Impoundments differ from naturally occurring bodies of water in several ways. Some of these differences arise from the design and purpose of the impoundment, and profoundly affect its limnological behavior. In general, the fundamental purpose of an impoundment is to store and supply water. There are two principal challenges: (1) the logistics of generating a water supply that is always avail able in sufficient quantities is complex, and (2) the ability to design and construct such systems within budgetary constraints often requires at least some compromise.
Fundamental differences in the ecological nature of lakes and impoundments may be understood by examin ing the behavioral distinctions between deep and shallow water systems. Shallow impoundments and shal low lakes are generally more similar than are deep impoundments and deep lakes.
The distinctions between shallow and deep impound ments relate not to absolute depth but to the consequences of basin morphometry and its ratio of depth to surface area. This ratio provides an index of vertical mixing. Lakes and impoundments defined as shallow are, generally speaking, polymictic and holomictic (fully mixed) for most of the time. Deep impoundments tend to develop persistent ther mal stratification for weeks or months at a time. The seasonal intensity and duration of this stratification is lar gely dependent on geography and water retention time, with longer water retention times favoring stronger strati fication. If retention time is short (e.g., less than 7-10 days), the high flow rates can prevent stratification even in deep impoundments (Figure 1).
Another variable that influences the depth of mixing in impoundments is water transparency. In more transparent waters, light penetrates more deeply than in colored, turbid or phytoplankton rich waters, and correspondingly the surface mixed layer, or epilimnion, will be thicker.
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