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CHAPTER 14 Fluvial Systems: catchments and rivers

Introduction

Flowing water links the terrestrial sector of the global hydrological cycle with continental -denudation, of which it is a prime agent, and later stages of geological fractionation. Tectonic uplift empowers fluvial erosion, on contact with water cycled by solar energy and concentrated in surface channels by catchment processes. Progressive sediment transfers occur between upper and lower catchments, and subsquently between lower catchments and marine basins. This is a continuous process in ‘short’ systems where coarse, raw fluvial sediments are swept as molasse into trenches and back-arc basins close to orogens. Flood plains form more enduring sediment stores in ‘long’ systems, where reworking continues mechanical and chemical sorting before onward transfer of mature sediments to marine basins.

The chapter commences with the recognition of the drainage basin as the fundamental fluvial landsystem. Its identifiable hydrogeological characteristics convert measurable hydrometeorological inputs to stream flow through a series of in-line stores and transfer processes. The hydrograph is valued as an aid in summarizing discharge characteristics and components. Stream flow is shown to represent an inevitable and more efficient means of moving water at the surface than overland flows. The concept of efficiency is central to all subsequent fluvial geomorphic processes. Sea level provides the principal base to which rivers work, and their long profile corresponds to a power curve, with exponential energy decay reflecting increasing age and lowering of, or distance from, source orogens.

Coalescence of tributary flows steadily increases the volumes of water and sediment which trunk rivers are required to move but also provides a corresponding energy saving as proportionally less water makes friction contact with the channel. Water movement in -channels is outlined prior to establishing core relationships between water and sediment discharge, channel geometry and stream velocity. Channel geomorphic activity is shown to represent a continuous attempt to balance these parameters, prior to placing them in context in catchment-scale fluvial landsystems.

Chapter Summary

Generation of channel flow

• The catchment, or drainage basin, is a landsurface unit which generates stream flow in a principal stream or trunk river. It forms a single accounting unit for the calculation of water and sediment balances.

• Catchment surface and subsurface components convert water, snow, ice and influent groundwater inputs into river discharge, evapotranspiration and effluent groundwater outputs.

• The three-dimensional landsystem is bounded by a watershed and is modelled as a series of in-line stores and transfer routes which delay the generation of stream flow. This moderates the episodic nature of inputs and generally sustains stream flow during dry spells.

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• Hydrometeorological transfers between the atmosphere and catchment are measured in terms of precipitation amount, type, frequency, intensity and duration; they strongly influence the rate and timing of onward transfers.

• Evapotranspiration returns a proportion of the water-equivalent inputs to the atmosphere, although for a significant component this occurs after water enters the drainage network.

• Hydrogeological transfers occur by gravity draw-down through each store as storage capacity is reached. Above-ground transfer is influenced by vegetation characteristics and subsurface transfer by the hydraulic conductivity of soil and rock.

• Stream gauging measures discharge over time and permits the construction of hydrographs which summarize the nature of its response to precipitation events and the components of stream flow.

Stream flow in channels

• Overland flow, which occurs when precipitation intensity exceeds infiltration rates,

is unstable and inefficient, and surface irregularities concentrate water in parts of the sheetflow.

• Horton overland flow on non-vegetated surfaces is distinguished from saturated overland flow, which emerges through soil towards valley floors.

• Channel initiation occurs where the erodibility of the landsurface and erosivity of the flow permit and commences as ephemeral rills or more enduring gullies.

• Hydraulic efficiency increases immediately, with less water in frictional contact with the surface, and remains a priority for stream flow in view of downstream potential energy decay.

• Stream flow encounters frictional resistance with the channel, solid sediment load, atmosphere and internally between ribbons of water as laminar or turbulent flow conditions develop, depending on water depth and channel roughness.

• Channel geometry describes the wetted perimeter and hydraulic radius of any stage of flow in a small channel segment of known length and slope and, with stream velocity, is used to calculate discharge.

• Channels are dynamic and their geomorphic activity and landforms are the response to constant changes in their power threshold, form and function.

• The drainage network is the sum of stream segments throughout a catchment, responsible for transferring incremental amounts of water and sediment downstream through a stream hierarchy and pattern which reflect catchment attributes and stage of development.

