The impacts of the proposed change on the release water quality must be evaluated. If a
significant change in the guide curve is proposed, impacts to the release water quality may be possible.
For example, increased residence time to allow settling of suspended solids may enhance thermal
stratification and subsequent depletion of oxygen in the hypolimnion. Withdrawing water from the
lower elevations of the reservoir could result in poor downstream water quality and possibly offset any
benefits achieved by the guide curve change. The delay in reservoir filling may also affect other areas.
These include recreation and access to fishing and swimming areas, as well as docks and marinas;
possible erosion of the exposed lake bottom due to wave action and the lack of a stabilizing plant
cover; the aesthetics of the exposed shoreline; management of aquatic plants; impacts on fish and
wildlife habitat; and effects on access to wetlands and spawning areas.
Applications of guide curve changes or modifications to guide curves have not been widely
reported in the literature. However, the control of hydraulic residence time has been reported from
laboratory studies to be a feasible technique in certain instances to enhance reservoir water quality
(Schiebe and Dendy 1978). In addition, this technique is a recognized method to enhance water quality
at Corps of Engineers' (CE) projects (HQUSACE 1987b).
A general evaluation technique for water quality forecasting for short and long-term impacts of
a guide curve modification may be found in "Management of Water Control Systems" (HQUSACE
1987a). This technique relies on the expertise of the water control manager in deciding on which
approach to take with a particular guide curve modification. The use of computer models to simulate
short- and long-term impacts is recommended.
A number of techniques that use numerical models have been developed to evaluate guide
curves for water quantity or volume perspectives (Newman and Loucks 1975). Recent investigations
have added water quality objectives to the development of guide curves (Hogan 1986). This technique
has combined numerical optimization methods with a numerical model that simulates the physical
processes in the reservoir to optimize the real-time operation of a reservoir.
This technique has been used to improve water quality during the initial filling sequence of new
reservoirs. Many new reservoirs, especially large ones, are filled in stages over a number of years to
minimize the inundation of new sediment and the oxygen demand of this material. Van Pagee et al.
(1982) identified a delayed filling period as one alternative to minimize the impacts of filling of a new
reservoir in the tropics region in Suriname. A similar technique is discussed in Engineer Manual 1110-
2-1201 (HQUSACE 1987b).