10. vaughan et al., 2009

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS

Aquatic Conserv: Mar. Freshw. Ecosyst. 19: 113125 (2009)

Published online 9 October 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/aqc.895

VIEWPOINT

Integrating ecology with hydromorphology: a priority for riverscience and management

I.P. VAUGHANa,*, M. DIAMONDb, A.M. GURNELLc, K.A. HALLb, A. JENKINSd, N.J. MILNERe,L.A. NAYLORf, D.A. SEARg, G. WOODWARDh and S.J. ORMERODa

aCatchment Research Group, Cardi School of Biosciences, Cardi University, Cardi CF10 3US, UKbEnvironment Agency, North West Region, Richard Fairclough House, Knutsford Road, Warrington WA4 1HG, UK

cDepartment of Geography, Kings College London, Strand, London WC2R 2LS, UKdCentre for Ecology and Hydrology, Wallingford, Oxfordshire, OX10 8BB, UK

eNational Fisheries Technical Team, Environment Agency, University of Bangor, School of Biological Sciences, Deiniol Road,

Bangor, Gwynedd LL57 2UW, UKfDepartment of Geography, University of Exeter, Cornwall Campus, Treliever Road, Penryn, Cornwall, TR10 9EZ, UK

gSchool of Geography, University of Southampton, Higheld, SO17 1BJ, UKhSchool of Biological and Chemical Sciences, Queen Mary University of London, London, E1 4NS, UK

ABSTRACT

1. The assessment of links between ecology and physical habitat has become a major issue in river research andmanagement. Key drivers include concerns about the conservation implications of human modications (e.g.abstraction, climate change) and the explicit need to understand the ecological importance of hydromorphologyas prescribed by the EUs Water Framework Directive. Eorts are focusing on the need to develop eco-hydromorphology at the interface between ecology, hydrology and uvial geomorphology. Here, the scope ofthis emerging eld is dened, some research and development issues are suggested, and a path for development issketched out.2. In the short term, major research priorities are to use existing literature or data better to identify patterns

among organisms, ecological functions and river hydromorphological character. Another early priority is toidentify model systems or organisms to act as research foci. In the medium term, the investigation of patternprocesses linkages, spatial structuring, scaling relationships and system dynamics will advance mechanisticunderstanding. The eects of climate change, abstraction and river regulation, eco-hydromorphic resistance/resilience, and responses to environmental disturbances are likely to be management priorities. Large-scalecatchment projects, in both rural and urban locations, should be promoted to concentrate collaborative eorts,to attract nancial support and to raise the prole of eco-hydromorphology.3. Eco-hydromorphological expertise is currently fragmented across the main contributory disciplines (ecology,

hydrology, geomorphology, ood risk management, civil engineering), potentially restricting research anddevelopment. This is paradoxical given the shared vision across these elds for eective river management basedon good science with social impact. A range of approaches is advocated to build sucient, integrated capacitythat will deliver science of real management value over the coming decades.Copyright # 2008 John Wiley & Sons, Ltd.

Received 23 February 2007; Revised 11 June 2007; Accepted 1 July 2007

KEY WORDS: ecology; geomorphology; hydrology; hydromorphology; rivers; Water Framework Directive

*Correspondence to: Dr Ian Vaughan, Catchment Research Group, Cardi School of Biosciences, Cardi University, Cardi CF10 3US,UK. E-mail: [email protected]

Copyright # 2008 John Wiley & Sons, Ltd.

INTRODUCTION

For over 40 years, issues of water quality have dominated river

research, management and conservation}driven by seminalpublications (e.g. Hynes, 1960), by widespread problems from

point or diuse pollution sources, and by major legislation (e.g.

the UK Water Acts of 1973, 1983; the US Clean Water Act

1977). Although interest in relationships between river organisms

and their physical habitat is also long-standing (e.g. Riley, 1921;

Percival and Whitehead, 1929), emphasis on this has generally

been less. This balance is currently being re-dressed for several

reasons. Globally, there is a need to understand the ecological

eects of a wide range of changes in physical habitat, as rivers are

increasingly exploited, regulated or otherwise modied through

ood-defence engineering, impoundment, river restoration,

climate change and the spread of alien species. Across Europe,

the Water Framework Directive (WFD; 2000/60/EC) has been

the major legislative driver by specifying that hydromorphology

should underpin good ecological status (European Commission,

2000). The improvements in water quality in Europe and North

America over recent decades mean that hydromorphic limits on

ecological quality are becoming increasingly apparent. Finally,

the recognition that these problems require multi-disciplinary

solutions has stimulated dialogue between physical scientists and

biologists whose shared vision is of more eective river science

and management.

River conservation has much to gain in this renewed push

for an improved understanding of ecologyphysical habitat

relationships. Hydromorphological integrity is central to

conservation since it provides the template upon which all

other ecological structures and functions are built. Furthermore,

in seeking good ecological status by sensitive management at

whole basin scales, rather than in the channel or riparian zone

alone, the WFD has become a highly signicant element in

wider river conservation. With hydromorphology an explicit

component of the Directive, the need to understand links to

ecology and conservation are clear.

Three important observations can be made regarding the links

between river ecology and hydromorphology. First, current

scientic understanding is generally poor, especially at the

quantitative levels required for eective prediction and

management. This is despite scientic literature stretching back

more than 80 years (Riley, 1921; Percival and Whitehead, 1929),

and comprising many thousands of peer-reviewed publications.

Numerous } mainly observational } studies have described

links between biological pattern, ecological processes, and river

form and physical processes, yet the underlying mechanisms are

often known only in outline. Relationships in riparian and

oodplain environments are less widely studied than those in the

wetted channel, highlighting the need to consider whole

catchments and river landscapes in the development of eco-

hydromorphic research (Eyre et al., 2002).

Secondly, improved understanding of ecologyhydro-

morphology is a pressing need if the timetable and aims of key

river legislation are to be met (e.g. for statutory regulation or

programmes of measures under the WFD). Major challenges

arise in distinguishing the inuences of hydromorphic

modications on organisms or processes from other potentially

confounding eects such as pollution (Allan, 2004). Biological

indicators of physical modication are still preliminary, rarely

described or poorly founded, while few biological models

diagnose how physical eects contribute to biological

departures from expected conditions (Davies et al., 2000). The

denition of expected or reference conditions as required by the

WFD is challenging given the inherent variability in both physical

habitat and biology (Nijboer et al., 2004).

