A predictive typology for characterising hydromorphology

Geomorphology 100 (2008) 32–40

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

A predictive typology for characterising hydromorphology H.G. Orr a,⁎, A.R.G. Large b, M.D. Newson b, C.L. Walsh c a b c

Environment Agency, Environment Centre Wales, Deiniol Rd, Bangor, LL57 2UW, UK School of Geography, Politics and Sociology, Newcastle University, NE1 7RU, UK School of Civil Engineering and Geosciences, Newcastle University, NE1 7RU, UK

A R T I C L E

I N F O

Article history: Received 22 November 2005 Received in revised form 5 April 2007 Accepted 16 October 2007 Available online 8 May 2008 Keywords: Geomorphology Hydromorphology Typology Water Framework Directive

A B S T R A C T Whilst traditionally poorly quantified, the link between physical habitat and ecological response in rivers is widely recognised, and is currently rising up legislative and policy agendas. In Europe, this is reflected in the Water Framework Directive which dictates that ‘hydromorphological’ condition of water bodies should be capable of supporting ‘Good Ecological Status’. Methods are developed that integrate river system hydrology, geomorphology and ecology (and the complex interplay between these three variables). Whilst hydrological and biological methods for characterisation are relatively well established, geomorphological methods are not. Effective characterisation of geomorphology (physical habitat) with full spatial coverage, at a range of scales, can be used to explore spatial interactions between habitat and biological data and potentially further our understanding of ecological response. Managers need to know what aspects of physical habitat and at which critical locations intervention will lead to greatest improvements in ecological condition. This requires information on hydromorphological character and condition. Existing applied approaches for capturing geomorphological data are highly dependent on intensive fieldwork, which is unlikely to be resourced at sufficiently extensive scales to meet management needs. This paper outlines a typology approach for characterising the physical template of rivers. It draws on a range of hydromorphological data to develop a framework for a channel typology; with data collation from secondary sources followed by targeted fieldwork to (i) assess to what extent individual channel types are characteristic of field conditions and (ii) to collect information on reach-scale variability within each type. Results suggest that characterisation of channel types based on stream power, floodplain width and stream order does result in a distinct set of channel types. Field survey subsequently found that these types had a characteristic suite of patch-scale habitat features (flow types). The approach was applied to a catchment where geomorphological processes exert a dominant control over physical habitat. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction: the importance of physical habitat This paper outlines the importance of river physical habitat as a driver of ecological response and the need to develop hydromorphological tools. As a result of innovative Europe-wide legislation in the form of the Water Framework and Habitats directives, tools are required to characterise physical habitat, assess hydromorphological condition and explore links with ecological response for all rivers across Europe. Ultimately these tools need to indicate where changes in physical habitat can lead to improvements in ecosystem health. Fully integrated ecological approaches are not yet operational and there is an urgent need to generate data to describe the physical template of rivers on which biotic function depends.

⁎ Corresponding author. Principal Scientist Climate Change, Environment Agency, Environment Centre Wales, Deiniol Rd, Bangor LL57 2UW, UK. Tel.: +44 1248 382233; fax: +44 1248 362133. E-mail address: [email protected] (H.G. Orr).

The importance and influence of river physical habitat on freshwater ecological response is widely recognised (e.g. Maddock, 1999; Urban and Daniels, 2006). A number of conceptual frameworks have been developed that emphasise this link, for example biotic and ecological integrity (Karr, 1981; Francis et al., 1993) and Ecological Health (e.g. Simon, 1999). It is clear that biota utilise specific habitat types and these are relatively well developed in terms of hydrological and hydraulic habitats and to some extent micro morphological habitat, particularly for fish (e.g. Moir et al., 2004; Schiemer et al., 2003). There is also evidence that larger scale morphological habitats determine biotic response (e.g. Montgomery et al., 1999; Pess et al., 2002). Water quality is known to be a significant pressure on freshwater ecological status; data and even the status of key species are widely available in many countries. However, data on the distribution or condition of physical habitats are rare. The impact of habitat degradation or loss is hard to study in freshwater systems as species are often highly mobile, are adapted to system stresses and may, therefore, be capable of utilising less optimal habitats. It is also possible that critical thresholds for habitat damage exist, beyond

