• ecological health evaluations must be based on information on physical, chemical, and biological components of the ecosystem;
• monitoring must be sustained for several years to document the status of the river/reservoir system, determine its natural year-to-year variability, and track results of water-quality improvement efforts;
• monitoring must provide resource managers with current, useful information;
• monitoring program design must be dynamic and flexible rather than static and rigid and must allow resource managers to adopt better monitoring techniques as they develop to meet specific needs; and
• monitoring is not primarily intended to address specific cause-and-effect mechanisms. Although monitoring results may provide sufficient information to identify cause-and-effect relationships, addressing these mechanisms usually calls for shorter-term, more-detailed assessment programs.

With these fundamental premises in mind, TVA's challenge has been to develop a sustainable monitoring effort that collects the right kinds of data at a minimum number of locations and frequencies, yet still provides enough information to reliably characterize ecological health. The four main activities of the program focus on physical and chemical characteristics of water; acute toxicity and physical and chemical characteristics of sediment; benthic macroinvertebrate community sampling; and fish assemblage sampling. Under a complementary program, TVA also collects aquatic macrophyte community information to provide a more comprehensive evaluation of each reservoir's condition.

Three areas in each reservoir were selected for monitoring: the inflow area, which is generally riverine in nature; the transition zone or midreservoir area, where water velocity decreases due to increased cross-sectional area, suspended materials begin to settle, and algal productivity increases due to increased water clarity; and the forebay, the lacustrine area near the dam (Figure 1). Transition zone and forebay areas include overbanks (flood plains that are inundated when rivers are impounded).

Embayments constitute an important reservoir area that support a variety of uses. Previous studies have shown that ecological conditions in reservoir embayments are controlled mostly by the characteristics of the immediate watershed and embayment morphometry (Meinert et al., 1992). The main body of a reservoir usually has relatively little influence on embayment conditions because typically there is only minimal water exchange. But monitoring the ecological health of the hundreds of embayments in the TVA reservoir system is well beyond the scope of this program.