Fluvial Design Guide - Chapter 4

Fluvial ecology

4.1 Introduction

The term ‘fluvial ecology’ refers to the relationships between aquatic organisms and environments associated with rivers or streams. It has long been known that river channel morphology has a strong influence on riparian plant and animal communities. Hence, changes to the form and function of rivers invariably have knock-on effects on ecology.

Some of these changes are brought about by natural processes such as drought and flood. However, others are a direct result of human intervention, specifically those activities identified in this guide relating to reducing flood risk, or associated with water resources or hydropower schemes.

This chapter highlights the inextricable link between channel form and ecology. It identifies general design principles and engineering approaches that avoid substantial deviation from the natural ecological state or, if this is not feasible, that minimise ecological damage and identify options for mitigation.

It is important to recognise that, while engineering works in the aquatic environment inevitably pose potential risks to the fluvial ecology, sympathetically designed schemes can also offer opportunities for ecological enhancement and biodiversity gain. The key to realising these benefits is early identification of the opportunities for enhancement, so that these can be factored into the design from the start, rather than added later.
 


4.1.1 Role of river morphology

After water quality, river morphology is arguably the main driver influencing aquatic ecology in fluvial environments. This has long been recognised in fisheries ecology, where gradient and stream order have been used to classify river reaches in terms of the fish populations they hold (for example, Huet’s Classification; Huet, 1949).

High gradient upland reaches (low stream order) are typically favoured habitats of salmonids (for example, trout and salmon), whereas the shallow gradient of lowland reaches (high stream order) are preferred by cyprinid fish (such as roach and bream). Other faunal communities (such as macroinvertebrates) vary considerably with changes in stream order. Similarly, considerable variation in aquatic flora can be observed with bryophytes tending to dominate low stream order, high gradient reaches and with a general shift to higher plants (such as crowfoots and pondweeds) in high stream order, lower gradient reaches.

Streams can be classified in many ways, with approaches often differing between countries. Although Water Framework Directive (WFD) typologies based on altitude, size and geology have been used in the UK, there has been recent development work to try out different methods. An example of this is Mimas, a national data centre based at the University of Manchester (SNIFFER, 2006), which is supported by the Joint Information Systems Committee and the Economic and Social Research Council. One approach to river typology is based on geomorphology and has been taken further by Rosgen (1994), who identified up to nine different stream types, with the broad differences resulting in contrasting habitats for aquatic ecology (see Figure 4.1).

River morphology defines these habitats, not only in terms of discharge (flow), velocity and depth of water, but also in the way that these physical parameters influence erosion, deposition, sediment size and sediment transport. These in turn influence the substrates available for algae, invertebrates and aquatic plant (macrophyte) colonisation. The latter have a profound influence on aquatic ecology in terms of habitat creation or ‘niche’ availability. Put simply ‘if the niche isn’t there, the beasts won’t colonise’. This applies to fish, invertebrates and water plants, and to the terrestrial animals and birds that depend on them.

Figure 4.1 Broad level stream classification delineation showing longitudinal, cross sectional and plan views of major stream types

(Rosgen, 1996, reprinted with permission from Wildland Hydrology)

Aa Very steep, deeply entrenched, low width/depth ratio and laterally contained
A Steep, entrenched, cascading with step/pool streams
B Moderately entrenched, moderate gradient, riffle-dominated channel
C Low gradient, meandering, point-bar, riffle/pool with broad floodplains
D Braided channel with longitudinal and transverse bars – very wide with eroding banks
DA Multiple channels, narrow and deep, with extensive well-vegetated floodplains and wetlands
E Low gradient, meandering riffle/pool stream with low width/depth ratio, high meander width ratio
F Entrenched meandering riffle/pool channel on low gradients with high width/depth ratio
G Entrenched 'gully' step/pool and low width/depth ratio on moderate gradients

4.1.2 Importance of ‘connectivity’

A river can be defined as a channel or corridor taking precipitation-derived water down a gradient towards the sea. With this in mind, the concept of ‘physical’ longitudinal connectivity is easy to grasp, as most of the water that initially starts off in the upper catchment ultimately ends up in the sea. However, river-borne sediments usually make an equivalent – albeit slower – seaward journey. Again, this has a profound influence on habitat availability and quality, as sediment forms a key component of many river habitats.

Many aquatic organisms make riverine migrations, either actively or by drifting with the current. Indeed, the lifecycle of some aquatic organisms relies on the use of different parts of the river system during specific life stages. In addition, the lifecycle of a number of migratory fish species (salmon and eel, for example) involves moving between riverine and marine environments. Consequently, the ‘ecological’ connectivity between reaches is of fundamental importance. Artificial barriers to longitudinal connectivity (for example, obstructions such as weirs) can have a devastating effect on aquatic ecology, removing species from a river section, or indeed catchment, with immediate effect. Similarly, natural barriers such as densely vegetated channels can act as a constraint to the movement of organisms within a watercourse.

Lateral connectivity is the connection of the river channel to adjacent wetlands, fringe habitats and riparian land. Adjacent wetlands serve a variety of important functions, not least by holding up floodwaters in natural flood retention areas or acting as simple ‘land sponges’. Although subject to some debate and considerable continuing research, such wetlands are thought to ‘smooth’ the ascending and descending limbs of the hydrograph and reduce flood peaks.

In addition to such hydraulic functions, lateral connectivity is also extremely important for aquatic ecology, with natural wetlands providing a vital habitat for many aquatic organisms. For example, ponds and wetlands adjacent to, and connected to, rivers provide fish species with areas of refuge that are important during periods of high flow.

The ideal scenario for such wetlands and offline habitats is for them to be connected permanently to the main river channel, as this facilitates colonisation by fish, plants and invertebrates. Additionally, they typically provide still-water conditions in which different aquatic and riparian plants (such as marginal reed-beds) can flourish. In turn, these support diverse invertebrate communities and offer a spawning medium and structural refuge for some fish species.

Wetlands also add considerable ecological value to the river ecosystem by providing an ideal habitat for wading birds and waterfowl, amphibians and aquatic mammals such as water voles. Indeed these wetlands are now regarded as nationally rare habitats in their own right and their substantial conservation value is promoted by organisations such as the Association of Rivers Trusts (http://www.associationofriverstrusts.org.uk) and Pond Conservation: The Water Habitats Trust (http://www.pondconservation.org.uk).
 

 

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