0169-555X/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.10.022

H.G. Orr et al. / Geomorphology 100 (2008) 32–40

which sharp reductions in biodiversity and associated function may result. A system suffering from a lack of ecological integrity is characterised by diversity losses, ecosystem function impairment and structural degradation. However, the relative importance of physical habitat degradation compared to other pressures such as diffuse pollution is still not fully clear. Whilst some practises such as channelisation are known to impact on ecological response it is not clear how much is too much, and it is often hard to assess which elements of physical habitat need to be improved to gain most ecological benefit. To establish baselines of current system status it is necessary to audit the amount, distribution and juxtaposition of specific habitat types within catchments. Such audits should aim to identify influences on reach- and catchment-scale ecological processes and consider whole river system resilience. No such system is currently in place within the European Nation States. The European Water Framework Directive (WFD) recognises and presents new challenges for the management of physical habitat pressures and aims to achieve ‘good ecological status’ in all water bodies by 2015. The WFD requires that hydromorphological condition supports assessments of ecological status together with chemical and biological conditions. A review of existing approaches to hydrological, biological and geomorphological typologies in rivers (Large et al., 2005) indicates that few integrated classifications exist and that none of the existing published typologies can simply be applied to rivers outside of the areas for which they were developed. The definition of ‘hydromorphology’ itself is subject to debate (e.g. Newson and Large, 2006) but for the purpose of this paper is assumed to be physical habitat as constituted by the flow regime (hydrology and hydraulics) and the physical template (fluvial geomorphology). Tools and techniques for determining flow regimes are relatively advanced and widely applied (e.g. Large et al., 2005). Developing similar tools in fluvial geomorphology is equivalent to deducing and predicting habitat availability since geomorphic processes determine the habitat template of rivers (Brierley et al., 1999) upon which a wide range of biophysical processes interact (Parsons et al., 2002). In theory at least, linking geomorphological processes and potential biological distributions should be possible, although this has not been widely undertaken (cf. Van Coller et al., 2000). Hydromorphological classification for implementation of the WFD requires three main elements: (1) characterisation, (2) condition assessment, and (3) identification of measures to improve condition. Characterisation requires knowledge of the characteristic processes and features of a river type in a semi-natural condition. Condition assessment requires knowledge of the dominant channel-forming processes and resilience (or otherwise) of characteristic features; in other words some awareness of the likely adjustment capacity of the channel to changes in driving forces or channel sensitivity to change. This includes information about channel modification that may be reducing the adaptive capacity of affected and adjoining reaches. Improvement measures might include those with known benefits to biological conditions, such as increased shade or the removal of structural barriers preventing the passage of migrating species. The definition of hydromorphological status raises questions about persistence, robustness and response to drivers such as land-use and flood-rich and flood-poor periods and future climate change. Whilst the WFD does not explicitly consider the implications of climate change, it is anticipated that effects of such change may be felt during the lifetime of the legislation (Wilby et al., 2006). As a result, tools are required that capture the distinctive character of channel hydromorphology and sensitivity to change. Importantly, since the aim of the directive is to improve ecological status where water bodies are less than ‘good’, characterisation and assessment techniques need to detect where ‘hydromorphological’ status could be improved. In this paper, we describe limitations of existing dynamic and static assessments, and present a case for the wider use of geomorphological typologies. Existing typologies are outlined and considered in terms of