Channel erosion and sediment transfer

• Stream power leads to channel erosion through the removal of soluble minerals, corrasion by entrained particles and fluid shear stress directed at the channel boundary by turbulent flow.

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• Fluid stressing, capable of eroding soft and well fractured rocks, is enhanced by cavitation caused by the implosion of bubbles against the channel boundary during rapid turbulent flow.

• Subaqueous erosion is augmented by bank caving above the water surface at low flow stages, through the removal of lateral support.

• Eroded debris and material delivered to the stream from adjacent slopes are entrained when stream velocity exceeds the entraining velocity for a given particle size, summarized in Hjülstrom’s diagram.

• Downstream sediment transfer occurs as suspended, bed and dissolved loads.

• Deposition occurs when stream competence falls below the critical velocity required to maintain movement.

• The variety of transport styles and particle sensitivity to competence lead to a considerable amount of particle sorting by size, which is reflected in fluvial sediments.

Fluvial landsystems

• The drainage basin landsystem is characterized by upper, erosion-dominated, and lower, deposition-dominated, components, with typically concave river long-profiles reflecting the downstream decay of potential energy.

• Bedrock channels are therefore more common in uplands where rivers incise steep-sided narrow valleys, or gorges, in areas of active (orogenic) or renewed uplift (rejuvenation).

• Upland fluvial sediments may be restricted to discontinuous pockets of raw, coarser sands and gravels with debris cones or alluvial fans where tributaries join less steep channels.

• Alluvial channels are more common in lowlands where silt–sand–gravel beds are spread over flood plains by meandering rivers and overbank floods.

• Straight channels are rare, occurring only when bed load is low, and meandering develops as channel geometry responds to changes in water and sediment discharge. It consumes surplus energy by lateral erosion and the larger wetted perimeter implicit in sinuosity.

• Meanders develop in close association with a riffle–pool sequence of bedforms of alternating deposition (riffle) releasing energy for erosion (pool), leading to increased sediment load, etc.

• Where streams divide at low flow around multiple bars or riffles and dissect them at high flow the channel becomes braided, or is said to be anabranching if stabilized by vegetation.

• Meandering, braiding and anabranching characterize the floodplain landsystem and rework its extensive fluvial sediment spreads. The enduring nature of the flood plain is also marked by aggradation or incision and terrace formation in response to changing sea level, climate and land use.

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CASE STUDY : Aspects of the River Severn Catchment, Wales and England Aims and Objectives Catchment studies are one of the most prominent areas of applied geographical and environmental studies, integrating the specific three-dimensional landsystem unit of a drainage basin with its human occupancy and land use. Catchment climate determines the hydrometeorological inputs whilst the basin landsystem components (soils, geology, slope and ecosystems together with human structures and activities) determine the hydrogeological volume, routeing and timescale of water transfers. Watershed models, calculation of the water balance and measurement of the hydrographic character are all necessary ingredients in the human management of the catchment This case study identifies the principal catchment characteristics of Britain’s biggest river ~ the Severn ~ and directs the reader to website resources of recent, current and continuing time-and-space data which create good opportunities for interactive analysis of its catchment hydrology and problematic management. Catchment character The River Catchment is Britain’s largest river in every sense. The trunk river is 347 km in length from the source on Plynlimon Fawr (756m OD) in the Cambrian Mountains, mid-Wales to the commencement of its tidal estuary ~ near the Haw Bridge gauging station between Tewkesbury and Gloucester in western England (Catchment Map available through the Centre for Ecology & Hydrology and National River Flow Archive listed under Web Resources below) Its catchment above Haw Bridge covers 9,895 km2 but at its maximum extent ~ including the Leadon (right-bank tributary) and Chelt (left-bank) below that ~ the overall catchment exceeds 11,000 km2. The highest point on its watershed is 827m OD on Cader Berwyn in the extreme north-west, and mean discharge as it enters the tidal stretch is 104.95 m3 sec-1. For comparison, the equivalent data for the River Thames is that its catchment extends over 9950 km2 above its tidal limit at Teddington Lock, Richmond-upon-Thames, flows for 345 km below a maximum watershed altitude of 330m OD in the Cotswold Hills, Gloucestershire and its mean discharge at Teddington is 65.59 m3 sec-1. The catchment and river possess two quite distinct characters either side of the Wales-England border. The Hafren, its Welsh name, rises on the east side of the Cambrian mountains within a few hundred metres of the source of the River Wye, also one of Britain longest rivers and which joins the estuarine Severn at Chepstow in south-east Wales. Although arising from the same upland moorland watershed, on Lower Palaeozoic rocks of low permeability, the Rheidol and Ystwyth reach the Irish Sea within 40 km to the west ~ little more than 10% of the distance the Severn travels and hence with a much steeper profile and faster mean flow. Nevertheless, the Severn and its upstream tributaries also have steep profiles, rapid mean flow and “flashy” hydrographs as a result of the high elevation of the Cambrian Mountains plateaux (Plates 1 and 2), high orographic mean precipitation of 2482 mm yr-1 at the Plynlimon gauging station (Plate 3) and generally 1700-2000 mm yr-1 in the mountains (Figure 1). The Severn is joined by one major left-bank tributary, the Vyrnwy, before it crosses the English Border downstream of Welshpool and ~ simultaneously ~ passes below the

