Sampling frequency reveals how each indicator varies over time. Physical and chemical indicators vary significantly in the short term; consequently, they are monitored monthly from spring to fall in reservoirs and every other month throughout the year in streams. Biological indicators better integrate long-term variations, and so reservoir and stream sites are sampled annually. Reservoir benthic macroinvertebrate sampling is conducted in early spring (February-April), and reservoir fish assemblage sampling is conducted in autumn (September-November). Benthic and fish community sampling in streams is conducted in late spring to early summer (May-June).
The net result of this monitoring design is that TVA collects data every year related to physical and chemical water-quality and biological conditions at sampling locations on 30 reservoirs (Figure 2) and 18 streams (Figure 3).
Aquatic macrophyte coverage is determined from large-scale (1 inch = 600 feet or 1 inch = 1,000 feet) color aerial photography taken during the late summer or early fall, which is the time of maximum submerged macrophyte growth. At approximately the same time as the over-flight, boat surveys are conducted to determine macrophyte community structure at selected sites. Using Mylar overlays attached to photographic prints, aquatic macrophyte colonies are delineated and labeled according to species, and areas are measured using an electronic planimeter. Reservoirs flown for aerial photography usually include Kentucky, Wilson, Wheeler, Guntersville, Nickajack, Chickamauga, Tellico, South Holston, and lakes in the Beech River project. For reservoirs where aerial photography is not carried out, standard field surveys and historical information are used to estimate aquatic plant community structure and coverage. Submersed aquatic plant populations generally are rare in tributary reservoirs because of the wide fluctuations of water surface elevations associated with their operation for floodwater storage. Known populations have been extremely small, short-lived, and of little significance. A detailed summary of TVA's Aquatic Plant Management Program is available in a technical report (Burns et al., 1994) that is updated and published annually.
Because no official or universally accepted guidelines or criteria exist upon which to base an evaluation of ecological health, TVA developed the following methodology for rating the overall ecological health of each of the 30 TVA Vital Signs reservoirs and 18 Vital Signs stream stations. This method combines infor-