 0.06 0.33 0.15 1.2

GHU-20 11.4 6.61 0.59 0.46 0.013 0.18 1.4

East Humber

GHU-11 187.6 12.31 0.68 0.29 1.3 2.54 1.6

GHU-5 581.2 30.25 1.05 0.23 1.9 8.74 2

GHU-2 832.2 46.37 0.93 0.34 12 17.01 2.5

Lower Humber

GHU-1 908.4 52.91 1.19 0.17 0.004 4.44 3.7

Black Ck. GHU-4 11.5 6.34 0.69 0.45 1.2 8.74 1.1 from TRCA Regional Watershed Monitoring Program, 2001 1 – at bankfull stage


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The upper reaches of the Main Humber River subwatershed are characterized by an extensive, highly branched network of short first and second order tributary channels contributing to the larger tributaries, Centreville Creek and Cold Creek (Figure 3). This irregular drainage network is the result of the hummocky topography and varied geology in this area caused by glacial action and the resulting deposits, as well as the extensive number of groundwater discharge locations. As the Main Humber River collects drainage from its tributaries, it becomes increasingly wide and sinuous and the slope of the channel decreases as it flows off of the Oak Ridges Moraine on to the till plain (Figure 4). The drainage of the West Humber River subwatershed, as well as Rainbow Creek, is much more regular than that of the Main Humber, as the relatively resistant till material and uniform slope of the landscape create relatively straight watercourses that are largely oriented in the same northwest-southeast direction (Figure 5). As the main branch of the West Humber River flows into the City of Toronto, it becomes relatively wide and shallow as the channel cuts into the underlying shale bedrock, adding angular substrate to the channel bedload (Figure 6). The East Humber River subwatershed has a typical branched drainage network with evenly distributed headwater tributaries of lesser number and greater length than those found in the Main Humber River headwaters (Figure 7). This can be attributed to relatively less extensive groundwater discharge, more regular topography and more resistant and impermeable surface soils. The main branch of the East Humber River becomes wider and more sinuous as it approaches the confluence with the Main Humber (Figure 8). Many of the historic tributaries of Black Creek have been eliminated as a result of past agricultural or urban development practices. Headwater channels that remain (Figure 9) are much larger and wider than headwater watercourses of similar drainage area in the East, Main and West Humber River subwatersheds due to the major increases in stormwater runoff and streamflow that have been caused by the extensive development in the Black Creek subwatershed. South of Wilson Avenue, the main branch of the creek has been channelized, realigned or hardened, leaving little or none of the original channel alignment and character (Figure 10). The Lower Humber River channel is very large as the result of the confluence of the West, Main and East Branches, and becomes a dominant feature as it flows through the City of Toronto (Figure 11). South of Bloor Street, the grade of the Humber River is controlled by the level of Lake Ontario, and flow becomes quiescent with a significant portion of the marsh complex at the mouth of the river still intact (Figure 12). As with Black Creek, most of the smaller tributaries that would have originally drained the portion of the watershed south of Steeles Avenue have been eliminated and as a result most of the additional flow input to the river in the City of Toronto is the result of storm sewer discharge. In many areas, the Lower Humber River was historically realigned, channelized or straightened to facilitate development. This confinement of the channel and increase in overall slope from a reduction in channel length has resulted in downcutting of the channel, and as a result a series of grade control structures have been constructed between Bloor Street and Lawrence Avenue to prevent further bed degradation and erosion.


