The Flow Environment

In fluvial systems the flow of water is a dominant and characterizing variable that influences diverse aspects of the stream environment (Hart and Finelli 1999). It affects channel shape and substrate composition and episodically disturbs both. Flow strongly influences the physical structure and hydraulic forces operating in the benthic and near-bed microhabitats occupied by much of the biota, and is important to ecological interactions, rates of energy transfer, and material cycling (Figure 5.2). Current velocity is a direct physical force that organisms experience within the water column as well as at the substrate surface. Organisms are directly affected when eroded from a substrate or as their energy reserves are depleted by the work of maintaining position. They are indirectly affected when the delivery of food particles, nutrients or dissolved gasses influences their metabolism and growth. Flow conditions are important to ecosystem processes through the delivery of nutrients and gases and removal of wastes, and possibly by influencing which species occur at a site. Definitions and methods of measurement were given in Chapter 2; recall that current is the speed of moving water (usually in cm s 1 or m s_1), and flow or discharge is volume per unit time (usually m3 s 1 or cfs).

Current velocity varies enormously, not only along a river's length and with the rise and fall of the hydrograph, but also from place to place within stream channels at meso- and microhabitat scales owing to bed friction, topography, and bed roughness due to large substrate particles and wood. The vertical velocity profile (Figure 2.8) is of fundamental importance to any consideration of the effects of current on organisms, as the flow conditions near the streambed may differ markedly from open-channel flow. When the depth of flow is substantially greater than the height of roughness elements, one expects an outer layer in which velocities vary little with depth and a logarithmic layer of declining velocity near the streambed (Figure 5.3). Under

FIGURE 5.1 The relative abundance of some macroinvertebrate taxa in four large New Zealand rivers within substrate size (left), depth (right), and velocity ranges. Aoteapsyche (Trichoptera); Colobursicus humeralis, Delea-tidium, and Nesameletus (Ephemeroptera). Error bars are 1 standard deviation. (Reproduced from Jowett 2003 )

FIGURE 5.1 The relative abundance of some macroinvertebrate taxa in four large New Zealand rivers within substrate size (left), depth (right), and velocity ranges. Aoteapsyche (Trichoptera); Colobursicus humeralis, Delea-tidium, and Nesameletus (Ephemeroptera). Error bars are 1 standard deviation. (Reproduced from Jowett 2003 )

smooth, laminar flow conditions, friction with the streambed results in a laminar sublayer of viscous flow very near the channel surface. In most natural circumstances, however, roughness-induced three-dimensional flows and turbulence characterize the near-bed environment where most stream organisms dwell (Hart and Finelli 1999).

Recognition of the complexity of flow conditions near the streambed has led to increasing efforts to measure current and hydraulic forces at scales most appropriate to the organisms. Using methods that allowed quantification of the flow environment of larval black flies at very fine spatial and temporal scales, Hart et al. (1996) established that the spatial distribution of Simulium vittatum was better predicted by velocity measured 2 cm above the bed than at 10 cm. When water velocity microhabitats were quantified at the scale of millimeters, rainbow

Ecological processes affected by flow

Dispersal

• Entrainment

• ln-stream transport

• Settlement

Predator-prey interactions

• Encounter probability

• Escape tactics

Competition

• Exploitation

• Interference

Benthic 4 organism

Habitat use

• Habitat structure

• Disturbance regime

Resource acquisition

• Resource distribution

• Capture efficiency

FIGURE 5.2 Multiple causal pathways by which flow can affect organisms. Potential interactions among pathways are not shown. (Reproduced from Hart and Finelli 1999.)

Outer layer

Logarithmic layer

Rough permeable bed U

FIGURE 5.3 Subdivision of hydraulically rough open-channel flow into horizontal layers. Flow velocities within the ''roughness layer'' are unpredictable based solely on knowledge of flow in the logarithmic layer. This figure is not drawn to scale. (Reproduced from Hart and Finelli 1999 )

Logarithmic layer

darters (Etheostoma caeruleum) in the Mad River, Ohio, were consistently found in microhabitat shelters where velocities were significantly lower than at adjacent (<5 cm distance) sites (Harding et al. 1998)

Characterizing near-bed flows creates an enormous measurement challenge and has led to a number of imaginative attempts over the past several decades to estimate or directly measure flow microenvironments. Approaches include application of boundary layer theory (Davis 1986, Vogel 1994); classification of flows and depths, and size and spacing of roughness elements (Davis and Barmuta 1989, Young 1992); predictions based on hydraulic engineering models (Statzner et al. 1988); and improvements in direct measurement at fine scales (Hart et al. 1996, Bouckaert and Davis 1998). Despite increasing sophistication these efforts have met with only partial success. Before discussing each in detail it is useful to introduce some language and equations pertaining to the velocity conditions and forces associated with flowing water.

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