5. Understanding your catchment

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5.1 Introduction

Before making any decision about what to monitor, or indeed what river restoration method is appropriate, the practitioner must have a good understanding of the hydrology, sediment load and the water quality of the watercourse since all can significantly affect both ecology and hydro-morphology elements. Understanding these aspects in your catchment is critical in terms of setting realistic objectives and determining your monitoring strategy.

The key aspects to understand are as follows:

Water and sediment quality Figure 3.3 has a major influence on invertebrates, fish and aquatic plants. Many species are intolerant of seriously polluting heavy metals, often associated with road runoff, but biological oxygen demand (BOD), dissolved oxygen levels (DO), phosphorous (P) and nitrogen (N) levels will also impact biodiversity abundance and diversity. These and other chemical constituents can be transported (and in the case of sediment also stored) within the watercourse. How and when this occurs is dependent on the river sediment and catchment type and flow regime.

Sediment loading Figure 3.4 is influenced by in-channel erosion and deposition processes. Sediment is also transported into the watercourse in a number of ways. The type and amount of sediment is dependent on landuse (e.g. ploughed fields, deforestation, urbanisation, mining), and the mechanisms by which it enters the watercourse which can be both natural and artificial (e.g. underlying geology through to the drainage network distribution). The extent and type of sediment can have a major impact on the success of a river restoration project; sediment inputs from the sub-catchment should be considered.

Flow regimes changes Figure 3.5 can significantly affect invertebrate species diversity; water- boatman, ramshorn snail and flatworms, as examples, will be dominant in slower, low flow conditions, as opposed to species which prefer fast flowing water such as most species of stone-, caddis- and mayfly. Often flow change is as a result of over- abstraction or in-channel impoundments, which affect the natural physical river process. Places which were originally fast flow streams may become silt ridden with a resultant decline in the fauna.


5.2 The importance of understanding your catchment’s hydrology, water quality and sediment

This section and the associated Appendices (5-7) provide guidance on the importance of considering hydrology, water quality and sediment issues in determining river restoration success as noted in Section 2.3.1 before considering the options of any project.

Figure 100.1.jpg


Figure 5.1 (a sub set of Figure 3.6) reiterates the importance of considering these aspects

Figure 5.1: Influence of high level forcing components (hydrology, water quality and sediment dynamics) on morphological and ecological responses to river restoration (explained in more detail in the following section)


5.2.1 Hydrology

A detailed explanation of hydrology, as well as some key data sources, is given in Appendix 5. In summary, the discharge (or flow) of a river is a function of the water‟s velocity and the cross-sectional area. Flow is measured in terms of volume rates, denoted as Q and expressed in terms of cubic metres per second or „cumecs‟ (e.g. 3.0 m 3 /s or 3.0 cumecs).

That is: Q = V * A

Where: Q= flow, V = velocity, A = cross sectional area ( i.e. width x depth)

These are the most important but relatively simple variables to measure in terms of hydrology.

In terms of river restoration opportunities the following are important to note:

High (peak) and low flows may impose limits on how both the natural processes and biodiversity may respond to in-channel works. Appreciating these limits will help predict what is likely to happen and inform your monitoring needs.

Catchment response (i.e. the hydrological response to the dominant geology and soils) will allow you to estimate the above peak and low flows, as well as the difference between them and the seasonal dynamics. For example, in a heavily urbanized catchment the river will be „flashy‟ and likely to rise quickly and dramatically in response to rainfall, and fall rather quickly too. In a more permeable chalk catchment there is less immediate runoff and rainfall (having infiltrated into groundwater) and so will be released more gradually to the river.


5.2.2 Water quality

Pollution, diffuse or from point sources, can also be a significant limiting factor on the kind of responses to restoration activities which can be measured especially in terms of ecological recovery. In broad terms these include the following (more details can be found in Appendix 6):

  • Organic pollutants comprising both biological as well as chemical agents.
  • Eutrophication (i.e. enrichment of water bodies with inorganic nutrients such as nitrates and phosphates to the extent that algal blooms quickly develop. This vast biomass out-competes macrophytes, causes the collapse of food webs and dramatically depletes dissolved oxygen, particularly when it begins to decompose).
  • Acidification often associated with mine drainage.
  • Heavy metals (e.g. mercury, cadmium and lead) from heavy industry have long residence times and can accumulate in sediment which is important to consider if sediment is to be disturbed as part of the restoration scheme.
  • Thermal pollution may also be an over-riding limiting factor in freshwaters, again changing the species composition of affected ecosystems.

5.2.3 Sediment movement

The processes of erosion and sediment deposition are particularly important in the generation of habitat diversity in rivers and measuring this change and the physical/biological interaction is often a key component of the appraisal of river restoration activities.