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generalized 200 m OD contour and onto permeable Mesozoic (Triassic) sandstones of the Cheshire-Shropshire Plain.

Plate 1 A typical landscape on the Cambrian Mountains’ summit plateau near Plynlimon Fawr, with thin soils and quite intensively grazed moorland vegetation shedding high rainfall quickly. (Photo: Ken Addison)

Plate 2 Off the eastern plateau edge, slopes become much steeper in narrow tributary valleys and shorten lag times to the rivers.(Photo: Ken Addison)

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Plate 4 Flume for stream gauging in a steep catchment on Plynlimon, mid-Wales. Its height, and baffles to dissipate stream energy, reflect the ‘flashy’ nature of upland stream flow. Photo: M.A. Fullen.

Figure 1 Abstract from Average annual total precipitation amount (mm) for 1971-2000, currently Figure 2:42 on page 68 of Jenkins, G.J, Perry, M.C. and Prior, M.J.O (2007) The climate of the United Kingdom and recent trends, UK Climate Impacts Programme, Exeter: Met Office Hadley Centre (Exeter EX1 3PB) ISBN 978-906360-01-6 enquiries@ukcip.org.uk © Met Office. taken from: UK CI08: The Climate of the UK and recent trends.

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Thereafter, the Severn undergoes two abrupt changes in direction which are probably attributable to significant Neogene developments of the British Landform and superimposed river systems, and certainly to their Quaternary ~ probably Late Pleistocene ~ disruption by glaciers. As it crosses the border, the Severn turns almost 90o east ~ away from its NNE line which would take it into the River Dee just 20 km north at Overton; the Dee itself also swings abruptly northwards from its easterly course out of Wales. The Severn swings again 40 km further east, by 60o to flow south towards the Gloucester and the Severn Estuary ~ this time diverted by the Ironbridge Gorge from a line which would have taken it into the River Trent 40 km away near Stafford (Plate 14.4).

Plate 4 The world’s first Ironbridge, completed in 1779, spanning the Severn in Ironbridge Gorge. The high arch is a functional element of the bridge, designed to allow Severn sailing barges carrying industrial raw materials and good to pass underneath at most stages (depths) of the river, which can rise quickly through the gorge. (Photo: Ken Addison) The entire region occupied in common by stretches of the Dee, Severn and Trent was breached by ice passing from the Irish Sea basin between the Cambrian Mountains to the west and Pennines to the east. A combination of glacial erosion on the flanks of the uplands, extensive lowland glacial deposition in the Plain ~ including the Wrexham-Ellesmere-Wolverhampton moraine and esker complexes, and subglacial meltwater erosion of Ironbridge Gorge radically reorganised Borderland drainage systems. The presence of an essentially-Devensian (< 125,000 ka) series of river terraces below Bridgnorth helps to confirm the Late Pleistocene age of these events. In all likelihood, the Severn and Dee were tributaries of an older “super-Trent” system draining eastwards out of the Welsh Mountains into the North Sea; now they both outflow westwards into the Irish Sea basin, after turning almost 180o. The Middle and Lower Severn meanders in its floodplains across two substantial topographic basins on permeable rocks, generally sandstones and mostly of Mesozoic age. Floodplain elevations everywhere are < 50 m OD downstream of Shrewsbury (mean flow c. 43 m3 sec-1) and in the rainshadow of the Cambrian Mountains with < 750 mm yr-1. The upper floodplain is centred around Shrewsbury and collects the major