Finally, the need to understand ecologyhydromorphology

linkages is accentuated by the prospects of climate change,

altered ow regimes and increased water consumption

(Jackson et al., 2001). It is vital that such changes are both

understood and clearly communicated to practitioners if river

environments are to be managed eectively in future. River

environments appear to be highly sensitive to climatic eects,

but features that mitigate impacts or increase ecological

resilience are poorly understood (Durance and Ormerod,

2007).

In this viewpoint paper, we consider the interface between

river ecology and hydromorphology: its scope, major research

and development issues, and sketch out a path for the

development of eco-hydromorphic research. Our aims are

to draw attention to the need for better science that links the

physical and ecological dimensions of rivers; to encourage

better collaboration; to prompt sponsoring agencies to

recognize the gaps; and to prompt bodies responsible for

river management to recognize the need. Some areas are

identied in which collaboration could be most eective.

Naturally, the issues raised reect the perspectives of the

authors, and are indicative rather than denitive. It is hoped,

nevertheless, that this stimulates a wider debate.

ECO-HYDROMORPHOLOGY: THE INTER-DISCIPLINARY INTERFACE

Several terms have been applied to the relationships between

organisms and physical habitats in rivers. Examples include

biogeomorphology (Viles, 1988), ecogeomorphology (Parsons

et al., 2003), ecohydrology or hydroecology (Wassen

and Grootjans, 1996), eco-hydromorphology (Clarke et al.,

2003) and geobiology (Noke, 2005). Here, we adopt eco-

hydromorphology, since it captures the main contributory

disciplines (ecology, hydrology and geomorphology). More

pertinently, this term is consistent with the WFD which uses

hydromorphology to describe the hydrologic and geomorphic

elements of river habitats (European Commission, 2000).

Eco-hydromorphology can be dened as the interactions of

the biological entities and ecological processes of a river with

the hydrological and geomorphological form and dynamics. In

this context:

* eco- encompasses riverine biota at all levels of

organization (from genes, through individuals,

populations and communities, to whole ecosystems), all

taxonomic levels and across all functional groupings (e.g.

primary producers, detritivores). It includes ecological

processes manifested in individuals through to entire

ecosystems (e.g. dispersal, reproduction, decomposition),

and which act over a wide range of timescales from the

immediate to the evolutionary (Townsend and Hildrew,

1994) and all spatial scales from local to lifetime

movements (Durance et al., 2006).* hydromorphology describes the geomorphology and

hydrology of a river system, their interactions, and their

arrangement and variability in space and time. Key

elements include the ow (sensu Po et al., 1997) and

I.P. VAUGHAN ET AL.114

Copyright # 2008 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 19: 113125 (2009)DOI: 10.1002/aqc

sediment regimes; channel and oodplain dimensions,

topography and substratum; continuity and connectivity

(longitudinal, lateral, vertical and temporal); hydrological

and geomorphological processes (e.g. sediment transport);

and the spatio-temporal arrangement of the hydromor-

phological components (European Commission, 2000;

Gilvear et al., 2004). Articial features (e.g. bank

protection works, weirs) and human modications to

processes are also included.* interactions } the mechanisms by which hydromorphology

and ecology aect one another. Hydromorphology may

inuence ecology, such as the eects of ow velocity on

macrophyte photosynthesis (Madsen et al., 1993) or the role of

hydromorphology in selecting assemblages of organisms with

appropriate traits (Townsend and Hildrew, 1994). Conversely,

organisms may aect hydromorphology, such as the impact of

biolms on ow characteristics (Battin et al., 2003) or the

impacts of invertebrates on sediment stability and release

(Edwards, 1968).

Eco-hydromorphology extends beyond ecology, geomor-

phology and hydrology into other contributing elds (e.g.

civil engineering, economics, social sciences) and the majority

of research is not labelled as being eco-hydromorphic (or any

of the other phrases coined) per se. Similarly, it spans both

pure and applied science, academia and regulatory agencies.

This diversity has increasingly been recognized, along with

the consequent needs to engage a disparate research and

management community, and to foster greater inter-

disciplinary collaboration (Gurnell et al., 2000; Hannah

et al., 2004). Unfortunately, there is little evidence that these

developments are reected in the composition of research

programmes. Hannah et al. (2004) assessed the authorship of

research papers involving the term ecohydrology (and

derivatives thereof) and found that collaborating university

researchers very rarely came from more than one academic

department. It seems that while the eco-hydromorphic

interface can be clearly dened, it is poorly developed, and

unless this situation can be addressed it could seriously

handicap the development of the science.

KEY RESEARCH AND DEVELOPMENT ISSUESIN ECO-HYDROMORPHOLOGY

Process and causality

Most eco-hydromorphic research has relied upon correlating

static ecological and hydromorphological patterns, using

space-for-time substitutions. Documenting such patterns is

typically the rst stage in the development of a research area,

from which more detailed mechanistic understanding can

develop (Gaston and Blackburn, 1999). In isolation, such

research creates a relatively weak science base, and so

eco-hydromorphology needs to move towards the use of

stronger inference (e.g. experimentation) and studying

underlying mechanisms wherever possible. The next step in

this direction involves the study of dynamics and process rates,

rather than static patterns. Ultimately, there is a need to

understand how eco-hydromorphic processes generate

observed patterns, and in turn how patterns inuence

processes. An example is the way in which vegetation and

sediments interact to shape river channels and ow

characteristics, and, in turn, vegetation recruitment and

sediment transport (Gran and Paola, 2001; Gurnell and

Petts, 2002). The ultimate aim is to achieve a causal

understanding of how the components of the ecology

hydrologygeomorphology interface interact.