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their transferability and applicability within a UK context. If all British river types had been subjected to the degree of empirical study undertaken by Adrian Harvey on upper piedmont streams in the Howgill Fells (e.g. Harvey et al., 1979; Wells and Harvey, 1987; Harvey, 1991, 2001) this could be a relatively simple matter but such contributions remain exceptional. An approach is proposed here for the development of a new typology for British rivers and a preliminary test applied to a case study catchment. 2. Existing options for characterising physical habitat Rivers and streams are individually unique, patchy, discontinuous and strongly hierarchical systems (e.g. Folt et al., 1998). Water body characterisation for the WFD is an iterative process that will evolve over time. The current guidance for the UK specifies only 11 river types based on geology and altitude defined for three different catchment sizes 10, 100 and 1000 km2 (UKTAG, 2004). These essentially comprise climatic, water quality and quantity controls on potential biological distributions. There is clearly potential for refined and improved typologies that could guide future research, monitoring and status assessment. The inclusion of geomorphological controls on habitat distribution and condition could offer far greater understanding of ecological response. However, integrated classification of rivers remains in a formative stage (Naiman et al., 1992), and 15 years on little has altered this perspective. In addition, much of the existing work on geomorphological adjustment has been over timescales that do not fit with the dominant (short term, i.e. annual to decadal) timescales of management. Full dynamic assessments of channel geomorphology are clearly desirable but currently impractical. Large-scale modelling (e.g. Kirkby, 1998; Morgan et al., 1998) is helpful in identifying the risk of sediment delivery from soil erosion and clearly informs policy development. Numerical models of whole catchments are useful for exploring geomorphological adjustment to climate and land-use drivers over long time scales and highlight the degree of inter-reach variability and sensitivity to change under different drivers (e.g. Coulthard et al., 2005). However, scale and process representation, e.g. lateral channel movement, currently constrain wider application (Macklin et al., 2006). Geographic Information System (GIS)-based models have been applied to slope-scale processes (e.g. Montgomery et al., 1998) however, scalingup from very small catchments to segment and catchment scales is not well advanced (Downs and Priestnall, 2003; Newson and Large, 2006). Direct observations of high resolution channel morphology data from remote sensing are currently limited and need considerable data extraction and analysis. A common theme arising from modelling work and channel classification systems is that incorporation of local controls on geomorphic condition, response and memory or history, govern the success or otherwise of predictive approaches (e.g. de Vente and Poesen, 2005). In addition, climate and geomorphic response are not closely coupled; parts of river systems will respond differently with significant time lags. The response may also be heavily conditioned by human interference and local conditions. Typologies are useful at a range of scales in order to simplify complex processes into a group with common characteristics. Thus, each channel-type description should be capable of explaining dominant processes operating within it. The difficulty in capturing local controls and the need for information at (a) the reach scale to underpin operational management and (b) the catchment scale for strategic decision-making has resulted in a proliferation of techniques that are largely based on field reconnaissance surveys (discussed in the following section). While some of these have led to the development of channel typologies, it remains to be seen whether robust typologies can be developed that can truly be transported to different landscapes and that clearly inform management. Linking climate drivers, the range of natural variability in geomorphic processes and biotic response remains a major challenge and developing robust process-

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H.G. Orr et al. / Geomorphology 100 (2008) 32–40

Table 1a Summary descriptions of previously applied large-scale typologies Typology

Basis

Where applied

Advantages

Constraints

Rosgen Rosgen (1994)

Dominant slope, width/depth ratio, planform pattern (including entrenchment) and predominant bed material size. Identifies a continuum of channel landforms and processes.

North America.

Provides a user-friendly tool as an alternative to hard channel design

Not process based and does not identify drivers of change in type. Not necessarily sensitive to disturbances.

Bed mobility variation is described. Applied to the transition between sand and gravel beds but also up to boulder size. Types can be assessed according to likelihood of change in basic geometries. Geomorphic condition is a measure of Focussed on Australian rivers River styles GIS interpretation of geology, soils, Brierley and Fryirs vegetation, climate and information on with potential for river how far a system/reach is from its (2000, 2005) catchment history. Nested hierarchical management, used for problem geomorphological potential or naturalness and indicates potential for solving, application and approach covers catchment to improvement in hydromorphological participatory management. geomorphic unit. River styles are status. characterised by common combinations of geomorphic units although individual units may not be unique to that style.