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tributaries of the Rea (right-bank), Perry, Roden and Tern (left-bank) which raise the mean flow to 60 m3 sec-1 in just 15 km as the river enters the Ironbridge Gorge. Exiting the gorge 30 km to the south at Bewdley with a mean flow just 1.5 m3 sec-1 higher, the river now enters the Worcester-Tewkesbury-Gloucester floodplain. Two major tributaries ~ the Avon (left-bank) draining the south-west Midland Plain and the Teme (right-bank), whose catchment sweeps in a long, concentric arc south of the Severn itself and also rises in Wales ~ contribute mean flows of 16.7 and 17.5 m3 sec-1 respectively, adding almost 60% to overall mean flow in just 30 km between Worcester and Gloucester. The hydrography, and therefore management, of the Severn is complex, the more so through the abrupt switch along the eastern scarp of the Cambrian Mountains from an essentially-upland to meandering flood-plain river. In Wales, it drains steep-sided valleys in high-rainfall, winter maxima conditions on -permeability strata through an almost entirely moorland, pastoral and occasionally forested catchment. It carries the water and sediment discharges associated with this, in “flashy” hydrographic style into England. Here, additional discharge is suppressed and regulated by much lower rainfall, with increasing tendencies towards summer and autumn maxima (particularly from the Avon sub-catchment), permeable strata and a more arable farming catchment. There are no major conurbations and relatively few large towns and industrial areas to seriously disturb catchment characteristics but the floodplains provide major river and groundwater abstraction sources for public water consumption well beyond the basin. Water management complexity is underlined by the extent of floodable land, flood risk and a recent steep increase in flood incidence ~ forecast to be exacerbated by climate change ~ in the Severn Catchment. Severn-Trent Water manages its water resources, taking a big chunk of mid-Wales out of Dŵr Cymru (Welsh Water) hands, whilst the national Environment Agency has responsibility for flood management. Only two reservoirs regulate discharge in the Cambrian Mountains and only one of these ~ Llyn Clywedog, impounding a maximum of 11 billion gallons ~ was purpose-built in the 1960s to hold back flood peaks on the left-bank tributary River Clywedog, lowering flood risk in Welshpool and Shrewsbury. The other, Llyn Vyrnwy, was constructed in the 1890s for the city of Liverpool. The hydrographs of the rivers leaving both lakes are inevitably quite different than those of adjacent upland catchments. With no other flood-control reservoirs, and with much higher annual and intra-annual flow variability that is suggested by mean discharge values alone, the floodplains between Welshpool and Shrewsbury and Bewdley and Gloucester are amongst the most flood-prone in Britain (Plate 5 and 6) and have experienced serious local or widespread flooding in the past decade ~ none more so than in 2007. Rising river stage (depth) at Bewdley is often taken as a marker for flood risk across England and Wales as a whole and communities along the Severn corridor are implementing hard (permanent) and soft (rapid-deployment and dismountable) flood defences (Plate 7). These are likely to be in increased demand if and when forecast increases in the amount and intensity of British precipitation kick in !

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Plate 14.5 Flooding in the lower reaches of the Shrewsbury flood plain above Buildwas, looking towards Ironbridge Gorge and the thermal power station. The power station makes a significant abstraction of Severn water for cooling, much of which is lost to the atmosphere as steam. Photo: Ken Addison

Plate 14.6 The river Severn floods in Shrewsbury, December 2000, picking out the incised meander around the medieval core of the country town of Shropshire, generated by high antecedent and intense rainfall in its Welsh catchment. The upstream Welsh (centre left) and downstream Englidh (centre right) bridges can be seen; the railway bridge crosses the river at the meander neck. Photo: by courtesy of the Shropshire Star.