River management would benet greatly from

understanding eco-hydromorphic mechanisms, as this would

provide a rm foundation for predicting the eects of

both management interventions and more gradual, longer-

term environmental changes. Currently, the challenge for

prediction stems from the novel combinations of

hydromorphic conditions that may be created. In the

UK, for example, climate change is generally predicted to

increase the seasonality of rainfall and runo (Kay et al.,

2006). This may produce novel environmental conditions,

which could have major ramications for eco-

hydromorphology. Similarly, the ecological responses to river

restoration may be dicult to predict, because specic

structural changes to geomorphology may be made without

altering the formative processes (Sear, 1994) } in eect thispartially or temporarily decouples recognized hydromorphic

processes and forms. To predict the eects of such changes,

models are asked to extrapolate outside currently observed

conditions, and these are the circumstances in which

correlation-based science and models often fail (Sutherland,

2006). In contrast, the underlying mechanisms that link

ecology, hydrology and geomorphology should remain

fundamentally unchanged, and so provide a foundation for

more reliable predictions.

Studying mechanisms, as opposed to correlations, presents a

major challenge. Temporal changes in patterns (e.g. channel

form or invertebrate assemblages) can provide detailed

information about process rates. Fluvial geomorphology

provides good examples, such as quantifying sediment

transport rates using temporal series of channel form records

(Fuller et al., 2002; Lane et al., 2003). However, the nancial and

logistical demands of research spanning many years frequently

restricts opportunities, despite widespread recognition of the

value of research carried out over large spatial and/or temporal

extents, such as the Long Term Ecological Research Network

(Symstad et al., 2003). Equally, management often requires rapid

appraisals (ecological or environmental), which may preclude

temporal monitoring. Progress requires concerted research

eort to address the two-way patternprocess interaction,

helping to identify, for example, how valuable patterns

recorded at a single point in time are for inferring information

about eco-hydromorphic processes.

The role of spatial structure

The spatial conguration of river hydromorphology, and how

it changes over time, has diverse eects upon river ecology

(Wiens, 2002). As a consequence, eco-hydromorphology is

more than an inventory of parts and processes. Potentially

important spatial structures can be identied across a wide

range of scales, from the properties of individual sediment

particles, through the arrangement of biotopes in a reach, to

the large-scale spatial context, including the conguration of

drainage networks and neighbouring catchments. Salmonids

are among the best-studied organisms in the context of spatial

structure, because of the long-standing recognition of both

INTEGRATING ECOLOGY WITH HYDROMORPHOLOGY 115


their migratory movements and the use of dierent habitat

types for spawning and for dierent developmental stages

(Bardonnet and Baglinere, 2000). The role of barriers}

especially articial features (e.g. weirs)} is an important topicin salmonid research (Morita and Yokota, 2002; Woord

et al., 2005). Spatial structure is a key characteristic, as the

signicance of such barriers depends not only on their

particular characteristics } their permeability to sh underdierent ow conditions } but also their location within the

river network (Fagan, 2002).

To elucidate the role of spatial structure in eco-hydro-

morphology, there is a need to identify: (i) the key spatial

characteristics for organisms and how these dier between

life-stages; (ii) how the activities of organisms spatially

structure the hydromorphology; (iii) how physical structure

inuences ecological, physical and eco-hydromorphic

processes; and (iv) ecologically meaningful ways of

quantifying structure. This applies both within the wetted

channel, and to the linkages between the channel and the wider

riverine landscape (Wiens, 2002; Brierley et al., 2006). An

example of these issues is provided by patch boundaries

and ecotones, which are thought to play important roles in

modulating eco-hydromorphic processes and the movements

of organisms (Ward and Wiens, 2001). The most obvious

boundary is the river shoreline, with terrestrialaquatic

interchanges making important contributions to riverine

ecosystems (Nakano and Murakami, 2001). Recent research

has focused upon shoreline length and complexity (Van der

Nat et al., 2002) and how it relates to a range of organisms

and ecological processes, through such mechanisms as increased

water retention and availability of lentic habitats (Reckendorfer

et al., 1999; Schiemer et al., 2001). This research suggests that

shoreline length could be a relatively simple, yet ecologically-

relevant measure for a range of organisms. In the opposite

direction, ecological processes aect shoreline structure (e.g. via

bank stabilization by growing vegetation and hydraulic eects of

large wood in the channel; Gurnell et al., 2002), providing an

example of ecological processes interacting with hydromorphic

patterns.

The capacity to study spatial structure has increased

dramatically. Technological developments make it possible to

capture patterns over larger areas and at higher resolutions

than ever before, and to analyse the large, complex data sets

that result (Ehlers et al., 2006). The accompanying theory

has also developed, and disciplines such as landscape

ecology provide valuable ideas (ONeill et al., 1988;

Gustafson, 1998).

Scale and variability

The changing uxes of organisms, materials and energy in

space and time, diering across scales, presents a research

challenge. This complexity is not discussed here, but some

generic points are made about scale and variability in eco-

hydromorphic research.

Scale

The results of research are conditional upon the scale(s) of

observation (Wiens, 1989). For example, the relationships

between organisms and their environment may appear to

change with the scale of observation in both space and time

(Wiley et al., 1997; Malmqvist, 2002). Hence, to understand a

process, the correct scale(s) must be chosen when studying it.

Organism body size and life cycle have major inuences, such

that smaller organisms with rapid generations (e.g. bacteria,

fungi and algae) interact with river structure over very dierent

spatio-temporal extents from mammals, birds and sh. At one

extreme, large populations of some organisms are contained

within just one river biotope. At the other, many individual

biotopes can be contained within the home range of a single

organism (Woodward and Hildrew, 2002).

Scaling eects also have ramications for monitoring,

assessment and management, all of which are likely to be

enhanced by selecting appropriate scales for the interactions

of interest. To support these aims, research designs should

include scales of management relevance (Vaughan and

Ormerod, 2003; Durance et al., 2006). In the UK, this could

include reach-scale (ca 500m) units, water bodies, catchments

and river basin districts. Equally, monitoring and

bioassessment need to be aware of the timescales over which

ecological, hydrological and geomorphological patterns and

processes respond to environmental change. An obvious

example is the potential lag in ecological responses to

hydromorphic changes, such as those in the ow regime

following impoundment (Martinez et al., 1994; Kruk and

Penczak, 2003). Management needs to be exible, adopting

scales that research reveals to be critical for eco-hydromorphic

processes.