Fluvial geomorphology Montgomery and Buffington (1998)

Division into geomorphic province, catchment, segment and reach scales and defines seven distinct channel types for mountain drainage basins.

based typologies is a useful step towards a framework for exploring these links. 2.1. Existing generic process-based typologies Many of the published fluvial classification approaches, based on geomorphic processes, focus on specific parts of the river systems or specific channel types. Pertinent examples are headwaters (e.g. Whiting and Bradley, 1993) and floodplain areas (e.g. Nanson and Croke, 1992). Classifications that describe morphological change across geomorphic zones tend to be highly dependent on intensive field reconnaissance, e.g. bar patterns related to changes in channel gradient and sediment supply (Church and Jones, 1982). Church (1992) developed a typology based on downstream change in key variables, and uniquely amongst classification schemes, addresses the issue of size and scale, including changes in width/depth ratios, bed roughness to flow depth and channel slope. Changes in morphological adjustment characteristics and modes of sediment transport are described, along with approximate quantitative thresholds for changes from one type to another (mainly slope- and discharge-based). Although these are largely based on unmodified (forested) channels in the Pacific Northwest, the governing relationships are worth exploring in other regional contexts. A number of attempts have been made to characterise types of fluvial adjustment. Many are focused on changes in meander

Channel types are not necessarily applicable to other regions. Affected by local controls including historic geomorphic features and coarse woody debris. Subjective and semi-qualitative assessment. Does not take account of large-scale drivers of local habitat variability. Well founded in geomorphological theory but heavily reliant on expert interpretation and thus relatively subjective. Approach is transferable but may not make best use of available data sources.

development and lateral change in channel planform (e.g. Kellerhals and Church, 1989; Hooke and Redmond, 1992). Downs (1995a) proposes a generalised channel adjustment typology that relies heavily on expert judgement and interpretation of field reconnaissance information. Although local changes in geology, geomorphology, vegetation and artificial modification will create local discontinuities, the three dominant variables that determine local channel morphology and behaviour are: channel gradient, degree of channel confinement and catchment hydrology (Reinfelds et al., 2003). As system change is a function of the excess of force over resistance, a truly process-based typology might be based on the spatial distribution of driving and resisting forces. A critical issue is the size of the disturbance and the system's ability to resist or accommodate the impact of the disturbance (Werritty and McEwan, 1997). Bank strength is likely to be a significant variable here (Eaton et al., 2004) but characterising bank resistance is highly dependent on local conditions and not widely attempted. Thus, a combination of system complexity (Phillips, 2003), singularity (Schumm, 1991) and limited data have so far prevented detailed analysis of the sensitivity of many rivers to a range of different driving variables (Downs, 1995b). Using total stream power or specific stream power as a guide to channel morphology (e.g. Knighton, 1999) is relatively practical with the use of GIS, detailed Digital Elevation Models (DEMs) and estimates

Table 1b Summary descriptions of large-scale UK fluvial data UK channel data

Basis

Where applied

Advantages

Constraints

Fluvial audit Sear et al. (1995) Orr et al. (2004)

Morphological survey but efforts have been made to incorporate adjustmentbased classification and local channel types to expressly inform local reachscale management and strategic assessment. Descriptive insights into channel form and features.

Aimed at assessment of Land Drainage Consent applications (the principal regulation of morphological change in England and Wales prior to the WFD).

Spatial connectivity of data is a key strength and detailed data collected can be used to monitor change. Derived typologies are process based.

A useful source of historic information in England and Wales.

500 m map-based surveys provide spatial continuity of information but are highly variable in quality. Habitat quality and modification scores derived based on divergence from reference conditions (similar sites in semi-natural condition). GeoRHS will provide an extension of the habitat quality maps to valuable floodplain habitats.

Subjective and semi-qualitative assessment. Requires rigorous and consistent methods of stream reconnaissance and considerable experience and awareness of processform linkages. Data not digitised — never quality controlled and thus care needed during interpretations.

River corridor surveys Gurnell et al. (1996)

RHS (River Habitat Survey) and GeoRHS Raven et al. (1998) Branson et al. (2005)

National UK database of 6000 500 m river bank survey of physical form, flow types (biotopes) and features sites on a stratified random basis within 10 km grid squares. of interest including channel modifications within channel and riparian area. Data are presence/ absence of key forms and structures. Newson et al. (1998) identify stream power as the most significant driving variable explaining type boundaries.

Provides a relative habitat quality score but does not describe process or process dominance and hence has no capacity for prediction. GeoRHS likely to be limited in the same way in terms of predictive capacity and illumination of dominant processes.

