Hydropower projects are constructed to pass water through turbines to produce two types of
power: baseload power and peak power. Baseload power is firm power generated to supply a portion
of a constant daily demand for electricity. Peaking power is supplied above the baseload to satisfy
variable demands during periods of heavy electricity usage. The power output of a project is
determined by the flow through the turbine and the head or pressures exerted on the turbine.
Therefore, it is advantageous to have hydropower reservoirs at maximum storage or full storage for
maximum hydropower generation. This is best accomplished by pumped-storage reservoirs which
maintain a full pool by pumping back into the reservoir following generation. Most hydropower
projects are not pumped-storage and are interconnected to a power grid so coordinating generation
releases for other uses is possible.
Water supply reservoirs are used to store water during periods of excess inflow for use during
other periods. Withdrawal may take place directly from the reservoir, or in downstream reservoir
releases. Water is generally provided to municipal, industrial, or agricultural users as reservoir storage
rather than a specific volume of water. Consequently, water supply can be obtained by a user from the
reservoir as long as there is sufficient water allocated in storage.
Reservoir projects operated for navigation purposes are directed at providing sufficient
downstream flow to maintain adequate water depth for navigation and/or providing sufficient water
volume for lockages. In many navigation projects, the reservoir pool is part of the channel, so pool
levels must be controlled to provide sufficient navigation depths within the pool and downstream area.
Downstream releases for navigation purposes may have a distinct seasonal pattern, with higher releases
required during the dry season.
Recreation activities in and around reservoir projects include camping, fishing, boating and other
water craft related activities, picnicking, swimming and hunting. These activities are related to the
quality of the fish and wildlife habitat, water control plan, and terrestrial management. Reservoirs used
for fish and wildlife conservation and enhancement may include features such as intake structures to
minimize entrapment and entrainment of fish and other aquatic species; outlet and emergency spillway
structures to minimize contact of aquatic species with waters supersaturated with dissolved gases and to
provide appropriate release water quality. Other structures and release strategies include fish ladders
and by-pass systems, maintaining vegetation, and providing for a minimal flow (low-flow augmentation).
Low-flow augmentation reservoirs provide releases that increase flow in the downstream channel for
downstream fish and wildlife purposes or for downstream water quality control.
Reservoir operations may be grouped by the depth (or layers) from which the released water
originates. Typically, these groups are surface, bottom, or mixed releases and release from more than
one layer is common. The mechanisms for release from these layers vary widely but a few typical
structures are provided for reference (Figures 1.2.12 through 1.2.15).


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