Understanding your catchment influences (both natural such as geology, hydrology and catchment response to rainfall events) and human impacts (e.g. land use, impounding structures such as weirs etc) will increase the project success confidence. Figure 5.2 provides some examples of the type of activities that can affect the frequency, quantity, type and distance of transport of sediment within the catchment.

Data sources to help define sediment effects in your catchment.

NOTE: Your local ecology or responsible agency fisheries officer can help you to understand the relevance of these documents if necessary.

River Basin Management Plans (RBMPs) provide an overview of sediment issues in specific catchments and are available through the EA, SEPA or NIEA. Pressures and risks maps due to sediment are included. Your local Environment Agency staff can talk you through these to help you understand them.

Proportion of Sediment-sensitive Invertebrates (PSI) index can determine whether or not the invertebrate assemblage is sediment tolerant, or intolerant and may identify whether excessive sedimentation is a significant issue. This data is available from the Environment Agency.

Sediment Matters is an Environment Agency handbook available in early 2011 with an associated e-learning package (Science Report SC080018/SR).

Guidebook of Applied Fluvial Geomorphology is a Defra Science Report FD1914, synthesizing several R&D projects, with associated e-learning package. It provides details on how to evaluate sediment, and where to look for additional information. The e-learning package provides you with an overview about understanding your catchment.c


Figure 5.2: From Guidebook of applied fluvial geomorphology (Sear, Newson and Thorne - 2010)

Sediment changes and impact on ecology

Sediment type, location and transportation are an intrinsic part of determining river form and associated habitats. It is generally an increase of fine silty deposit that is detrimental to habitats. However, it should also be recognised that in some instances large deposits of boulders and gravels may be a problem (e.g. where there is sudden bank failure or release of gravel from an upstream source) where this interfaces with existing redds or freshwater pearl mussel beds. The following outlines some of the key elements of importance in this respect.

Fish Changes in the fish community – some species are adapted to sediment-rich waters, others are not. Smothering of spawning gravels, of particular impact on salmonids. Changes in extent of bed-rooted vegetation through excessive siltation and the associated physical and biological habitat structure. Turbidity, which reduces visibility for visual hunters and can directly damage some species‟ gills.

Invertebrates Changes in invertebrate community. Smothering of less mobile species such as bivalve molluscs (e.g. pearl mussels). In-filling of gravel interstices (gaps between stones), and associated habitat loss. Changes in plant communities which represent both food and physical habitat.

Plants Changes in species and functional types present. Changes in resistance to up-rooting in high flows. Changes in substrate nutrient availability, as well as possible toxic effects of contaminants. Floodplain deposition which may smothered plants (although conversely this may be a benefit in introducing new seed source and organic sediment).

Wetland Birds Changes in prey type available.

5.3 Inter-relations between biodiversity and physical habitat

Figure 3.6 provides the framework for considering the physical and biological factors which interact in river ecosystems. The following section outlines some of the major interactions and relationships between these aspects in the context of river restoration. The importance of understanding these when deciding what should be monitored should not be underestimated - though many and highly complex, the more these interactions are considered, the more powerful your monitoring. In essence the more complex your physical habitat, the greater diversity of species you might expect (see Figure 5.4).


5.3.1 Fish

Fish distribution is linked with the hydro-morphological characteristics of the river. Species, such as Bullhead, are indicative of fast flowing turbulent waters, whereas others (e.g. Bream) are found in much slower-flowing waters. This is true from the reach to the micro-habitat scale – even strong swimmers will always search out low energy zones such as margins, backwaters and the lee of boulders. Fish will often move considerable distances up and down the river to meet habitat requirements of different life stages. Spawning may require suitable macrophytes or gravels; juveniles will require refuge from fast flows and predators; and for example different stages will feed on different types of invertebrates.

Figure 5.3: European bullhead requires turbulent flows and stones for spawning (Cottus gobio)(courtesy of James Holloway)


5.3.2 Invertebrates

Invertebrates exist in a wide range of aquatic habitats from silted debris-rich pools to cobbles and boulders in fast flowing upland rivers (see Figure 5.5 and 5.6). Channel form and flow are important factors in determining invertebrate habitat. For those invertebrates with an aerial stage, emergent plants are particularly important as a means of leaving the water, and so channel margins are a key factor.


5.3.3 Plants

Plants exist in river channels, marginal areas, banks and the wider riparian zone and floodplain. Channel shape often determines plant habitat, dictating the water availability at any point on the bed whilst substrate stability is critical, with stable areas colonized much more consistently than unstable areas (see Figure 5.7). The presence and type of vegetation is a primary influence on habitat for all river biota, and particularly the invertebrates. As well as providing physical structure and influencing flows, plants are both direct and indirect (as a substrate for algae) food sources. In addition they provide cover for young fish to avoid predation, whilst many invertebrates lay their eggs within the plant complexes.