In lieu of an a priori knowledge of the important scales,

multi-scale studies (in time and/or space) provide a way of

discovering them or insuring against missing critical scales.

Multi-scale studies are relatively well established in river

systems as a consequence of the explicitly hierarchical

organization of rivers (Frissell et al., 1986). Studies focusing

on individual sites and reaches could benet from the addition

of larger-scale factors, as they provide context for individual

sites and may allow the conclusions from small-scale studies

to be more readily generalized (Wiens, 2002). Equally, short

duration studies may benet from longer-term monitoring

data to provide context (Bradley and Ormerod, 2002).

The ultimate aim is to achieve cross-scale understanding in

eco-hydromorphology, revealing the potential ramications of

processes or management interventions on organisms and

processes across spatial and temporal scales. Damming a river,

for example, has eects both locally (e.g. habitat

fragmentation and altered physical structure) and across the

entire river system (e.g. eects on river regime or migratory

organisms) (Allan, 1995). New methods for data analysis may

need to be devised or introduced to eco-hydromorphology to:

(i) reveal cross-scale pattern and process relationships; and (ii)

facilitate scaling up of results from the small spatio-temporal

scales often amenable to research, to larger scales relevant to

management (Phillips, 2005).

Variability

The variability and dynamics of river environments present a

serious research challenge, yet need to be understood for

successful river management (Thoms, 2006). In the rst

instance, empirical study is required to characterize the

variation that occurs (including rates and magnitudes of

change) and this needs to be followed by an understanding of

the role of variability in eco-hydromorphic processes. From a

conservation viewpoint, the observed variability is important



to the functioning of river ecosystems (Thoms, 2006), and so

needs to be preserved as part of conservation management.

Eco-hydromorphic variation has been studied over a wide

range of spatial and temporal extents} the latter often in thecourse of long-term monitoring, such as ow gauging (Sheer

et al., 2003), retrospective aerial photograph/map analyses of

geomorphology (Tiegs and Pohl, 2005) and annual river bird

surveys (Carter, 1989). Long-term monitoring can provide

invaluable information about the behaviour of river systems,

as can palaeohydrological investigations that extend the study

of river variability over hundreds or thousands of years, or

even into longer geological and evolutionary time-scales (Sear

and Arnell, 2006). Methodological improvements, both in

terms of data collection (e.g. increased accuracy and precision

of remote sensing) and analysis (e.g. spatio-temporal statistics)

have greatly increased the potential to study variability, yet

major gaps in understanding remain (Thoms, 2006).

Alongside the characterization of variation, frameworks/

paradigms are needed to incorporate it into

eco-hydromorphology. Concepts such as meta-stability,

where a system is considered to vary within certain

boundaries } unless subjected to major perturbations } are

valuable (ONeill et al., 1989). Much emerging theory in eco-

hydromorphology places variability at its core, such as the

concepts of ood pulses (Junk et al., 1989), ow pulses

(Tockner et al., 2000) and the natural river regime (Po et al.,

1997). This trend needs to continue, acknowledging variability

in river ecology, hydrology and geomorphology, and their

interactions.

The ramications of eco-hydromorphic variability are less

clearly understood. In part, this stems from the diculty of

capturing temporal variation in the short time-scales employed

in most research. When spatio-temporal variability is

examined explicitly, more detail can be revealed about the

underlying processes. For example, Langhans and Tockner

(2006) revealed marked dierences in leaf litter decomposition

rates on the river Tagliamento oodplain according to the

durations and frequencies of inundation. In the Colorado

River basin, Cooper et al. (2003) found variations in the

patterns of Populus and Tamarix recruitment that

corresponded to the frequency and magnitude of high and

low ow events revealed by long-term gauging data. Making

the links between variability and processes is therefore a clear

research priority.

The spatio-temporal variability observed in rivers has been

greatly altered by several thousand years of anthropogenic

changes in catchment vegetation cover and land use (Birks

et al., 1988), and} more recently} by direct modications tohydromorphology (e.g. ood defences). For most research and

management processes, the dynamics within these existing,

modied systems are of great interest. However, understanding

natural systems is also important, for example in studying

hydromorphic processes and acting as references against which

modied dynamics can be compared (Tockner et al., 2003), but

to do so requires the eects of widespread modication to be

circumvented. Coupled palaeohydrological and palaeo-

ecological studies examining the behaviour of rivers prior to

major human modications provide one possibility (Brown,

2002). Another approach is through the direct observation of

relatively undisturbed rivers. In Europe, such systems are rare

} and therefore valuable (e.g. Fiume Tagliamento; Tockneret al., 2003). Both approaches have weaknesses, stemming from

incomplete information in reconstructing the past or questions

over the generality of model rivers, but nevertheless can

provide unique insights into the behaviour of natural rivers.

Eco-hydromorphic responses to environmental change

An understanding of how river systems respond to environ-

mental (external) changes is of fundamental research interest

and vital for successful management. Changes may consist

either of short-term pulse disturbances of varying magnitudes

(e.g. weed cutting, ood events), or longer-term press

changes, such as shifts in land use or climatic changes

(Brunsden and Thornes, 1979). Depending upon the

particular river system and disturbance, eco-

hydromorphology could show no change, temporary

displacement followed by a return to pre-disturbance

conditions, progressive adjustment to the new levels of the

drivers or a rapid shift to a dierent set of conditions/stability

domain (Brunsden and Thornes, 1979). Understanding the

ecological responses to such disturbances should enable

practitioners to appraise alternative policies based on their

likely impacts on the river system and to set upper limits to

potentially damaging activities (Groman et al., 2006). An

example of the latter are the attempts to nd an acceptable

level of urban development within a catchment before changes

in ecology are observed (Allan, 2004).