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Figure 3 Cold Creek at Castlederg Sideroad (RWMP site GHU-32)

Figure 4 Main Humber River at Kirby Road (RWMP site GHU-21)


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Figure 5 West Humber River (main branch) at Mayfield Road (RWMP site GHU-17)

Figure 6 Lower West Humber River at Islington Avenue (RWMP site GHU-3)


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Figure 7 East Humber River west of Bathurst Street (RWMP site GHU-22)

Figure 8 East Humber River at Langstaff Road (RWMP site GHU-11)


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Figure 9 Black Creek between Steeles Avenue and Finch Avenue (RWMP site GHU-4)

Figure 10 Black Creek south of Wilson Avenue


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Figure 11 Lower Humber River at Scarlett Road (RWMP site GHU-2)

Figure 12 Humber marshes


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4.2 City of Toronto Study Sites In support of the City of Toronto’s Wet Weather Flow Management Master Plan, a reconnaissance level geomorphic survey of thirteen (13) sites was carried along City of Toronto portions of the Lower Humber River main channel, West Humber River and Black Creek during the fall of 2000. Detailed evaluations of erosion and stream bank stability were carried out at six (6) of the thirteen sites. This information was used to recommend stream restoration works along selected reaches as part of the preferred management strategy (XCG Consultants Limited, 2003). Through this preliminary assessment, the reaches of the West Humber River were the only sites examined that considered to be relatively stable. Reaches evaluated along the main channel of the Lower Humber were all observed to be, although not fully unstable, in a process of adjustment to a changed stream flow or sediment supply regime, presumably from upstream development and/or the historic realignment and alteration of the channel. Reaches of Black Creek evaluated through this study were evaluated to be unstable, reflecting the nearly full urbanization of the Black Creek subwatershed and the fact that very few existing urban areas contain modern stormwater management controls. Reaches where stream banks have been “engineered” (e.g., channels have been lined with concrete to improve conveyance of high stream flows or where armoring of stream banks with gabion baskets has been done) were delineated through interpretation of aerial photography. Although the smaller tributaries including Albion Creek, Berry Creek and Silver Creek have not yet been explicitly evaluated, given the nature of their tributary catchments (fully urbanized with little or no stormwater management controls), it was assumed that channels in these subwatersheds would be unstable due to similar controlling conditions as on Black Creek (XCG Consultants Limited, 2003).

To help address observed instances of channel instability and erosion damage to the valley system and improve the quality of aquatic habitat, stream restoration works were recommended for several reaches within the City of Toronto including portions of the Lower Humber south of Eglinton Avenue, the West Humber around Kipling Avenue and several reaches of Black Creek. Stream restoration works that were proposed to be implemented over a 15 year timeframe included: restoration of degraded sections of the streams using natural channel design techniques; stream bank revegetation; removal or modification of a number of in-stream barriers to fish movement; reforestation and wetland creation; and restoration of the Humber River Marsh. A total of 10 kilometres of stream were proposed for restoration, 24 fish barriers were to be removed or modified, 12 kilometres of stream were to be revegetated, 35 hectares of valley lands were to be reforested and 5 hectares of wetlands were to be created. However, the scope of the Wet Weather Flow Management Master plan was strictly conceptual with respect to stream restoration, and significant additional study is required to determine if the proposed restoration measures are both feasible and appropriate.


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4.3 Subwatershed and Drainage Study Sites

Rainbow Creek In 1988, in support of the City of Vaughan’s Rainbow Creek Master Drainage Plan, a detailed watercourse inventory was carried out to assist in the development of storm drainage design criteria for area of proposed urban development (Cosburn Patterson Wardman Limited, 1991). The field survey identified forty (40) eroding sites or erosion scars within Rainbow Creek subwatershed. At the time the erosion sites did not represent immediate threats to any existing public infrastructure or private property, but over time were thought to have the potential to result in the loss of tableland. The study did not determine whether the erosion observed was the result of natural geomorphic processes or from human-induced changes such as land development or channel alteration