Figure 5.7: Interactions between generic vegetation types and location in a river cross-section (adapted from Judy England)


5.3.4 Mammals and birds

Whilst there are no strictly aquatic mammals in UK rivers, those such as Otters (Figure 5.8) and Water Voles spend much of their lives in and around the water. For Otters, the rivers and streams provide food (fish) and river banks provide shelter and a corridor to move along often over tens of kilometres. Similarly, vertical banks provide the right conditions for Water Voles to burrow into and aquatic plants in the marginal and slower flowing areas provide food and nest material. American Mink is also now a key player in many river corridor ecosystems in the UK. Numerous bird species rely on watercourses for food, nesting and shelter, and as habitat corridors. Their presence is dictated by the diversity of river habitats and the abundance of the associated species of fish, plants and invertebrates that are vital sources of food.

Figure 5.8: European Otter

5.3.5 In-channel morphology

Habitat diversity appears to be the chief determinant of species richness in studied streams. Factors influencing habitat distribution include flow velocity, substrate and the presence of wood, detritus and vegetation. All of these affect the key macro-invertebrate assemblages which support higher levels in the food web.

Woody debris provides a food source, habitat structure and both resistance to erosion and local areas of scour. Debris dams also have a significant influence upon morphological process and ecology.

Water velocity at any given point has a direct influence on the macro-invertebrates present and similarly there is a relationship between velocity and substrate composition. Highest velocities are associated with peak „channel forming‟ flows which result in the most significant geomorphological activity.

Increases in the amount of silt and sand in a river lead to increased instability of the sediments, which often adversely affects fauna.

5.3.6 Banks and margins

Though the importance of in-channel habitat may be clear, banks and margins often support the bulk of river biodiversity. Marginal vegetation is utilised by macro- invertebrates for egg-laying and emerging, and as a link between aquatic and terrestrial environments for many animals, thus acting as a focus for reproduction and recruitment as well as providing a refuge from high flows.

Riparian zones are also a source of leaf litter, and the structure of the community at a site has been shown to be significantly influenced by the amount of detritus present. This plant material influences habitat structure, but is primarily a food source for „shredder‟ invertebrates which are a key component of river ecosystems.

5.3.7 Floodplains

Floodplains are important to consider in the context of river restoration since they increase the structural diversity and provide habitats for a wide range of fauna and life stages. Linking the river to the floodplain has an impact on out of bank flows and ultimately, where sediment, water and nutrients are stored both over the short and longer term. Connection is also important for providing refugia for many species and feeding areas for over wintering birds. Some of these inter-relationships are shown in Figure 5.9 below.

5.4 Interacting components – understanding the connections

The following Figures (5.10 - 5.12) are aimed at helping to understand the connections between the physical and biological processes. The 3 examples follow those outlined in Section 4 and defined as SMART objective: a) Restore floodplain dynamics by reconnecting to the river; b) increase in-channel habitat variability; c) increase salmonid spawning opportunities upstream of a weir). The key point here is it to provide a check to ensure that all links are appropriate. Also refer back to Figure 3.6 for a whole suite of options.


5.5 Policy and legislation

It is important to be aware that there are a number of environmental policies and legislative instruments, which vary across the UK countries. Whilst the Water Framework and Habitats Directives are among key drivers for river habitat enhancement across Europe there are, in addition, a number of country-specific regulations for England and Wales, Scotland and Northern Ireland. The following provides information about where to find additional information, but this is primarily referred to in this document so that those aspirations of river restoration are aware that the appropriate statutory organisations should be approached at the inception stage to ensure the smooth running of the project.

A summary of many regulations can be found on the NETREGS website that identifies all the new environmental regulations for Scotland and Northern Ireland during the past 12 months.

5.5.1 England

Further England-specific information can be found on the Defra website, the Natural England website, and the Environment Agency website.

5.5.2 Northern Ireland

Further Northern Ireland-specific information can be found on the Rivers Agency (DfI Rivers) website and the Northern Ireland Environment Agency (DAERA) website. In general however, the Water Framework Directive is being used as a mechanism to deliver environmental gain.

5.5.3 Scotland

Further Scottish-specific information can be found on the SEPA website and NatureScot website, relating specific projected areas to the WFD. Most importantly, in the context of river restoration, are the Water Environment's (Controlled Activities Regulations (2005)) under which all river-related projects must comply. Given the country's strong interest in its fisheries, there are also a number of protection acts including the:

  • Sea Fisheries (Shellfish) Amendment (Scotland) Act 2000
  • Salmon and Freshwater Fisheries (Consolidation) (Scotland) Act 2003
  • Aquaculture and Fisheries (Scotland) Act 2007

5.5.4 Wales

Further Welsh-specific information can be found on the Natural Resources Wales (NRW) website.