H.G. Orr et al. / Geomorphology 100 (2008) 32–40

of discharge (at least in the UK). Data are still required on channel width and most applications of the stream power approach have relied on spot measurements of width from representative reaches (e.g. Lawler et al., 1999). A typology that incorporates the dominant system processes may be particularly useful in defining the potential for more ‘natural geomorphic processes’ in heavily modified or degraded reaches. In addition to such classification, many regions will also require information on the location and extent of artificial channel modifications. For example, in the UK 60% of all channels assessed by River Habitat Surveys (a stratified random sample of 3.5% of UK channels) were modified. Middle and lower reaches of rivers may be more intensely modified; surveys of 100 km of channels in a semi-natural rural catchment in the English Lake District found 40% of lower reaches to be modified (Orr et al., 2004). The primary benefit of process-based typologies is to illuminate the drivers of morphological variability (see Goodwin, 1999, for a detailed review). Where data are available, an analysis of stream power at the catchment scale may help to guide sampling and reconnaissance surveys usefully augmenting observation-only type classifications (Newson et al., 1998). River types, and thus most typologies, are likely to be locally or at least regionally specific (Montgomery, 2001). However, modes and styles of adjustment and some threshold relationships are likely to be more broadly applicable.

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3. Applied large-scale typologies The process-based classifications discussed above have not generally been applied in a regulatory environment or related to ecological processes. The best known approaches developed for application from a range of regional settings are summarised in Table 1a. While elements of the schemes may be transferable, the extent to which most rely on field reconnaissance survey means that schemes in their entirety are unlikely to be directly transferable. In addition to the key variables that determine channel morphology in natural river systems; the reviewed typologies also collectively indicate key variables that will help to identify local discontinuities discussed in Section 2.1. Variables relate to measures of bank strength (riparian vegetation, land-use, channel modifications) or ecologically relevant habitat descriptions such as flow types or physical biotopes (e.g. Newson and Newson, 2000). Given that this paper aims to develop geomorphological typologies in the UK it is useful to also consider specific UK-based approaches and in particular the availability of data. Classifications are described which have already (RCS, RHS) or are likely to (GeoRHS) generate relatively large datasets on physical habitat and geomorphology (Table 1b). Most of these are essentially proforma-based surveys, useful for assessing habitat and physical wealth and quality. In general, these approaches record presence and absence of features within a limited survey area.

Fig. 1. Nested hierarchical approach to channel typology development (part modified from Montgomery and Buffington, 1998, and Brierley and Fryirs, 2005).

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H.G. Orr et al. / Geomorphology 100 (2008) 32–40

The collection of features may provide opportunities for colonisation or use by specific biota that seek the specified physical and flow interactions. However, there is no explanation as to why these features exist, how transient they are, or how they may change under different scenarios. Spatial connectivity is lacking in many of these data, attempts to join them together may merely be an inefficient way of collecting large amounts of data and, at worst, be an ecologically invalid approach. Despite this, development of more robust, overarching, typologies may allow substantial improvements in interpretation and use of these data; particularly if channel type can indicate the wider representativeness of local site surveys. 4. Developing a new typology for British rivers Fluvial action in British rivers is generally less than 10,000 years old; slope is a local driver, and rivers are short and steep on a world scale, reducing the number of representative classes in typologies drawn from abroad (Newson and Large, 2006). Rivers have been heavily modified for 2000 years, and the fluvial system is ‘supplylimited’ for sediment. Such limitations enhance the importance of local sources in channel sediment transport patterns and morphological response. The approach proposed here is hierarchical and follows research-led approaches developed in the United States and Australia but adjusted for the uniqueness of the British river context, and aims to identify British river types. In addressing these differences, the typology is data-driven and so will not merely apply existing channel types from elsewhere, although it is likely that similar types will be observed (e.g. Montgomery and Buffington, 1998). This allows a ‘bespoke’ continuum of channel types to be defined.

