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 1.2.7 RESERVOIR ZONATION
Water moves laterally in lakes as a result of inflows and outflows. Reservoirs may be functionally
divided into three regions in which these flows have different characteristics. Headwaters are often
dominated by processes similar to and as a result of the riverine inputs to the region. If inflows have a
density greater than lake surface waters, the inflows will tend to plunge beneath the lake surface. Often
a trash line of floating debris will indicate such a plunge point. If the inflow water is less dense, it will
flow over the lake surface. If inflows are greater density than the lake surface but less dense than lake
bottom waters, they may form an interflow which extends some distance into the lake or perhaps
throughout the lake. Such interflows are common where plunging inflows attain depths similar to the
penstock opening depth on the dam impounding the lake. In all instances, substantial inflows can greatly
influence thermal structure in the lake.
The region of a reservoir where the inflows dominate the lake characteristics is considered the
riverine zone of a reservoir. In this region, the types of processes occurring are more like a river than
a lake.
In the deeper regions reservoirs are often dominated by processes typical of open-water
(limnetic) environments. The region in which the lake gradually changes from riverine to limnetic
dominance is aptly termed the transition zone . Care must be taken when identifying these zones in
lakes because lake levels, flows, seasonal thermal and chemical processes can vary in ways to cause
the positions of these zones to change. This is especially true of the transition zone.
In the deepest region downstream from the transition zone and where strictly limnetic processes
dominate is the lacustrine zone . This zone extends to the dam in reservoirs. The three zones often
change size or position depending on the trends of rainfall and resulting flows. For example, high flows
from occasional storms may extend the transition zone additional kilometers downstream in West Point
Lake. And in Richard B. Russell Lake, reversed flow with pumped storage gives the forebay lacustrine
zone properties similar to the transition zone. Increased flows during the winter can greatly diminish the
lacustrine zone and greatly enlarge the riverine and transition zones.
Inflows are important sources of materials to lakes. Materials input from external sources are
termed, allochthonous materials. Some materials can have their source internally (oxygen and carbon,
for example) and when they have an internal source they are termed, autochthonous materials. These
terms apply to all materials whether they function as sources of turbidity, nutrients for plants, or food for
fish and other animals. You will be tested on your ability to spell these two terms.
1.2.7.1 Chemical Processes
A budget accounting for the inputs and outputs of materials can be constructed in a manner
similar to the energy budget or the household budget. In some ways chemical budgets are actually
1.2-18
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