Multi-Pur pose Reservoirs 23 



tion of two or more uses, such as flood control, navigation, the generation 

 of electric power, irrigation, and recreation. Not all of these values are 

 assigned to one reservoir; usually irrigation is a western assignment, 

 navigation may be a localized or general assignment, and the generation 

 of power requires a dependable and constant source of water. 



Some of these uses appear to conflict with one another. For example, 

 flood conti-ol demands an empty reservoir to give maximum flood storage, 

 whereas navigation, irrigation, and the generation of power require a 

 full basin, since they depend upon the release of water from the reservoir. 

 Furthermore, although recreation may ride along with a changing water 

 level, it is gone when the reservoir basin is dry ( as in some flood control 

 projects ) . However, generally speaking, needs are seasonal, so there may 

 not be intensive competition for water at any one time. 



These apparent conflicts of purpose are resolved by assigning a range 

 of levels to specific uses. First, the basic purpose is taken care of by 

 setting a conservation-pool level or elevation near the bottom of the 

 reservoir. Until this "absolute minimum" water level has been exceeded, 

 no large amount of water will be released. In a reservoir of 24,000 to 

 30,000 acres, water at the conservation-pool level might create a lake of 

 3000 to 6000 acres. Then, other fractions of the reservoir's storage capacity 

 may be assigned to power, navigation, or irrigation. Usually the flood 

 control function belongs to the upper layer of reservoir capacity which is 

 drained off after each flood as rapidly as the river channel below the 

 dam will permit (bank full), so that the upper lake in the reservoir will 

 be available for storing water once again should another flood occur. 



In the operation of a multi-purpose reservoir, water in the river channel 

 below the dam is never allowed to exceed the top of the river banks ( as 

 long as any storage capacity exists in the reservoir), or to fall below a 

 certain minimum flow in drought periods, even if it means using some 

 water assigned to the conservation pool. This minimum flow is so small 

 in relation to the capacity of the conservation pool that there is little 

 danger of ever draining the latter. The controlled release of water into 

 the river channel below the dam insures a constant supply for agrarian 

 users and towns situated on the river and maintains the fish population 

 that inhabits the river below the dam. 



Experience has shown that fish congregate in waters below tlie outlets 

 of these large reservoirs through upstream migration, and extensive sport 

 fisheries have developed in these tailwaters. The type of fishery that 

 results is dependent upon ( 1 ) the temperature of the water released from 

 the dam during summer, ( 2 ) what fish are available in the stream below, 

 and ( 3 ) which ones may be stocked in it. The temperature of the water, 

 in turn, depends upon the vertical location of the outlet gates on the face 

 of the dam. 