The typology is developed in two stages. Initially a desk-based broad typology is developed from readily available GIS-based digital datasets. Secondly, field reconnaissance surveys are used to collect reach-scale data on the identified channel types. The aim of the second stage is to assess whether the types are actually different from each other on the ground and to identify characteristics of each type at reach and geomorphic unit scale (cf. Fig. 1). 4.1. Methodology and case study The 1366 km2 catchment of the River Eden, northwest England was selected for testing the methodology (Fig. 2). This catchment covers a range of geomorphological landscapes, total annual rainfall ranges from less than 650 mm in the valley to more than 2000 mm on high grounds. Predominant land-use is extensive pasture and rough grazing. The methodology is described in detail in Orr and Walsh (2007) and thus only summarised here. A digital channel network was divided into arbitrary lengths (500 m), largely to ensure compatibility with the resolution of other available data on channel widths and discharge estimates. Each channel length was then assigned values for the following variables: stream order (shown to relate closely to catchment area (Orr and Walsh, 2007)), floodplain width and specific stream power. Specific stream power (discharge × slope, divided by bankfull width) used slope derived from a 50 m DEM and bankfull discharge was estimated using the hydrological model SHETRAN, which was set-up for the catchment in a previous study (Walsh and Kilsby, 2007). Floodplain width was extracted from national digital maps of the extent of the 1 in 100 year flood (source: Environment Agency). Bankfull channel widths were taken from a relationship established between GIS-

Fig. 2. Location of the case study catchment: the River Eden, Cumbria, UK.

H.G. Orr et al. / Geomorphology 100 (2008) 32–40

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Fig. 3. Channel types derived from stream power-driven typology (SP=specific stream power).

derived catchment area (based on a 50 m DEM) and field-based measurements of channel width. It is clear that small channels dominate the Eden catchment (83% of channels have a stream order of 1 or 2) and that, to be meaningful, any typology needs to reflect this distribution. Correspondingly, stream order was selected as the first level of the typology. The frequency distribution of all variables: stream power, floodplain extent and slope were examined for each stream order, and major breaks in the slope of the graphs were used to define channel-type boundaries. The typology was thus defined by visual inspection of the data; statistical analysis (e.g. cluster analysis) would be preferable but at this stage the aim was to derive a simple typology for testing with field data. As a result a total of eleven channel types were identified (Fig. 3). At this stage channels can be described by their size, steepness and degree of confinement. Preliminary analysis included information on solid geology and landuse; however it was clear that at this crude scale neither factor affected the distribiution of channel types. Clearly such factors are critical to assessments of water quality and, hence, ecological distributions but these may more effectively be used to subdivide channel types at a later stage.

percentage area of each biotope recorded in the reach summary table. Biotopes or flow types were defined according to the standard River Habitat Survey Manual (Environment Agency, 2003). Other qualitative and descriptive information were also collected for comparison with the desk-based typology. Field data show that, in terms of patterns of erosion, deposition and floodplain width, the defined channel types are distinctly different from each other. Although flow types are not unique to channel types, a clear dominance and combination of types is associated with each type (Table 3). In other words large-scale predictions of channel morphology do have some relevance at the small scale. Combining the desk-based typology and the field data enables a fuller description of the channel types (Table 4). It was notable in the Eden catchment that extensive modification (channelisation) was mainly evident on lower energy channels. The extent of erosion (Table 3) may illustrate a possible effect of differences in slope and stream power. Derived parameters such as

4.2. Field testing and reach and geomorphic unit unit-scale variability

Variable

Attributes

Floodplain and valley width Bankfull width and depth Flow depths of dominant biotopes Vegetation cover Distribution of riparian trees Bed vegetation cover Dominant bed material Dominant bank material Dominant bank structure Sediment transport patterns Style of adjustment Management issues/pressures Sediment sources Dominant flow types Grain size of recent deposits Reason for reach change

– For dominant biotope e.g. riffle, glide Time and date % both banks Continuous, semi-continuous, isolated % macrophyte and filamentous algae Other grain sizes present Left and right bank Left and right bank e.g. side bars e.g. meandering, enlargement, incision e.g. dredging e.g. banks, hillslopes % of each type – e.g. bed material size change

