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34 Chapter 2 Water Sources: Surface Water

Figure 2.6 Hoover Dam, Clark County, NV
(Courtesy of the National Resources Conservation Service and USDA)

4. Harnessing water power
5. Low-water regulation for navigation
6. Preservation and cultivation of useful aquatic life
7. Recreation, for example, fishing, boating, and bathing
8. Control of destructive floods
The greatest net benefit may accrue from a judicious combination of reservoir func-
tions in multipurpose developments. The choice of single-purpose storage systems should
indeed be justified fully.
Storage is provided when stream flow is inadequate or rendered unsatisfactory by
heavy pollution. Release of stored waters then swells flows and dilutes pollution. Storage
itself also affects the quality of the waters impounded. Both desirable and undesirable
changes may take place. Their identification is the responsibility of limnology, the science
of lakes or, more broadly, of inland waters.
If they must receive wastewaters, stream flows should be adjusted to the pollution load
imposed on them. Low-water regulation, as such, is made possible by headwater or up-
stream storage, but lowland reservoirs, too, may aid dilution and play an active part in the
natural purification of river systems. Whether overall results are helpful depends on the
volume and nature of wastewater flows and the chosen regimen of the stream.

2.2 SAFE YIELD OF STREAMS
In the absence of storage, the safe yield of a river system is its lowest dry-weather flow;
with full development of storage, the safe yield approaches the mean annual flow. The eco-
nomical yield generally lies somewhere in between. The attainable yield is modified by (a)
evaporation, (b) bank storage, (c) seepage out of the catchment area, and (d) silting.
Storage-yield relations are illustrated in this chapter by calculations of storage to be pro-
vided in impounding reservoirs for water supply. However, the principles demonstrated are
also applicable to other purposes and uses of storage.