SOURCES OF WATER 3 



ward from a line drawn through the eastern part of the Dakotas, 

 middle Nebraska, western Kansas and central Texas the rainfall 

 decreases to less than 20 inches yearly, all of the Great Plains 

 region being characterized by small rainfall. In the Black Hills, 

 the Bighorn Mountains and the higher sections of the main chains 

 of the Rocky Mountains the rainfall is 20 or 30 inches yearly; 

 and in the high Sierra, the Cascades and the Coast Ranges it is 

 70 inches or more, reaching a maximum of 150 inches in the Coast 

 Ranges of Oregon. In the Great Basin region, between the 

 Sierra Nevada and the Wasatch Mountains, the rainfall is less 

 than in any other section of the country, in places being as low as 

 2 or 3 inches year. 



Run-off. Only a small part of the precipitation on most 

 areas is disposed of directly by run-off, by far the greater part 

 of the flow of the surface streams being supplied by waters that 

 have first been absorbed by the ground, rather than by waters 

 shed directly from the surrounding slopes. In arid regions, where 

 the surface deposits are porous, the run-off is relatively small, but, 

 owing to the fact that rain in these regions falls chiefly in sudden 

 downpours, the annual run-off is not so small as would be indi- 

 cated by the small annual precipitation. In humid regions and 

 in places where the surface is composed of impervious materials 

 the run-off is greater. Frozen, snow-covered and ice-covered 

 ground yield especially large flood flows. Over frozen areas 

 nearly all the rain water may at once join the streams, whereas in 

 some sandy regions practically all the precipitation is absorbed by 

 the soil. In the eastern half of the country the run-off will prob- 

 ably not average more than 20 per cent of the rainfall. In the 

 West, although the percentage of run-off in small areas is at 

 times great, it is on the whole less than in the East, for much of 

 the water that is not taken up directly is later absorbed from the 

 streams by the dry, sandy soils. 



Evaporation. Owing to the great humidity of the atmosphere 

 during storms, evaporation while rain or snow is falling is small. 

 Snow may remain on the ground a long time, and, as a rule, is 



