Many reservoir projects operate under some type of water control plan that relies on seasonal
change of reservoir elevations. This sequence of reservoir elevation change is usually termed a guide
(or rule) curve. Depending on the project authorization, the guide curve may permit rather large pool-
level fluctuations on an annual basis. For example, operation of a flood control project usually involves
drawing down the reservoir level during the fall and winter and filling during the spring and early summer
to a stable pool through the summer and early fall.
In some cases where the summer pool elevation is maintained at a relatively stable elevation,
water quality may be a concern. Water quality concerns in the reservoir which result from poor quality
inflows may be averted by pooling up later in the year. In another approach, the retention time of the
reservoir may be modified by adjusting the guide curve. Constituents of water quality that may be
affected by this technique include inflow with undesirable qualities, nutrient loading of the reservoir (and,
in turn, the effects on algal growth and fisheries), turbidity, and sedimentation.
For example, inflows with a high sediment load may be delayed in the upper reaches of a
reservoir and allowed to settle out before reaching the outlet works. Therefore, some modification of
the guide curve to reduce or increase the residence time may enhance the water quality of the reservoir.
Modification of the guide curve is a technique that relies on changing the hydraulic residence
time of inflows in the pool to control poor quality inflows. This can be accomplished by maintaining a
small pool through most of the flood control season and allowing undesirable inflows entering the reser-
voir to be flushed through. The reservoir is then allowed to fill to summer pool elevation later in the
season, when the inflow water quality is typically better. If inflow quality is governed more by discharge
than season, maintaining a larger pool with a greater retention time may allow suspended material to
settle in the headwaters, and thereby improve the quality of the main portion of the reservoir.
Another variation of this technique relies on changing the minimum pool elevation of the
reservoir and allowing additional storage for water quality maintenance or conservation of a quality
resource, such as cool water for downstream release. Adjusting the guide curve may also have a
significant benefit to the wildlife of the reservoir by creating additional habitat at critical times or
controlling aquatic plant growth. An example of a modification of a guide curve is shown in Figure


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