low dam, limited storage, minimal retention time, low hydraulic head and no positive control over water
storage. Discharge is a function of inflow. Mainstem dams are projects located on a major waterway
as opposed to a tributary to the river and include run-of-the-river dams. Mainstem dams are
characterized by a retention time of about 25 days and a maximum depth between 50 and 100 feet
(USEPA 1993). These dams (with the exception of low dams) have water control capabilities and can
be utilized for most of the project purposes listed above. Transitional dams display an even greater
retention time (up to about 200 days) and have a maximum depth between 100 and 200 feet. The
increased retention time and greater depth result in the potential for management opportunities for water
control and increased development of water quality patterns. Storage dams are typically more than
100 feet deep and have a retention time greater than 200 days. Maximum hydraulic head at storage
dams provides for hydropower generation and storage capabilities provide for maximum flood control.
In some areas, dry dams (a dam with no permanent reservoir) are used for flood control. Lock and
dam combinations are used primarily on large rivers for navigation and a minimal reservoir or pool is
maintained.
While dams and reservoirs are distributed fairly evenly throughout the continental United States,
large projects are primarily the responsibility of the U.S. Army Corps of Engineers, the Bureau of
Reclamation, and the Tennessee Valley Authority. Each of these agencies have primary responsibilities
in various drainage basins. For example, Walker (1981) points out that the CE projects are distributed
mostly between 30 and 45o latitude which is lower in latitude than most natural lakes and the area of
glaciation (Figure 1.2.10). The distribution of these projects by type (e.g., reservoir, lock, dry dam) is
depicted in Figure 1.2.11.
1.2.13 PROJECT PURPOSES
Most reservoirs are authorized as multiple-purpose projects with storage allocated for two or
more purposes. Whether operated separately or as a system (several reservoirs in a river system),
demands of the multiple purposes often result in conflicting uses for the reservoir storage. Changes to
the water control plan have to be investigated if considerable deviations are required to balance the
conflict. The addition of water quality enhancement as a project purpose sometimes contributes to the
conflict but can also be used to support a compromise if water quality is enhanced.
Flood control consists of storing excess water (that volume that is above the downstream channel
capacity) during flood periods for later release. These later releases may be made during periods of
flow below the downstream channel capacity. Since a major factor in flood control reservoirs is
maintaining available volume or empty storage space for storage of flood waters, the flood control
purpose is generally the least compatible with other project purposes.
1.2-32
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