Various lengths of the identified channel types were subjected to field reconnaissance survey (Table 2); this was to try and ascertain how much survey was required to verify channel types. Within a channel type, reach boundaries were noted in the field, these were determined by observed changes in local slope, floodplain width or major land land-use. For each reach, data were summarised for sixteen different variables selected specifically to capture the intrinsic physical and biotic character of the reach in question (Table 2). In addition, large-scale base maps (1:5000) were used for mapping key features relating to system condition, namely erosion, deposition, artificial structures and modifications (all dimensions were recorded for individual features). The spatial coverage of geomorphic or hydraulic unit-scale features – biotopes – was mapped, and the

Table 2 Reach-scale variables recorded by field reconnaissance

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Table 3 Summary statistics for channel types based on field survey Type Total channel length (km)

No. of surveyed reaches (% of type)

Stream power

Erosion extent (m2/ km)

Slope

Main biotopes (in Mean order of dominance) floodplain width (GIS)

Mean floodplain width (field)

Mean bankfull width (m) (GIS)

Mean bankfull Sizea width (m) (field)

1 2 3 4 5 6 7 8 9

110 60 70 210 70 110 70 130 60

18 (11) 2 (1.5) 2 (1.7) 7 (2.6) 10 (7.1) 51 (27.1) 5 (3.4) 2 (0.5) 18 (19.9)

b 20 b 20 b 20 b 20 N 20 N 20 N 20 N 20 b 50

144

Glide/run/riffle

36 274 48 221 28 2,904 209

Present Absent Present Absent 0–40 m N 40 m Present Absent 300 m (mean)

125 Absent 27 20 56 100 57 Absent 64

1.74 3.58 2.61 1.63 3.78 4.99 3.44 1.65 9.76

9.5 1.1 8.3 7.6 5.9 11 16 1 11

Small Small Medium Small Medium Medium Medium Small Large

10

60

10 (13.6)

N 50

117

150 m (mean)

109

13

13

Large

11

50

0

350 (mean)

b 2.5 b 2.5 N 2.5 N 2.5 b 7.5 b 7.5 N 7.5 N 7.5 1 (mean) 4 (mean) 2 (mean)

a

Glide/riffle/run Riffle/glide Chute/glide/riffle Glide/run/cascade Run/cascade Glide/run Run/riffle/glide/ cascade Glide/run

Table 4 Channel-type descriptions (from a combination of desk and field data) Channel type

Description

1

Small, partly confined sinuous channels, cobble and coarse gravel beds. Low energy, heavily modified Small, floodplain channels, gravel bed, low energy, heavily modified — possibly former lake beds/bog. Medium, partially confined meandering channels, cobble bed. Limited modification. Small, unconfined floodplain meandering channels, coarse gravel bed, low energy. Medium sized, partially confined plane cobble bed channels, short steep bedrock and boulder reaches, dominated by meandering sections. Unmodified. Medium sized, partially confined meandering channels, gravel/cobble bed. Limited modification. V-shaped incised valley, confined, steep, boulder and bedrock channels, largely colluvial. Unmodified. Incised gorge, extensive bedrock banks and bed material. Unmodified. Medium to large flood plain rivers, partially confined floodplain reflected in meandering and sinuous reaches. Highly variable bed morphology. Partially modified (b 10%) Medium to large unconfined floodplain meandering channels, cobble bed. Partially modified (b 10%) Large unconfined meandering floodplain channels, gravel bed. Varied levels of modification.

3 4 5

6 7 8 9

10 11

30

Large

Size refers to the catchment area (small b 20 km2, medium b50 km2, large b 1112 km2).

stream power were not estimated in the field (this would have required intensive measurements of flow discharge and channel geometry), however the extent of erosion served as an indication of erosive power. Sites which were shown to have floodplains (long-term depositional processes) also seemed to be largely depositiondominated in the short term (Table 5). An erosion/deposition ratio greater than one denotes erosional types which do not have floodplains. Downstream change in specific stream power can be compared with field data to determine whether channel segments are erosion-, deposition- or transfer-dominated. This also provides an indication of channel sensitivity in the face of potential change in external drivers, e.g. wider hydroclimatic change. Determining whether a reach is erosion-dominated can be a simple derivation based on the balance of erosion and deposition features in the reach (approximate spatial extent was recorded for each reach in the field). However, where erosion is extensive, due to artificial influences such as livestock damage, process dominance may change. For example, a naturally depositing reach could become a net source of sediment as a result of livestock damage to banks. An alternative methodology is to note the amount of erosion and deposition per unit of channel area as merely being an indication of the relative importance of erosional and

