Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 103

natural lakes, and presumably reservoirs are not much different, except
that natural lakes usually do not release deep waters downstream. Occa-
sionally, the capacity of natural lakes is increased and outflow is regulated
by adding a dam. Unlike natural lakes, reservoirs are deep near the dam
and generally become shallower near the deltas of the rivers that feed them.
Reservoirs are often limited by the surrounding topography, so they have
a lower mean depth than many natural lakes. Low mean depth can lead to
increased mixing and associated suspended solids.
Reservoirs fill the drainage basins of rivers and streams, and each arm of
a reservoir moves up into a former stream channel. Thus, a typical reservoir
has a dendritic or tree-like shape (Figs. 6.1 and 6.9). A dendritic shape results
in a high value for the shoreline development index. The shallow mean depth
and high shoreline development index indicate that many reservoirs are very
productive unless turbidity limits light for photosynthetic production.
STRATIFICATION
The factors influencing density of water that were discussed in Chap-
ter 2 and the heating effects of light discussed in Chapter 3 have profound
effects on mixing in lakes. These effects influence the biogeochemistry, bi-
ology, and physical geology of lakes. A primary factor creating stratifica-
tion of lakes is the difference in density resulting from temperature or salin-
ity variation. The classical understanding of lake stratification is based on
consideration of cold-temperate lakes, so this seasonal sequence of stratifi-
cation is considered first.
During the early spring in a cold-temperate lake, the water is isother-
mal, or approximately the same temperature from top to bottom (Fig.
6.10). An isothermal lake can be completely mixed by wind, leading to
spring mixing. The entire lake will continue to mix as long as the wind
continues to blow. As the spring season progresses, the surface of the wa-
ter is warmed by solar energy. The surface waters of the lake heat the most
because the infrared radiation (heat) is absorbed quickly with depth. If you
have ever swum in cold water on a calm, sunny spring day, you are famil-
iar with the phenomenon of the top several centimeters of the water being
much warmer than the deeper water. Such stratification is only temporary
because the wind can mix a shallow layer of warm water into the lake.
When a series of calm, warm days occurs, the lake stratifies. The sur-
face waters of the lake heat enough so that the wind cannot completely
mix the warmer, less dense water into the cooler water below. The top of
the stratified lake is called the epilimnion . The zone of rapid temperature
transition is the metalimnion or thermocline . The bottom of the lake at
fairly constant temperature is called the hypolimnion (Fig. 6.11). The strat-
ified layers will stay distinct until a prolonged period of cool weather oc-
curs. The period with distinct layers is called summer stratification . Pro-
longed summer stratification is a combined function of the very slow rate
of diffusion of heat across the metalimnion and the continued heating of
the epilimnion. Because there is minimal mixing across the metalimnion,
no eddy diffusion of heat occurs; only molecular diffusion occurs. The slow
rates of molecular diffusion were discussed in Chapter 3.