The component disciplines of eco-hydromorphology have

made valuable progress in studying river responses to

environmental change. Eco-hydromorphic responses to high

ow events and ooding provide a case in point. Numerous

studies document how channel forms have changed following

ooding, and then shift to a dierent state or recover their

original form (e.g. Schumm and Lichty, 1963; Myers and

Swanson, 1996; Sloan et al., 2001). Similar documentation of

ood eects has been carried out on river ecology (e.g. Power

and Stewart, 1987; Boulton et al., 1992), as well as

experimental disturbances intended to mimic ooding (e.g.

Melo et al., 2003). Ecological or hydromorphological

responses to ooding (rapid recovery or shift to a dierent

form) have been conceptualized over recent decades, invoking

ideas including stability, sensitivity, resistance, resilience,

stability domains separated by thresholds, nonlinear

dynamics and complexity theory (Leopold and Wolman,

1957; Holling, 1973; Schumm, 1979; Phillips, 1992; Downs

and Gregory, 1995; Gunderson, 2000; Stallins, 2006). By

combining empirical observations and conceptual models, the

mechanisms underlying these behaviours are being elucidated,

such as the roles of feedback mechanisms and thresholds in

maintaining or switching between alternative system states

(Dent et al., 2002).

Developments in ecology, hydrology and geomorphology

are tempered by the limited development at their interface.

While much of the work on environmental change focuses on

hydromorphic and ecological interactions, it often suers from

the common problem of expertise concentrated in a single

discipline (Hannah et al., 2004). The conceptual frameworks

for studying the responses to change have developed in

isolation, such that analogous physical and ecological

concepts are given dierent terms and classied in dierent

ways. For example, while thresholds and process (or stability)

domains are dened and used in similar ways, the concept of

landscape sensitivity encompasses the ecological notions of



resistance and resilience in a more holistic concept (Brunsden

and Thornes, 1979; Gunderson, 2000). One conclusion is that

there is a clear need to unite dierent disciplines in the course

of studying environmental change, to share a common

language and concepts (Benda et al., 2002). In terms of

specic research and management aims, there is a need to

identify: (i) the responses of dierent elements of eco-

hydromorphology to dierent forms of environmental

change (e.g. brief perturbations or prolonged changes); (ii)

the nature of any thresholds in the external drivers (e.g.

climate) of river systems beyond which major changes occur;

(iii) the proximate drivers and more gradual changes that

reduce resistance/resilience (or increase the sensitivity) to

environmental changes (e.g. reductions in water quality

reducing the ecological resilience to hydromorphic

disturbance); (iv) the trajectories of eco-hydromorphic

changes, both in response to perturbations and in recovering

to the previous state; and (v) indicators of when eco-

hydromorphology is in the vicinity of a threshold, to trigger

management interventions (Groman et al., 2006).

Covariance between eco-hydromorphic elements

Many elements of the riverine environment covary. Urban land

adjoining a channel, for example, may be associated with

modied water quality, altered ow regime, structural changes

to the channel (e.g. channelization, bank reinforcement) and

disruption of processes such as sediment supply (Paul and

Meyer, 2001). Concomitant ecological changes in such

situations (e.g. reduced taxonomic diversity or increased

decomposition rates; Paul and Meyer, 2001) could be a

response to any or all of the changes associated with the land

use. Their eects could be additive, subtractive or synergistic.

Covariance between elements of river systems is both a

challenge and a benet. The challenge occurs in establishing

causal relationships among correlated variables. In the case of

urban development, this may include separating the roles of

hydromorphic modications from reduced water quality.

Where feasible, experimentation could break some of the

correlations and identify key factors, such as the distinct

inuences of substratum and ow velocity on macrophytes }

two variables that are normally correlated (Chambers et al.,

1991). However, experimentation creates articial conditions,

potentially limiting generalizations unless all of the important

factors are addressed.

The potential benet of covariance stems from the oppor-

tunities to describe general eco-hydromorphic relationships.

To pursue the urbanization example, if consistent relationships

between water quality and hydromorphic variables could be

characterized, it might be feasible to combine them in general

ecologyurbanization relationships (Paul and Meyer, 2001).

In this way, the observed covariance may simplify eco-

hydromorphic study. A sound principle underlies this idea,

because, assuming that correlations have been correctly

identied, the covariance between eco-hydromorphic

components is indicative of some common causal process(es)

} albeit unresolved and potentially far removed from theobserved variables (Shipley, 2000). Studying the covariances

between eco-hydromorphic components could assist in the

generation of causal hypotheses.

Two implications of multicollinearity for data analysis are

noteworthy. The rst is that many studies will benet from

being placed in a broader context, identifying a wider set of

covarying eco-hydromorphic elements to avoid drawing

premature conclusions about the relevant variables. Second,

careful choice of methods is crucial. Factor analysis or

ordination methods can help to describe collinearity and

identify underlying structures/variables that have common

causes (Vaughan and Ormerod, 2005). Path analysis and

structural equation modelling may provide a more relevant

framework for dealing with inter-correlated variables (Pugesek

et al., 2003). Statistical variable selection methods (e.g. stepwise

regression) should be avoided, as they are unreliable in the

presence of collinearity and present the temptation to make

inferences about the main variables based upon statistical

signicance (Graham, 2003).

Development of tools and methods

The need to develop novel or more rened tools and methods

reects (i) the complexity and uncertainty surrounding river

research and management, (ii) the diverse audience} in termsof opinions, interests and technical understanding } that

needs to be engaged, and (iii) attempts to link ecology and

hydromorphology more directly, rather than simply

correlating patterns. Numerous issues could be suggested:

here, this is restricted to three examples.

Linking ecological and hydromorphic processes inresearch

Identifying and redressing technical limitations is a clear

priority to facilitate stronger coupling of physical and

ecological processes. At present there is often a mismatch in

the capacity to capture the ecological features and the physical

environment at the empirical level, and this may limit progress

in some aspects of research. For example, laser scanning can

rapidly quantify channel morphology over several biotopes

(hundreds of square metres) at high resolutions (potentially

sub-centimetre), and using repeat imaging, quantify sediment

transport (McCarey et al., 2005). Matching biological

sampling to such data presents problems, both in terms of

obtaining thorough coverage of such large areas and capturing

how organisms such as benthic invertebrates interact with

physical habitat at such high resolution (Peckarsky, 1991).

