FOKEST AND EANGE EESOUBCES OF UTAH 87 



streams and lakes. Ground water moves very slowly, especially 

 when it reaches the nearly horizontal bed rock or other impervious 

 material, and it may accumulate in varying depths in the overlying 

 gravel and other incoherent material. This is the supply that fur- 

 nishes the water in wells. The slow movement also accounts for the 

 regular flow of springs, since the water may not reach such outlets 

 for man} 7 " months after it first enters the ground on the watershed, 

 and the water table may hold a large amount of water and give it 

 forth very slowly. 



Observations made on high-mountain watersheds in central Utah 

 show that during the summer, when water loss by evaporation is 

 high and growing plants are using a great deal of water, the precipi- 

 tation is insufficient to contribute to the ground water supply and 

 underground run-off. During the heavier rainstorms in summer 

 a part of the water runs off on the surface of the ground and finds 

 its way directly into the streams. This is the only portion of the 

 summer rainfall contributing to the usable water supply. That 

 which goes into the soil penetrates only a relatively few inches, and 

 between rainstorms it is exhausted by evaporation and through use 

 by growing plants. It is only during the period of melting snow 

 in the spring when water becomes available very rapidly and when 

 the vegetation is not yet growing that the underground water sup- 

 ply is replenished and the streams receive most of the surface run-off. 



At the lower elevations, where the snowfall is comparatively light, 

 melting usually takes place so slowly and intermittently that the soil 

 can hold all of the water until it is returned to the air by evaporation 

 and transpiration. At the lower elevations sudden melting of snow in 

 the spring or heavy rains in the summer may contribute some sur- 

 face run-off directly to the streams. This constitutes virtually the 

 whole contribution of the lower watershed to the usable water supply. 

 The 7,000- foot elevation or contour in Utah is approximately the 

 dividing line between the areas that contribute very little or no water 

 except occasional surface run-off and the high mountain watersheds 

 with heavier precipitation and winter-snow storage. 



CITY AND TOWN WATER SUPPLY 



Much of the town and smaller city water supply is taken directly 

 from springs. Logan, a small city, has one of the larger water sup- 

 plies taken from springs. Some of the smaller towns obtain their 

 water by pumping from wells. Ogden is a notable example of a fair- 

 sized city that obtains its water supply from artesian wells. Wells 

 and springs afford the best and purest, although usually not the most 

 abundant, water supplies. Cities as large as Salt Lake City take 

 their water supply directly from the open streams or from reservoirs 

 into which the streams are diverted. The watersheds of such streams 

 require protection against pollution and erosion. 



WATER FOR IRRIGATION 



By far the greatest use of water in Utah is for irrigation. (Fig. 

 45.) It has already been pointed out that most of the farm land in 

 Utah is irrigated. The Utah Agricultural College estimates that the 

 mean annual run-off of streams of L T tah is about 9,000,000 acre-feet. 



