160 



STORAGE RESERVOIRS 



Limitations to Storage. There is, however, a limit to storage 

 and in no case can sufficient impounding be maintained to give 

 to any stream the power representing anything like its maximum 

 flow. The excess run-off from any watershed varies greatly from 

 year to year, and it is generally considered to be the best prac- 

 tice to base the reservoir capacity on the run-off for the minimum 

 year, as impounding the water in years of heavy run-off for holding 

 over in storage to dry seasons is generally considered uneconomical, 

 among other things on account of the loss due to evaporation. 

 In general, there are two factors determining the practicable 

 amount of storage. The first consideration is usually the topog- 

 raphy of the locality. In some localities a sufficiently high dam 

 may be built at a very reasonable cost, and it may provide storage 

 for an immense volume of water and thus greatly enhance the 

 minimum power of the stream. In other cases the conditions 

 may be entirely the reverse. A further practical consideration 



is the value of the 

 land. Even with 

 favorable topographic 

 conditions the cost of 

 acquiring lands to be 

 flooded may be so 

 great as to make any 

 great amount of stor- 

 age impracticable. 



Location of Reser- 

 voir. The relative 

 location of the pro- 

 posed reservoir site in 

 the drainage area 

 must, of course, also 

 be considered, and 

 likewise its location 

 with respect to the 

 point of distribution 



so that proper outlets and conduits can be provided at a reason- 

 able cost. 



Before accurate surveys are justified, it may become desirable 

 to approximately determine the quantity of water that a proposed 

 reservoir may hold. This is usually done by means of contour 



FIG. 83. Contour Map of Reservoir Site. 