West Humber In fluvial geomorphology investigations conducted for the West Humber Subwatershed Study (Aquafor Beech Ltd. et al., 1997), it was concluded that stream channels have been highly modified in the past through transition from forested conditions to agricultural land uses accompanied by agricultural drain construction, culvert and bridge construction, and channel re-alignment. Riparian vegetation was found to have been altered significantly in the past and generally lacking mature forest canopy coverage. The West Humber Subwatershed Study Final Report identified reaches within the Main, East, and Lower Branches, where stream channel rehabilitation work should be a management priority. Similar to the Toronto Wet Weather Flow study, further detailed study is required to determine if the recommended stream rehabilitation is practical and appropriate given current and evolving conditions in the subwatershed. Fluvial geomorphology studies have been completed for reaches within the West Humber subwatershed (West, Main and East Branches) in support of development planning for the Eastgate and Vales of Castlemore secondary plan areas in the City of Brampton (Aquafor Beech Limited et al., 1997). A field inventory of erosion sites was completed in May and June of 1994 to document baseline conditions of erosion features present at that time. The erosion site inventory identified 119 erosion features and criteria were applied to classify each feature according to level of priority for mitigative action. Of the 119 erosion features, 17 high priority features were identified that represented sites where high potential existed for further stream bank erosion to cause damage to existing structures, loss of tableland; excessive sediment loading to the stream; negative impacts to downstream sensitive aquatic habitats; and, negative impacts on the storage capacity of the Claireville Reservoir flood control facility. As in the Rainbow Creek study, it was not determined whether the causes of erosion at these locations were natural or anthropogenic. Detailed assessments of geomorphic characteristics were completed for eight (8) representative reaches of the West Humber that flow through the Eastgate and Vales of Castlemore secondary plan study areas to characterize the stability of the fluvial system. All of the reaches assessed in detail showed evidence of past alteration. Reaches upstream of


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Mayfield Road were found to be dominated by local disturbances in riparian vegetation and channel hydraulics from livestock, agricultural practices and drainage works. In reaches downstream of Mayfield Road, channel instabilities observed were found to be more often related to alterations in upland stream flow regimes and direct modification of the channel and riparian vegetation. Various degrees of instability were observed in all four (4) channel segments surveyed, downstream of Mayfield Road. Centreville Creek Fluvial geomorphology studies have been completed for Centreville Creek and Cold Creek tributaries of the Humber River in support of subwatershed planning studies initiated by the Toronto and Region Conservation Authority in partnership with the Region of Peel and local municipalities. These subwatershed studies have been undertaken to assist regional and local municipalities with implementing their environmental policies, to set criteria for implementation of best management practices and to identify priority regeneration initiatives to improve the health of the natural system.

Within the Centreville Creek subwatershed, rapid assessments of geomorphic parameters were completed for 24 reaches, including evaluations of channel condition and stability (Parish Geomorphic Limited, 2003). Detailed assessments of geomorphic parameters were completed for 3 of the 24 reaches and critical erosion threshold values were estimated. Most of the reaches assessed were found to be in transition, adjusting to a changed stream flow regime. Generally, channels were observed to be adjusting through widening and aggradation. The study did not examine the causes responsible for this presumed transitional condition. It is important to note rapid assessment techniques rely on qualitative observations of watercourse channels characteristics that may not necessarily indicate transition or instability on all channels, particularly those that exhibit more rapid natural rates of change. Cold Creek Within Cold Creek subwatershed, rapid assessments of geomorphic parameters were completed for 36 reaches, including evaluations of channel condition and stability (Aquafor Beech Limited and Parish Geomorphic Limited, 2004). Detailed assessments of geomorphic parameters were completed for 4 of the 36 reaches and critical erosion threshold values were estimated. Results of these analyses showed that much of the Cold Creek drainage network has been directly altered or influenced by human or agricultural activity in the past. However, the stream channels were found to be relatively stable.