Developing the temporal dimension of eco-hydromorphic

measurement is vital if the links between processes are to be

made more directly than using simple space-for-time

correlations between patterns. Signicant technological

advances have been made, such as the potential applications

of passive integrated transponder (PIT) tagging to monitor

organism or sediment particle movements at higher temporal

resolutions than previously (Lucas and Baras, 2000; Lamarre

et al., 2005).

Nevertheless, fundamental limits remain on the ability to

measure eco-hydromorphic patterns and processes in both

space and time. For example, in a rapidly changing ow

regime with concurrent change in bed morphology, it is at

present impossible to quantify spatial patterns at meso-habitat

scales (ca 0.5m resolution), let alone microhabitat (Cliord

et al., 2005). In lieu of such abilities, modelling presents a way

forward. Physically based numerical modelling frameworks are

capable of accurately representing ow structures in 13

dimensions and over time (Parsons et al., 2004). Sear et al. (in



press) advocate the use of coupled ecologicalgeomorphic

models within an experimental design to explore the

relationships between hydromorphic processes and the

resulting habitats. Agent-based models provide a versatile basis

on which to relate the behaviour of organisms or ecological

processes to physical habitat structure or processes (Booker

et al., 2004; Rashleigh and Grossman, 2005). Many modelling

frameworks are adept at handling nonlinear systems. Using

observations of ecological patterns and physical forms to

calibrate process rates and system behaviour in models,

followed by further observation to test the models, provides a

powerful way of reconciling the desire to study processes with

the relative ease of recording patterns.

Addressing uncertainty

The importance of uncertainty in research and management

has long been recognized, yet rarely addressed adequately.

Uncertainty derives from a range of sources, including

measurement errors, the weak science-base for much of eco-

hydromorphology, conicting evidence about a phenomenon,

and issues } especially in the future } that can never be

known (Van Asselt and Rotmans, 2002). It is vital that

uncertainty be addressed explicitly in many situations, to avoid

over-estimating condence in conclusions or predictions, or

setting unrealistic goals for management (Clark, 2002). River

restoration projects provide a good example, being inherently

complex and involving a high degree of uncertainty from a

range of sources } especially when projects are viewed over

geomorphologically relevant timescales (Sear et al., in press).

Explicitly acknowledging uncertainties provides a way of

managing unrealistic stakeholder and societal expectations

(Clark, 2002; Rogers, 2006).

Frameworks are required that consider uncertainty, along

with tools with which to describe or quantify it (Clark, 2002).

On one level, analyses that handle known uncertainties such as

measurement error can be adopted readily. Recent

developments using Bayesian statistics, information-gap

theory and other methods illustrate how this may be possible

in eco-hydromorphology (Regan et al., 2005; Halpern et al.,

2006). At a higher level, frameworks for placing management

within the context of all uncertainties} known and unknown

} are being developed (Johnson and Brown, 2001).

Decision making and communication tools

The need for decision making and communication tools is a

consequence of: (i) the diversity of stakeholders involved in

river management; (ii) acknowledging the complexity and

inherent uncertainty in managing river systems; and (iii) the

concomitant capacity for conicts of interest (Clark and

Richards, 2002). Potential conicts occur frequently, such as

the desire to increase water abstraction for drinking and

agriculture, compared with the desire to minimize human

impacts. This has driven the development of a range of

decision support and communication tools. For example, cost-

benet analyses are increasingly being performed, often in

terms of improvements in water quality (Eisen-Hecht and

Kramer, 2002; Mourato et al., 2005), but also relating to eco-

hydromorphology. Kuby et al. (2005) developed a modelling

method for dam removals in the Willamette River basin in

Oregon, which compared the potential benets for salmonid

migration against socio-economic losses (water storage and

hydropower) under dierent scenarios, and which suggested

signicant habitat connectivity benets for relatively low levels

of dam removal. Willis and Garrod (1999) considered the

balance between water abstraction and the potential losses of

recreational income associated with low ows, and showed

that recreational angling in particular could provide nancial

justication for many types of low ow alleviation.

Developments in decision-making need to be coupled to

initiatives aimed at making the tools, and the outputs from

them, readily accessible to the relevant parties and policy

makers. Arnold et al. (2000) described education programmes

for making GIS-based decision support systems for catchment

land-use more accessible to local planners. Ultimately,

decision-making partnerships are required that involve all of

the stakeholders } scientists, managers, land owners and

wider society (Rogers, 2006) } in the same way as researchrequires an inter-disciplinary approach.

THE DEVELOPMENT OF ECO-HYDROMORPHOLOGY

Eco-hydromorphology is in its infancy, despite decades of

relevant research, and it is valuable to consider the framework

within which it could develop (Figure 1). Except for specic

eco-hydromorphic interactions that have been well studied,

much of the research eld still appears to be concerned with

identifying gaps in understanding and framing key research

questions, with expert opinion and some initial literature

reviewing helping to clarify the gaps (Figure 1). In the

preceding discussion some general research gaps have been

highlighted, and in the conclusions some key research aims and

questions are suggested.

Three approaches could be employed to address the research

questions: formal literature reviewing, use of existing data

resources, and dedicated data collection and experimentation

(Figure 1). With research spread across at least three separate

disciplines and many decades, reviews of the riverine eco-

hydromorphic literature are extremely important to synthesize

current understanding and to introduce ideas from the

individual disciplines across eco-hydromorphology. The

opportunities for data mining and other post hoc analyses are

considerable, given the extensive data resources }

ecological, hydrological and geomorphological } that havebeen collected over many years. The overlaps between such data

sets in space and time provide a basis for valuable eco-

hydromorphic investigations, along the lines of the rebeccaproject (relationships between ecological and chemical status of

surface waters), which uses existing data from across Europe,

mainly to examine the linkages between water quality and

benthic macroinvertebrates. Dedicated research programmes

are likely to be the only way of addressing many of the research

questions, but should be supported by literature reviewing and

opportunistic data analyses wherever possible.

