The Fluvial Hydrosystems

Rivers are a popular source of fascination because of their changing moods from headwater stream to large lowland rivers and from quiet summer brooks to raging flood torrents. The character of a river changes continually, from day to night, from day to day, from season to season and from year to year. It is this dynamism that attracted the attention of naturalists and geologists in the 19th century, and that today provides a focus for research by biologists, fisheries scientists, geomorphologists and hydrologists. However, the interest in rivers is more than curiosity; most nations are dependent on sustainable river management.

Fluvial hydrosystems are the product of physical, chemical and biological processes operating throughout a river’s drainage basin and over a range of time-scales from a year to tens of thousands of years. A drainage basin is the area that gathers water from precipitation and delivers it to the river (Figure 1.2). Defined by a topographic divide, the basin is occupied by a drainage network which collects the runoff from hillslopes, together with its load of sediment, particulate organic matter and solutes. Thus, a river may be seen as the artery of a drainage basin conveying water, minerals and organic matter to the sea. A drainage basin perspective is also important because the flow regime and sediment loads determine the morphology of the channel which has a strong influence on the structure and function of fluvial hydrosystems as first recognized by Hynes (1970, 1975). However, drainage basins are complex geomorphological systems with a history. This chapter describes the characteristics of drainage basins and examines the ways in which the basin influences fluvial hydrosystems over a range of time-scales.

The terms hydrological and hydrochemical dynamics imply the motion of water and associated solutes under the influence of external forces and mass exchanges. They are also suggestive of process operations and of energy exchange within a defined systems framework. This chapter examines such concepts from two distinct but complementary perspectives. The first section examines in greater detail the fundamental dependency of upland water quality on dynamic hydrological pathways, chemical budgets and catchment characteristics. Here emphasis is placed on micro- to mesoscale processes, and on the role of heterogeneity within basins. The second section considers the aggregate effect of multiple headwater systems routed and mixed via the channel network and the significance of channel-floodplain interactions. This will underline the importance of considering macroscale controls versus anthropogenic impacts from both a spatial and temporal perspective. Throughout the discussion a twofold division into headwater streams and large rivers provides a useful vehicle for examining the significance of scale and connectivity.

Small headwater tributaries usually flow within steep-sided V-shape valleys. Further down the river network the valley sides tend to be less steep and the valley bottom is often infilled with sediments. Here the river is separated from the valley sides by more or less extensive flood-plain. Sometimes bordered by terraces, remnants of former active floodplains form stair-like features up valley sides. However, this generalized change in the downstream character of rivers and river valleys masks considerable variability.

Chapter 4 described the basic mechanisms of river dynamics, emphasizing longitudinal and transverse variability in morphology and flows of water and material. This chapter applies information on river morphology and dynamics to examine the physical structure of hydrosystems. The interplay between river channels and the set of environmental variables determines geomorphological ‘styles’; the basis of ‘functional sectors’ with their distinct patchworks of special habitats.

The distribution of plant species and their productivity in a fluvial hydro-system is dependent on the complex interactions between hydrodynamic processes (e.g. flow velocity, shear stress, the nature and stability of the substrate etc.), hydrochemical processes (e.g. nutrient cycling, pH etc.) and the use of solar energy by their photosynthetic processes (influenced in turn by water transparency, shade etc.). Diverse adaptive strategies permit plant communities to colonize a range of patches in the fluvial unit, but they are sensitive environmental indicators (Naiman and Décamps, 1990). However, environmental changes affecting plant distributions in fluvial hydrosystems are now recognized as complex phenomena, giving rise to transient states in plant communities which may persist for decades.

From the source of the river, the longitudinal gradient of ecological conditions corresponds to a spatial sequence of faunal communities. This distribution of species is the result of (a) adaptive strategies to physical parameters such as, water temperature, flow velocity and shear stress, and bed-sediment size, and (b) strategies which optimize the utilization of food resources and available living space. Along a river, the flow of water determines both the connectivity between, and the general characteristics of, the different sectors. Similarly, in the floodplain the aquatic environments (cut-off channels and permanent backswamps) are fed from the main river either directly during floods or indirectly, by exchanges with the alluvial aquifer. However, hydrological connectivity between aquatic environments within the floodplain is discontinuous both in space and time, and the importance of this linkage depends not only on its strength and duration (Figure 7.1) but also on its timing. A variety of aquatic environments are created by the differing degrees of connectivity and give rise to a mosaic of interlinking habitat patches and communities. Three concepts have been proposed to explain the functioning of the river and its floodplain: the River Continuum Concept (Vannote et al., 1980), the Nutrient Spiralling Concept (Webster, 1975; Newbold et al., 1981) and the Flood Pulse Concept (Junk et al., 1989).

Of all aquatic organisms, fish are among those that possess the greatest mobility, thus offering the possibility of rapid occupation and exploitation of the various biotopes in the fluvial mosaic. For those fishes that migrate in order to reproduce, the longitudinal course of the river represents the principal means of access to spawning grounds. Long-distance migrants may ascend the entire length of the river to reach its headwaters; whereas non-migratory species, such as some cyprinids, use the longitudinal course of the river merely as part of the fluvial network to move between various transversal parts of the hydrosystems (e.g. side channels, ox-bows) for different periods in their ontogeny.

The distribution of the different units within a hydrosystem is related to the geomorphological mechanisms involved in their creation (Chapter 5). In addition to the effects of the environmental constraints specific to each unit (Chapters 6, 7 and 8), the composition of the populations in each unit also depends on their spatial relations because of exchanges and interactions between them. Some species need to be close to different complementary units for their development, or for re-establishment after disturbance. The hydrosystem as a whole has a resilience, with some units within it acting as refugia or foci for recolonization.

In the oxbow lakes formed by channel cut-off along meandering rivers, flowing waters become stagnant and then become invaded by submerged aquatic vegetation (hydrophytes), which is itself dominated and then replaced by emergent vegetation (helophytes). These helophyte communities, such as reed beds, will in turn be invaded by marshland shrubs, which themselves may be replaced by hygrophilic forest communities. Likewise, herbaceous plants will become established on a newly deposited sand bar, followed by shrub thickets, which will tend to eliminate them, and these will in turn be replaced by larger trees. Of course, these changes in vegetation are accompanied by changes in the conditions of the habitat (e.g. nature of the soil, water depth) and animal populations. This is a very general phenomenon known as ecological succession.

For thousands of years rivers have been altered by many forms of human activity. Deliberate changes of fluvial hydrosystems have resulted from the power of technology to satisfy the many demands upon water resources. River regulation by dams for domestic and industrial water supply and for irrigation, which today often include intercatchment transfers, and for flood control has markedly altered the flows, sediment loads and water quality characteristics of many rivers (Petts, 1984). Channelization for navigation and land reclamation has further altered the ecological character of fluvial hydrosystems by isolating the channel from its floodplain (Brookes, 1988). These direct impacts have been compounded by indirect impacts associated with the range of land-use changes that have altered the catchment areas over the period of human occupation — several thousand years in some cases.

River management should involve resource utilization without deterioration of the natural basis (Mellquist, 1992), a concept promoted in the Brundtland Report (1987). In practice, river management involves important choices (Boon, 1992). First, for rivers that are essentially pristine, there is an overwhelming case for preservation. The challenge is to allow natural changes within fluvial hydrosystems (those caused by floods, droughts, erosion and sedimentation — and variations in the frequency and duration of these processes with changing weather patterns) whilst protecting the river from artificial influences. In most cases, however, the pressures for land and water development, and the resulting problems of waste disposal, will require management to limit artificial changes within the catchment and to mitigate the impacts of human actions.