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Plate 14.7 Temporary flood defences on the river Severn in Ironbridge, Shropshire, consisting of heavy-duty polythene sheeting stretched over aluminium A frames, provides a cheap and rapid-response means of protection at congested sites. Photo: Ken Addison. Catchment hydrology data sources and their use The principal UK hydrological data source is the National Water Archive (from which the data summary above comes)~ comprising the National River Flow Archive and National Groundwater Level Archive, maintained by the Centre for Ecology and Hydrology’s base in Wallingford, Berkshire under the overall management of the UK’s central government-funded Natural Environment Research Council. Other important agencies include the UK Environment Agency, Environmental Data Index (UKED) and catchment-specific management agencies or schemes (such as the Severn & Avon Wetlands Partnership) and the UK Meteorological Office. Their data and graphics enable, for example, comparative studies of the hydrographic data and river hydrographs from different parts of the catchment, according to the location of gauging station and the many meteorological, geological, land-use, river-network and management variables across the catchment. Learning Objectives

• Explain the role of drainage basins characteristics, including those modified by human activity, in converting precipitation into river discharge to the sea.

• Appreciate how the processes of water and sediment transfer shape the geomorphic character of individual river channels and the river networks of the catchment.

• Understand how any significant human socio-economic activity inevitably changes hydrographic characteristics of the basin and may create water management problems.

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Essay titles

1. How do the cross- and long-sections of stream channels adjust to variations in stream discharge, sediment load and potential energy?

2. Outline the likely impacts on stream flow, sediment yield and channel

morphology of a catchment due to changes in land use from woodland to arable farming and then suburban development over the past century.

3. What may meandering, braiding and terracing by floodplain rivers tell us about

stream power and climatic change? Discussion topics

1. Use the principal website data sources, together with appropriate topographic and geological maps and UK Meteorological Office data, to explore and explain Severn basin hydrographic characteristics ~ including short-term variability and analysis of change over time.

2. Consider the rôle of the drainage network in the efficient evacuation of water

from a catchment.

3. After decades of straightening and smoothing river channels, why are many rivers managers now seeking to reverse the changes?

Further Reading Bridge, J.S. (2003) Rivers & Floodplains: Forms, Processes and Sedimentary Record.

Oxford: Blackwell Publishing. A richly-illustrated and comprehensive cover of fluvial processes and landforms, bridging the interface between geomorphology with useful sections on sedimentology and fluvial stratigraphic records.

Downs, P.W. and Gregory, K.J. (2004) River Channel Management: Towards Sustainable Catchment Hydrosystems. London: Arnold. An excellent introduction to the issues of sustainable channel/catchment management in the light of river channel sensitivity and responsiveness to change.

Robert, A. (2003) River Processes: An Introduction to Fluvial Dynamics, London: Arnold. A short but useful text, concentrating on river channel processes and channel morphology rather than rivers and floodplains in any wider sense.

References Boulton, G. S. (1992) ‘Quaternary’ in P. M. D. Duff and A. J. Smith (eds) Geology of England and Wales, London: Geological Society, 413–44 Butzer, K. W. (1976) Geomorphology from the Earth, New York: Harper & Row Department of the Environment Water Data Unit (1983) Surface Water: United Kingdom, 1977–80, London: HMSO

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Gregory, K. J. and Walling, D. E. (1973) Drainage Basin Form and Process: a geomorphological approach, London: Arnold Institute of Hydrology (1980) Low Flood Studies, Wallingford: Institute of Hydrology Knighton, A. D. (1998) Fluvial Forms and Processes : a new perspective, London:

Arnold Leeder, M. (1999) Sedimentology and Sedimentary Basins: From Turbulence to

Tectonics, Oxford: Blackwell Science L’vovich, M. I. (1979) World Water Resources and the Future, Chelsea, MI: American Geophysical Union Mackay, G. A. and Gray, D. M. (1981) ‘The distribution of snow cover’ in D. M. Gray and D. H. Male (eds) Handbook of Snow: principles, processes, management and use, Toronto: Pergamon Press, 153–90 Newson, M. D. (1992) Land, Water and Development, London: Routledge Newson, M. D. (1981) ‘Mountain streams’ in J. Lewin (ed.) British Rivers, London: Allen & Unwin, 59–89 Ouichi, S. (1985) Response of alluvial rivers to slow active tectonic movement, Geological Society of America Bulletin, 96. 504-515 Selby, M. J. (1985) Earth’s Changing Surface: an introduction to geomorphology, Oxford: Clarendon Press Strahler, A. N. and Strahler, A. H. (1992) Modern Physical Geography, New York: Wiley Ward, R. C. (1975) Principles of Hydrology, second edtion, London: McGraw-Hill Ward, R. C. and Robinson, M. (2000) Principles of Hydrology, fourth edition, London: McGraw-Hill

