River and Lake Characteristics of Reservoirs

Reservoir Longitudinal Zonation

Reservoirs display characteristics of both rivers and lakes because they are formed by the damming of a river:

• Similar to rivers, reservoirs have longitudinal and lateral gradients in certain features:
• Current velocity decreases from the upper portion (where the river enters) toward the dam, and from mid-channel to the banks.
• Width generally increases from upper to lower portions of the reservoir.
• Channel depth increases from upper to lower portions of the reservoir, and decreases from mid-channel to the banks.
• Similar to lakes, large reservoirs can have vertical (depth) gradients of several important characteristics (Cole 1983):
  • Light -- Light is attenuated by travel through water, constraining the growth of plants and algae to shallower, termed the euphotic zone.
  • Temperature -- Solar radiation, like light, is absorbed rapidly by water. Deeper reservoirs can stratify into layers: a warm layer near the surface (epilimnion), a cold layer near the bottom (hypolimnion), and a temperature gradient between them termed the thermocline.
  • Oxygen -- Oxygen is dissolved in water by surface exchange with the atmosphere, and from photosynthesis of aquatic plants and algae; at greater depths (and reduced light) oxygen can be depleted (anoxic).

The longitudinal transition from upstream river to downstream lacustrine characteristics can be seen as a series of zones in a reservoir (Soballe 1992). See figure below: