DRAINING A VALLEY. 



229 



than one dam ; some subordinate ones may be required 

 to prevent overflow at lateral points. This will be dis- 

 covered when the contour lines of the level of the valley 

 are run, as they should be for every three or six feet of 



Fig. 110. A VALLEY RESERVOIR. 



elevation. From these contour lines the capacity of the 

 reservoir may be calculated for each level. In the case 

 here illustrated, it will be observed that two valleys run 

 together, and meet where two spurs of high land approach 

 near each other. This combination of favoring circum- 

 stances frequently occurs in mountain regions, or among 

 the lateral spurs and foot hills of higher ranges. In the 

 mountain ranges and hills which lie within our territory 