Evidence appraisal is vital (Figure 1). The three basic

research methods can be arranged around a simple hierarchy

(Figure 2), representing a ladder of evidence in terms of the

depth of understanding and strength of the evidence.

Literature reviews and analyses of existing data can be

assigned to broad sections of the ladder, the precise positions



depending upon the rigour of both the methods used and those

of the contributing studies or data resources (Figure 2).

Literature reviews are potentially far more valuable than any

individual study owing to the accumulated mass of evidence,

especially if comparable studies are performed under non-

identical conditions, helping to reveal how general the ndings

are over space and time (Lindsay and Ehrenberg, 1993).

These should result in a critically appraised body of eco-

hydromorphic research, which could, in turn, feed into both

pure and applied outputs (Figure 1). Eco-hydromorphology

could contribute to basic science in a wide range of areas, both

in terms of general theory (e.g. diversity and ecosystem

functioning, ecologyphysical habitat interactions) and in the

ways that rivers dier from terrestrial and marine systems

(FBA, 2005). In applied areas, eco-hydromorphic research

should help to generate an evidence base for river management

(Sutherland et al., 2004) and underpin management tools (e.g.

decision support frameworks or bioassessments of

hydromorphological pressures). From all of these outputs,

and indeed from preceding stages in the framework, feedback is

anticipated to the early stages, representing further renement

of the knowledge gaps and key questions (Figure 1).

CONCLUSIONS: PRIORITIES FOR THEDEVELOPMENT OF ECO-HYDROMORPHOLOGY

Research

Numerous research gaps can be identied, and several that are

considered priority areas for eco-hydromorphology have been

discussed. Some of the most valuable short-term work could

focus on making the best use of existing resources} literatureor data. Multi-disciplinary reviews would not only help to

clarify the extent of eco-hydromorphic science, but could also

stimulate development of the area through the combination of

initiatives from the dierent disciplines. Opportunistic data

analyses and data mining will generally be limited to static,

correlative research, yet this should help to provide initial

answers to questions and provide quantitative hypotheses to

guide more detailed research. Another early priority is to

identify model systems or organisms that could act as research

foci, as many research questions will require time series data,

creating a minimum start-up period before they can be

addressed.

In the longer-term, numerous research lines requiring new

data collection can be identied. In this paper, pattern-

processes linkages, spatial structuring, scaling relationships,

system dynamics and responses to environmental change have

been considered, and a few of the many research possibilities

within them suggested } many others could be proposed.Over-arching priorities are to: (i) aim for mechanistic

understanding wherever possible (cf. simple, correlative

investigations); and (ii) try to approach research areas from

multiple disciplines, to draw expertise from dierent areas and

avoid unnecessary duplication of research eort resulting from

a lack of communication between elds. Given current

emphasis on climate change, abstraction and river

regulation, research into eco-hydromorphic resistance/

resilience and responses to environmental change are likely

to be particular priorities.

Monitoring and methods

Numerous issues can be identied relating to the methods and

tools required for research and management (Table 1). In the

short term, most of the aims identied focus on the appraisal

of current methods and ways in which they could be improved

(Table 1). In the longer term, the focus moves to the

development of specic eco-hydromorphic methods, rather

than the adaptation of methods developed for more specialized

purposes.

Carefully designed river monitoring programmes are

extremely important and have much wider value than their

monitoring role. The opportunistic analysis of existing data

sets can address a range of questions, yet is heavily dependent

upon the data that are available (Figure 2). Routine

monitoring programmes are potentially the most important

resource in this respect, with national coverage and temporal

time series; indeed, they may be the only such resource,

unequalled in their sample sizes and coverage. Unfortunately,

many monitoring programmes may have developed in a

piecemeal fashion and involve biases in eort or sampling

scheme (e.g. biases toward road networks). Appraising and

rening existing monitoring programmes, with a view to

optimizing their scientic rigour, would permit more valid

conclusions to be drawn for river monitoring and management,

as well as bringing widespread benets to eco-hydromorphology.

Management and practice

Suggested management priorities are a combination of

organizational and opportunistic measures, coupled with

Knowledge gap

Key questions Expertopinion

Informal literaturesearch

Literaturereviewing

Analysis ofexisting data

New datacollection

Science base Detailedpolicy

Legislation

Basic science Tools Practice &management

Evidence appraisal filter

Figure 1. A framework for the development of eco-hydromorphicresearch and management.



No evidence-base

Exploratory data analysis, expertopinion.

HYPOTHESIS GENERATION

Stronger correlative methods e.g.independent testing,validation in

other rivers or with otherorganisms

Before-after comparisons

Dedicated, replicatedexperimental manipulations

Qualitativeliteraturereview

Systematicreview Strong, general evidence-base

Data mining

Experiments e.g.data collected

before and afterflood defence works

Literaturereviewing

New datacollection

Analysis ofexisting data

Figure 2. A ladder of evidence describing the relative strengths of literature reviewing, analyses of existing data and new data collection. Newinvestigations form the main body of the ladder, with stronger research methods and scientic inference encountered on higher rungs. Results frommost investigations can contribute to the overall evidence base (dotted lines). Literature and data-based research are plotted on the same scale, such

Table 1. Some monitoring and methodological priorities in riverine eco-hydromorphology

Short term* Review current ecological and hydromorphological monitoring programmes:

what potential is there to add (i) hydromorphological measurements to water quality or ecological monitoring schemes, and (ii) ecologicalvariables to hydromorphological surveys (e.g. uvial audit)?

coverage, sample sizes and other concerns.* Appraisal of hydromorphological survey methods (e.g. River Habitat Survey, physical habitat components of the US EPAs rapidbioassessment protocol): how relevant are they to ecology? Do they capture underlying processes and spatial structure? do they operate at ecologically relevant scales? do dierent methods complement one another? e.g. eld surveys, remote sensing and ow gauging what elements are missing and could be added to increase the eco-hydromorphic relevance?

* Improved description of eco-hydromorphic variability, to help distinguish background variation from overall system changes.* Evaluate appropriate research methods in eco-hydromorphology: opportunities for experimentation, eld survey designs, data analysis toolsfor stronger inference (e.g. structural equation modelling)

* Strengths and weaknesses of dierent surveys and analysis methods, and guidance regarding when each should be used.