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5.0 WATERSHED REPORT CARD RATING

The objective for management of fluvial geomorphology issues, as defined in the Humber River watershed strategy, Legacy: A Strategy For A Healthy Humber (MTRCA, 1997) is to “protect the form and function of the Humber River and its tributaries”. While the 2000 Humber watershed report card did not include indicators and targets for watershed conditions pertaining to this objective the following are recommended to be used in future reporting and evaluation:

Objective: Protect the form and function of the Humber River and

its tributaries

C

Indicator Measure Target Rating

Channel morphology

Channel geomorphic surveys at RWMP sites

Maintain or restore natural channel structure and rates of morphologic change1

Information not yet available

Erosion index Erosion indices and flow frequency

Maintain or restore pre-development erosion potential and flow regimes2

Information not yet available

Natural cover in stream corridors

Portion of riparian zone with riparian vegetation

Greater than 75% of riparian areas with natural cover

C

Risk to public and private property from channel erosion and evolution

Number of locations where infrastructure, buildings and private property are located in stream corridors at risk from channel erosion and evolution

Reduce or eliminate buildings, infrastructure and private property at risk from channel erosion and evolution3

Baseline not yet

established

1 Target channel structure and baseline rates of change to be defined from RWMP site data as well as

other local and provincial reference data sets. 2 Based on long term stream gauge measurements and additional gauges recommended for installation;

Baseline stream flow regimes are described in TRCA, 2008b. 3 Baseline to be determined by conducting an inventory of all structures at-risk.

Sufficient information to assign a rating regarding the indicator of channel morphology is not possible at this time. Information from more recent assessments of channel structure and rate of change at RWMP sites is not available to be compared to findings from the 2001 assessments, so a rating cannot be assigned according to this measure. Furthermore, it is premature to attempt to calculate stream bank erosion rates using the erosion pins installed in 2001, as changes have likely been too small to accurately measure. It is proposed that baseline stream bank erosion rates be calculated based on the assessment of fluvial geomorphology monitoring sites completed in 2007. Notwithstanding the above, there is


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information from previous studies and qualitative observation to indicate that the channel morphology of Humber River Watershed watercourses has been highly impacted through both direct channel alteration and the altered hydrologic regime resulting from urban development. At present, these impacts are most apparent in the City of Toronto and in particular Black Creek, but it is expected that detailed study would identify many of the same impacts on watercourses within and downstream of developed areas of Brampton, Vaughan, and possibly King City and Caledon as well. Characterization of baseline erosion indices and flow frequencies has been completed through analysis of long term stream gauge measurements (TRCA, 2008b) and estimated for selected other locations in the watershed based on outputs of a continuous simulation hydrologic model (HCCL, 2008; TRCA, 2008c). A rating for the indicator of erosion indices and flow frequency cannot be assigned at this time because baseline conditions have just been established. The existing amount of natural cover within the riparian zone for the watershed is 60.7% (TRCA, 2008) and a rating of C for this measure has been assigned, based on the proportion of the target achieved, considering that the target is 100%. This rating suggests that a significant amount of work remains to be done to protect stream channels in the Humber, including regeneration of natural vegetation along stream banks.

A rating for the indicator of risk to public and private property from channel erosion and evolution cannot be assigned because a baseline inventory of structures at risk has not yet been completed. Such an inventory is to be completed as follow up to completion of the watershed planning study.

In the absence of ratings for other indicators of condition, the updated watershed report card rating for the fluvial geomorphology objective is based on available information regarding riparian zone vegetation and observational knowledge of channel condition throughout the watershed. Once information regarding additional indicators becomes available, future watershed report card ratings will be based on an average of ratings assigned for each indicator and our ability to compare between periods.

6.0 MANAGEMENT CONSIDERATIONS

Watercourses are complex, dynamic systems that evolve over long periods to achieve a balance between climatic and geologic processes in a watershed. Human development and alteration of watercourses can disrupt this sensitive balance, resulting in instability and extreme conditions which impact natural systems and humans alike. The process of channel adjustment to these disruptions can continue for decades or even centuries, prolonging impacts and causing the loss of species that rely on the impacted watercourses for habitat. A key management principle for protecting the form and function of stream channels is respecting the complexity of geomorphic processes and the potential for disruption of these processes following changes in land use or land cover. Striving to maintain watercourses and their valley corridors in a natural state, and preserving their pre-development hydrologic regime and sediment supply to the greatest extent possible should be a primary objective as experience has shown that it is very difficult to recreate the form and function of natural watercourses that are impacted by urbanization. That said, a myriad of interruptions and modifications have already occurred to watercourses within and downstream of urban areas in


