primarily available from the upstream impoundment, agricultural or other areas in the adjacent
watershed, and point sources. Carbon is also present in soluble forms through leaching and breakdown
of particulate carbon.
Tailwaters are more than conduits for transporting particulate and dissolved materials.
Biological processes occur in flowing waters and influence the cycling of transported material. Running
water is a richer habitat than still water due to currents which prevent the accumulation of a shell of
depleted resources around organisms by providing a constant supply of fresh material (nutrients,
oxygen) for metabolism. Biological processes in tailwaters involve benthic communities (microbial,
invertebrate, and vertebrate), macrophytes, periphyton, phytoplankton, zooplankton, and fisheries
communities. Other biota (birds, reptiles, amphibians, etc.) are usually considered on a site-specific
Biological processes are often inseparable from physical and chemical process as is illustrated
in evaluating the dynamics of dissolved oxygen in tailwaters. The consumption of dissolved oxygen in
rivers is most often a biological process which breaks down organic matter (carbon). This process
often results in a decrease (sag) in dissolved oxygen concentrations downstream from the input of
organic matter. As illustrated in Figure 1.3.22, the "sag" in dissolved oxygen concentrations can be
Figure 1.3.22 Measured and computed dissolved oxygen concentrations
demonstrating "sags" in longitudinal concentrations. (From Krenkel and
Novotny (1980), reprinted with permission of Academic Press).


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