spring diatoms to be replaced by greens and cyanobacteria. This was presumably due to competitive
interactions for phosphorus and silica (which is needed by diatoms). The idea was formalized by Claire
Schelske and Gene Stoermer as a hypothesis to explain the succession of plankton in the Great Lakes
and a response to eutrophication in Lake Michigan.
We now know that under stratified conditions, vertical zonation of processes and biota occurs in
reservoirs. The photic zone is inhabited by planktonic organisms and fish. Sometimes a layer of
organisms accumulates near the thermocline and this is called a metalimnetic layer. Water density and
viscosity changes interact with the organism densities and shapes to help form these layers. The layers
can be sites of production as well as mineralization of organics.
Longitudinally, production in a large reservoir tends to peak around the region of the transition
zone, diminishing toward the headwaters as they become more riverine and toward the lacustrine
waters of the forebay as they become more depleted of nutrients as well as suspended materials.
However additional water movements such as upwellings in the lacustrine zone can reverse this trend
and cause increased production.
Reservoirs are the product of a manmade structure which was constructed for specific purposes
resulting in the control of the hydrology of the river. These purposes include flood control, hydropower
generation, water supply, navigation, low-flow augmentation, recreation, and fish and wildlife habitat.
Each of these purposes results in a different type of reservoir operation for storage and discharge and
often must be balanced based on water load and water demand. While general operating guidelines
have been developed for reservoir operations, interest in optimizing water control for water quality
management has recently increased. The objective of this section is to provide a general overview of
reservoir distribution, purposes, and operations and to provide a background for enhancement
techniques presented later.
1.2.12 DISTRIBUTION AND TYPOLOGY
A dam can be defined as constructed impoundments that are either (1) 25 feet or more in height
and greater than 15 acre-feet in capacity, or (2) 6 feet or more in height and greater than 50 acre-feet
in capacity (USEPA 1993). Using this definition, there are more than 7,700 dams in the United States.
These dams vary in construction and complexity from earthen dams with simple outlet works to large
concrete dams with elaborate outlet works. The impounded water upstream from the dam is referred
to as a reservoir (pond, lake, and pool are also terms commonly used).
Dams and reservoirs are located to provide water control based on need(s) resulting in a wide
distribution but often with logical groupings. These groupings typically include the following types of
projects: run-of-the- river, main stem, transitional, or storage reservoir. A run-of-the-river dam has a