2

150–1200 m (range)

depositional features. The qualitative comments compiled for each reach during field survey can be used to confirm dominant processes for individual reaches, with artificial influences removed. An attempt to explore this is shown in the longitudinal profile for a 14 km length of Helm Beck, a tributary of the River Eden (Fig. 4). Eroding and depositing sites were denoted according to the amount of erosion and deposition per unit channel length. In general, the eroding sites map onto reaches predicted to have high stream power, whereas depositing sites map onto those with lower stream power. For this river, a number of sites that did not fall neatly into this division included those that had been heavily modified (channelised) or were heavily poached by livestock (nearly 30% of sites). The threshold between erosion and deposition dominance seems to be around 30 W m− 2, which corresponds to published thresholds of 25–30 W m− 2 for erosion (e.g. Brookes, 1988). Floodplain width is included in Fig. 4 to show that low stream power tends to correspond with larger floodplains. 4.3. Discussion Identifying channel types from secondary sources does appear to offer some insight into dominant processes, and with targeted field data can be a resource-efficient way of capturing information at all spatial scales. Further analysis of the data may indicate downstream changes in width/depth ratios and characteristic morphological adjustment and channel channel-type boundaries. These data have been collected and discussed briefly in Orr and Walsh (2007). It is possible to identify reach variability in terms of the degree of confinement, channel geometry and range of flow types similar to those identified by Montgomery and Buffington (1998). Our typology captures the driving forces of channel morphology but not the resisting forces (see discussion in Section 2.1). Further work is

Table 5 Contemporary balance of erosion and deposition (active features observed in field survey 2005) Channel type

Erosion (m2/ Deposition km) (m2/km)

Erosion/ deposition ratio

Mean floodplain width (m)

1 3 4 5 6 7 8 9 10

144 36 274 48 221 28 2904 209 117

0.17 0.27 0.9 0.16 0.35 0.08 9.46 0.53 0.33

125 27 20 56 100 57 Absent 64 109

867 132 304 301 628 339 307 391 354

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Fig. 4. Predictability of eroding and depositing reaches using specific stream power derived for the Helm Beck, Cumbria, UK.

required to capture structural resistance variables which in many cases can be provided by an assessment of bank and riparian vegetation. Bank strength is captured to some extent in the UK by RHS data although this is not explicit and requires further analysis and testing of the national database to quantify this variable (see also Newson et al., 1998). In the future, a combination of the desk-based approaches outlined here and targeted field work, potentially including RHS and GeoRHS, may enable some further insights for a wider range of channel types. There are a number of uncertainties and assumptions made here, particularly relating to digital datasets. The derivation of channel slope is dependent on the scale of the DEM used and is known to be a difficult parameter to capture accurately. Stream order is a relative measure depending on the quality of the channel network data used (very small streams were under-represented in our typology). Stream power is derived from slope and estimates of discharge could be derived from a variety of sources. For example, not every catchment has a hydrological model capable of deriving flows, however, systems such as Low Flows 2000 (Holmes et al., 2005) could be used in the UK. However, Low Flows 2000 is not designed for estimating high flows — that is, Q1 (the flow that is equalled or exceeded 1% of the time) needed to estimate bankfull discharge (Young et al., 2003). Stream power also uses a measure of bankfull width which is highly variable (Orr and Walsh, 2007). No catchment substrate dataset is available so that channel roughness cannot currently be taken into consideration unless field surveys have been completed. 5. Conclusions Dynamic modelling options in geomorphology currently have limited applicability, classification systems are generally region- or location-specific and, whilst a number of methods exist for characterisation at reach scales, conceptual and theoretical frameworks for scaling-up have not been widely applied to date. Despite these issues, it is possible to derive a first order geomorphological classification of river channels from existing and widely available data. A stream power-driven approach does appear to reflect some of the variability in morphological channel types and indicates dominant processes observed in the field. This should enhance the use of existing taxonomic approaches and available large-scale datasets by providing a rational context for the habitat types and habitat quality scores provided by RHS and the forthcoming GeoRHS.

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