Longer term* Targeted environmental sampling to document ecologyphysical habitat relationships. e.g. along gradients of hydromorphological pressures* Comparison of eco-hydromorphological assessment tools and models in demonstration catchments* Long-term monitoring at both near-pristine and modied sites}quantify variability and compare between natural and modied systems* Developing specic eco-hydromorphological methods cf. modifying existing ecological or hydromorphological methods:

measures that address causal mechanisms and processes multi-scale analysis tools frameworks for up-scaling from small-scale experiments/monitoring to scales of management relevance

* Adopt more mechanistic/dynamic approaches to modelling and data analysis e.g. agent-based models to simulate ecological interactions withphysical habitat



some specic research and development aims (Table 2). Much

of current management, developing environmental standards

for hydromorphology and trying to identify actions that might

threaten or enhance river ecological quality, relies upon expert

opinion, with relatively little underpinning science. In the more

medium term, there is a need for research and development

that will enhance the capability for detecting, diagnosing and

predicting hydromorphological eects on ecological status.

Across Europe, this understanding will be vital into the next

WFD cycle (i.e. from 2015 onwards). The hope is that with

the development of eco-hydromorphology, management will

evolve into an increasingly evidence-based, science-led process.

Final point: capacity development

Eco-hydromorphological expertise is widely scattered across

the three main contributory disciplines (ecology, hydrology

and geomorphology) and beyond (e.g. civil engineering,

chemistry, social science). In some areas there has been a

dramatic loss of research capacity, such as among freshwater

ecologists in the UK (FBA, 2005; Raven, 2006). In other areas,

policy or regulatory requirements have tended to focus on

site-specic management (e.g. engineering or hydraulic works),

rather than considering the wider catchment. A change

in emphasis, through such initiatives as Making Space for

Water (in the UK) and the WFD, requires the skills of

geomorphologists, sedimentologists, hydrologists and allied

elds working at catchment scales to deliver eective riverine

planning and management (European Commission, 2000;

Defra, 2004). The dispersed nature of this wider eco-

hydromorphic community } both research and management} has the potential to restrict both its research anddevelopment capacity, and its ability to inuence policy.

A major priority in developing eco-hydromorphology, and

river research and management more generally, is to reverse

the declines and overcome the fragmented distribution of

expertise. A thriving community needs to be built and

sustained, linking the expertise that is available across

disciplines and institutions (including universities, research

institutes, regulatory bodies and conservation organizations).

Regular conferences and workshops, with broad invitations,

are one way to achieve this. Web-based initiatives could also

prove to be an eective way of building and maintaining links

across subject and geographical boundaries (Foote, 1999).

There is a strong case for promoting large-scale projects to act

as foci for collaborative eorts, to attract nancial support and

to raise the prole of eco-hydromorphology. The studies based

in the Hubbard Brook Forest, USA (Bormann and Likens,

1979) and at Llyn Brianne in upland Wales (Edwards et al.,

1990; Durance and Ormerod, 2007) provide illustrations of

how eective and enduring demonstration catchments can be.

The addition of urban catchments for this purpose would not

only improve understanding of urbanization eects, but also

make it easier for the value of the research to be appreciated by

the wider public. Focusing resources on such catchments is a

long-term commitment, but this brings the benets of

understanding processes and change over a range of timescales.

Finally, it is vital to recognize the international nature of the

eco-hydromorphological community. In Europe, the WFD

provides an obvious focus for international collaborations, as

member states need to address similar challenges. Other

European developments, such as the EU Water Initiative,

could also play a role. Globally, similar questions are being

asked and challenges faced. This is evident from a series of

United Nations initiatives, including the International

Hydrological Programme, the World Water Assessment

Programme and Millennium Development Goals. All of

these involve consideration of sustainable use of water

resources: eco-hydromorphology is a key aspect in achieving

this. The extent of global interest reveals both the

opportunities for international collaboration, and the degree

to which such collaboration could benet research at the

ecologyhydrologygeomorphology interface.

ACKNOWLEDGEMENTS

This paper resulted from a workshop (Linking PhysicalHabitat Structure to Riverine Biodiversity) held as part of theUK Population Biology Network (UK PopNet), funded by theNatural Environment Research Council (Agreement R8-H12-01) and English Nature. Further funding was provided by the

Table 2. Some priorities for management and practice in eco-hydromorphology

Short term* Dening the role for ecohydromorphology in dierent areas of catchment management* Collate best available evidence for ecologyhydromorphology relationships } mainly literature on correlative ecologyhydromorphologyrelations

* Establish a widely accessible repository for best practice and guidance for assessing habitat status, managing hydromorphological pressures(e.g. ood defence works), and restoration and conservation schemes

* Devise forums for involving the full range of stakeholders and wider society in decision making* Develop standard monitoring protocols for interventions (e.g. weir installation, river restoration) to ensure that as much information aspossible can be derived about the eects on ecology and hydromorphology

* Develop tools for handling uncertainty (e.g. decision support tools) and ways of setting management priorities (e.g. identifying important reachesfor the functioning of hydromorphic processes or those readily improved according to socio-economic criteria)

Longer term* Causal understanding of ecologyhydromorphology relationships (cf. simple speciesenvironment correlations):

natural eco-hydromorphic relationships responses to point hydromorphological modications (e.g. weirs, local engineering works) responses to diuse or distal hydromorphological modications (e.g. land use, sediment loading, climate change)

* Appraise conservation interventions and river restoration using long-term monitoring* Research the concepts of resilience and thresholds as they apply to ecology, hydromorphology and their interactions, e.g. extent of urbandevelopment or impaired connectivity (longitudinal, channeloodplain or channelhyporheic zone)

* Integrate socio-economic concerns, e.g. cost-benet analyses* Understanding the eects of combined stressors (e.g. morphological, chemical and climatic) on river ecology



Environment Agency. We wish to thank all of the workshopparticipants for valuable discussion and ideas, as well asProfessor Philip Boon and two anonymous referees, all ofwhose comments enabled us to make valuable improvementsto the manuscript.

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