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the Humber River watershed, and rehabilitation of urban rivers and streams incorporating principles of fluvial geomorphology has the potential to improve their condition. However, due to the altered urban sediment and flow regime, as well as constraints presented by insufficient corridor widths and the need to protect property and infrastructure in urban areas, such efforts will generally not be able to restore watercourses to a natural, self-regulating state. Long-term monitoring and maintenance will be required to maintain the integrity and function of rehabilitated urban streams. Stream channels in the Black Creek subwatershed and drainage areas of tributaries to the Lower Humber (i.e., Albion Creek, Berry Creek, Emery Creek, Humber Creek and Silver Creek) in the City of Toronto are unstable and adjusting to a changed stream flow regime. It is likely that other streams within and downstream of development areas in Brampton, Vaughan, Caledon and King City are in similar condition. This is the result of both past channel alteration and urban development; the former has directly affected channel morphology and stability while the latter has acted to alter the hydrologic and sediment transport regime of downstream watercourses. In urbanized portions of the watershed where changes in the stream channel are either putting property or infrastructure at risk, or are negatively impacting aquatic community health, careful planning of rehabilitative work is required that incorporates detailed knowledge of fluvial geomorphology. A critical part of such rehabilitative work will be restoring more natural patterns of stream flow in urban watercourses as it is unlikely that an altered urban flow regime can sustain a stable channel with desirable ecological and physical characteristics. In high density urban areas, restoration of more natural patterns of stream flow may only be achieved through the slow and incremental process of urban redevelopment and incorporation of stormwater controls to reduce run-off (XCG, 2003) unless major financial commitments are made to retrofit controls to existing development on a large scale. While not likely to restore natural flow patterns in impacted areas, some level of improvement may be realized in the short term by improving the level of control provided by existing stormwater ponds through retrofitting, implementing lot level and conveyance stormwater controls, and restoring natural cover in upstream drainage areas. Reaches within the Black Creek and Lower Humber subwatersheds in the City of Toronto, and within the West Humber subwatershed in the City of Brampton have been identified through previous studies as being in need of rehabilitative work to address issues associated with channel erosion and to improve aquatic habitat. However, as noted above, it is difficult to conduct stream restoration in areas that are highly impacted and constrained by urban development. All restoration opportunities identified in such planning-level studies should be reviewed and the condition of the subject watercourses and upstream drainage areas assessed in detail before executing any works. Future urban growth areas and associated stormwater controls should be designed to maintain or improve the pre-development hydrologic regime to the extent possible, with a focus on maintaining the duration of erosive stream flow conditions. In urbanizing areas, where opportunities exist to influence land use planning and design decisions, urban land use concepts and associated stormwater management controls need to be designed with consideration of the form, function and sensitivity of affected stream channels, to prevent accelerated rates of stream bank or channel erosion. Detailed assessments of channel


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morphology on a catchment or subwatershed basis need to be completed prior to or as a part of the earliest stages of the planning process. Such assessments need to include estimates of the stream flow conditions at which channel forming erosive forces occur. Monitoring will be essential to determine whether the objective of maintaining natural channel morphology and stability is being achieved. Geomorphic indicators provide effective means of monitoring the health of a watercourse in relation to stream flow and sediment supply. The monitoring of geomorphic processes is a critical part of demonstrating the effectiveness of stormwater management best practices and regeneration initiatives in maintaining or restoring natural patterns of stream flow for the protection pf baseline erosion rates and aquatic habitat conditions. To improve our understanding of the current condition and sensitivity of Humber River stream channels, and of the stressors affecting them, additional subwatershed or catchment scale analyses are needed to complement available information which is generally limited to a small number of sites that may not characterize the larger watershed area. This understanding could then be correlated with knowledge from other disciplines such as hydrology and biology to better understand the relationship between land use, physical channel structure and ecological stream health in the Humber River watershed. Such an understanding would aid in predicting cumulative impacts of anticipated or potential land use changes, and to recommend management approaches that are more effective at minimizing alterations to existing processes.


