Lakes and reservoirs have distinct structures that arise by the basin geology, morphometry, hydrology, and meteorology that affect the distribution of light, heat, and water circulation and exchange. Changes in instream water quality are inevitable when free-flowing water becomes impounded as deeper, still waters within project reservoirs, and then back to riverine environments downstream of sluice gates, spillways, and generator tailraces. If the water supply was artificially impounded as part of the original hydroelectric project development, water and power demands along with the need for flood control very often dictate the regulation and control of stream flow. Variations in flows and the large size and variations in reservoir storage volumes often disrupt natural processes that would occur if water were not impounded, or that occur in the present day upstream and downstream of the impoundment.
In general, the water column in most reservoirs has a characteristic temperature structure that is independent of the size of the basin. In summer, thermal stratification occurs, with warmer water overlying colder and denser water, much like summertime thermal stratification found in many temperate lakes (Figure 1). Depending on the flow and temperature of water entering the reservoir and the particular configuration of the reservoir inlet and outlet structures, this vertical zonation very often prevents water exchanges between warmer surface waters (epilimnion) and bottom waters (hypolimnion). Depending on the overall aquatic and upland productivity of the system, this zonation may result in hypolimnetic oxygen depletion due to the decay of natural organic matter, with subsequent increases in metals, and increased rates of nutrient exchange at the sediment water interface. Releases of this impounded water can effect downstream populations of fish and other aquatic organisms due to decreased levels of dissolved oxygen; increased levels of metals, nutrients, and turbidity; or altered temperatures.
Various approaches for evaluating reservoir temperature and chemical structure are used by researchers, the most common of which are discussed below, including lake and reservoir water sampling, sediment sampling, and bioindicator monitoring.
The following references are recommended for additional information on reservoir temperature and chemical structure:
Horne, A. J. and C. R. Goldman 1994. Limnology. McGraw-Hill. New York.
Wetzel, R. G. 2001. Limnology: lake and river ecosystems. Academic Press, San Diego, California.