Web Resources ~ General UK Catchment http://ceh.ac.uk/data/nrfa/index.html and http://www.nerc-wallingford.ac.uk/ih/nwa/index.html

The principal websites for the Centre of Ecology & Hydrology (CEH), National Water Archive (NWA) and National Water Flow Archive (NWFA), providing hyperlink access and information on their component River Flow and Groundwater Archive of data sets for British catchments. The aim of these UK Designated Data Centres is to provide a focus for the Natural Environment Research Council's environmental holdings and provide information and advisory services to a wide range of users. NWA data holdings range from the catchment scale, eg detailed climatological and hydrological data for a network of experimental catchments, to national (flood event data, annual reviews) and international coverage (world flood archive). Onward linkages to other data sets (including digitised rivers map, a Digital Terrain Model of the UK, hydrology of soil types map, digital representation of average rainfall and evaporation records and flood studies reports) can be made available by contacting the NWA. The access route to these centres may change from time to time.

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~ Severn Catchment-specific Web Resources By intelligently navigating the principal websites above, it is possible to obtain maps, data-sets, information, hydrographs, reports etc. for most river catchments in the UK. It is not usually possible to go straight into the specific pages referenced below; instead, entry through the entry web page opens a list of options and the particular ones to look for ~ either on the entry page or click-on links listed there ~ are: River Flow Data : Time Series Downloads. UK Gauging Stations Network (with click-ons → Regional Maps → named and located gauging stations → their catchment details, descriptions, data, sample hydrographs, gauged daily flows etc.). The following related pages provide examples of useful types of data sets and graphics you can find but they have to be searched for by title from the main sites, rather than targeted directly. In other words, the page addresses below are the reference address, not the search address. http://www.nwl.ac.uk/ih/nrfa/webdata/eam.html This specific web page consists of the Severn catchment in its series of UK catchment maps, which also show the principal gauging stations for which other parts of the overall website provide meteorological and hydrographic data. Clicking on any active gauging station brings up station details.

http://www.nwl.ac.uk/ih/nrfa/river_flow_data/explanatory_notes.html http://www.nwl.ac.uk/ih/nrfa/river_flow_data/about_the_data.htm As these web addresses suggest, they provide useful summaries of what the NWA data show and how to use them.

http://www.nwl.ih.nrfa.webdata/054001/g.html This web page ~ of the River Severn at Bewdley, Worcestershire ~ shows the data which is typically available for all individual gauging stations and includes local catchment details affecting runoff, annual hydrographs continuously from 1921-2005 (for this station) and daily mean discharges with summary statistics for the most recent complete year (2005). http://www.environment-agency.gov.uk/regions/midlands/567079/567090/893833/89 The UK Environment Agency and the Department for Environment, Food and Rural Affairs (DEFRA) open up other website opportunities to review, inter alia, flood-risk areas and strategic approaches to river and flood management. This particular address opens up their River Severn Strategies site. Other Web Resources http://www.environment-agency,gov.uk The Environment Agency is the UK’s principal environmental protection organization, with a wider brief than just rivers, river and coastal management and flooding. Nevertheless, it is an excellent source of contemporary issues and events and provides direct access to a wide range of data and information sources, government consultancy and policy documents covering catchment and coastal management.

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http://www.metoffice.gov.uk/ This accesses the homepage of the UK Meteorological Office, the prime source of current and past weather data and related information and services. The principal access to data records is http://metoffice.gov.uk/education/index.html http://www.epa.gov/OWOW The United States Environment Protections Agency’s Office of Wetlands, Oceans and Watersheds website, replicating for the USA many of the services and sources of information on its field of responsibility as the UK Environment Agency. The website provides a good balance of regional, national and international contemporary interest in contemporary catchment management, conservation and protection issues. http://www.iahs.info The International Association of Hydrological Sciences (giving its English name) promotes the study of all aspects of hydrology through the initiation of international collaborative research and publication of results. As high-level research organization, its principal value is to provide access to newsletters and reports of its various associated International Commissions dealing with a wide range of hydrological fields including Groundwater, Snow & Ice, Water Quality and Water Resources.