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7.0 REFERENCES

Ashmore P. and M. Church. 2001. The Impact of Climate Change on Rivers and River

Processes in Canada, Geological Survey of Canada Bulletin 555, Ottawa. Aquafor Beech Limited. 2007. Stormwater Management and Watercourse Impacts: The Need

for a Water Balance Approach. Prepared for Toronto and Region Conservation Authority.

Aquafor Beech Limited, Beak Consultants Limited, Candevcon Limited, 1997, West Humber

River Subwatershed Study Final Report and Technical Appendices, City of Brampton. Aquafor Beech Limited, Parish Geomorphic Limited, 2004, Cold Creek Fluvial Geomorphology

Study, Toronto and Region Conservation Authority, June 2004. Bengtsson, L. and G. Westerstrom. 1992. Urban Snowmelt and Runoff in Northern Sweden

Hydrological Sciences Journal. Vol. 37 (3) pp. 263-275. Booth, D.B., 1990. Stream channel incision following drainage basin urbanization. Water

Resources Bulletin. Vol. 26, pp. 407-17 Booth, D. B. and C. R. Jackson. 1997. Urbanization of Aquatic Systems - Degradation

Thresholds, Stormwater Detention, and the Limits of Mitigation. Journal of the American Water Resources Association. Vol. 22 (5).

Cosburn Patterson Wardman Limited, 1991, Rainbow Creek Master Drainage Plan - Phase 1,

City of Vaughan, December 1991. Foster, G., 1999. Pools, riffles and channel morphology of erosional streams in Southern

Ontario. In Stream Corridors, Adaptive Management and Design; Proceedings of the Second International Conference on Natural Channel Systems. Niagara Falls, Ontario.

Fuerstenberg, R. R. 1997. The impacts of urbanization on streams in King County, Washington. In Workshop proceedings urban stream protection, restoration and stewardship in the Pacific Northwest: are we achieving desired results? Fraser River Action Plan, Department of Fisheries and Oceans, New Westminster B. C.

Hammer, T.R., 1972. Stream Channel Enlargement Due to Urbanization. Water Resources Research, 8, 1530-40. Knighton, D. 1998. Fluvial Forms and Processes, Arnold, London. Metro Toronto and Region Conservation Authority, 1997. Legacy: A Strategy For A Healthy

Humber, Prepared for the Humber Watershed Task Force.


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Parish Geomorphic Limited, 2002, Regional Monitoring Program Fluvial Geomorphology Component – Etobicoke Creek, Mimico Creek and Humber River Watersheds, Toronto and Region Conservation Authority, June 2002.

Parish Geomorphic Limited, 2003, Bankfull Characteristics and Erosion Thresholds For TRCA

Regional Monitoring Program Detailed Sites, Toronto and Region Conservation Authority, May 2003.

Parish Geomorphic Limited, 2003, TRCA Fluvial Geomorphology Study and Erosion

Assessment – Centreville Creek, Toronto and Region Conservation Authority, February 2003.

Toronto and Region Conservation Authority (TRCA), 2007. Listen to Your River – A Report Card

on the Health of the Humber River Watershed. Prepared for the Humber Watershed Alliance.

Toronto and Region Conservation Authority (TRCA), 2008a, Humber River State of the

Watershed Report – Aquatic System. Toronto and Region Conservation Authority (TRCA), 2008b, Humber River State of the

Watershed Report – Surface Water Quantity. XCG Consultants Limited, 2003. Toronto Wet Weather Flow Management Master Plan Study

Area 3 Humber River - Final Report, City of Toronto.

humber river state of the watershed report - fluvial ...trca.on.ca/dotasset/50113.pdf · fluvial...

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