FOREST AXD RANGE RESOURCES OF UTAH 89 



be farmed if water for irrigating the land were available. Every 

 possible step should be taken to conserve and utilize efficiently what 

 water there is. 



THE VALUE OF DIFFERENT KINDS OF VEGETATION ON WATERSHEDS 



The denser the vegetation, the more of a physical obstruction to 

 the flow of water it forms. The more extensive its network of 

 roots, the more effective it should be in checking erosion and rapid 

 run-off. Dense coniferous forests afford great protection. The ex- 

 tensive root systems bind the soil and open it up to absorb water 

 more readily. The needles and other material that accumulate on 

 the ground serve as a blotter and hold back more of the water so 

 that it may soak into the soil more readily. The trunks of the trees, 

 down logs, and the larger roots form physical barriers against run- 

 off. More open stands of coniferous timber, deciduous trees, and 

 large shrubs act in a similar manner but to. a less marked degree. 



Grasses, weeds, and low shrubs, are also effective and are highly 

 valuable in checking erosion and regulating run-off. When water 

 falls or originates from snow in sufficient quantities to cause surface 

 run-off on a slope where there is a fairly dense stand of grasses, 

 weeds, or low shrubs, the run-off can only find a meandering course 

 down the slope. The individual plants serve to turn the small 

 streams of run-off first to one side and then to the other, checking 

 their velocity, and thus preventing the water from forming rills of 

 any size. Those formed do not erode deeply because the soil is held 

 by roots. Consequently the ability of the run-off to carry a load 

 of soil from the watershed is kept at a minimum. Even though 

 rains continue until the soil becomes saturated and the water runs 

 off the surface, it must run off more slowly because of the obstruc- 

 tion afforded by the vegetation. As a result a sheet of comparatively 

 clear water is discharged gradually from the hillside into the streams. 



Now contrast the results on a similar slope where the vegetation 

 has been more or less depleted or destroyed. The water in seeking 

 lower levels soon accumulates in small rills, which, in the absence of 

 any obstruction, run directly down the slopes. In these compara- 

 tively straight channels, the velocity of the water, and hence its 

 power to carry soil and other sediment, is greatly increased. It has 

 been found that if the velocity of a stream is doubled its carrying 

 power is increased 64 times. In other words, if the velocity of a 

 stream just able to carry along a rock that weighs a pound is 

 doubled, it will be able to carry a rock that weighs 64 pounds. The 

 soil, gravel, and rocks, torn loose and carried down by the water, 

 further increase the volume and the corrosive action. Small gullies 

 soon join to form larger ones, in which the volume of water is con- 

 centrated, and the carrying power increased still more, until even- 

 tually a veritable torrent, capable of moving large rocks, is being 

 poured into the streams which have probably already been swollen 

 from other tributaries. The result is a flood, usually of short dura- 

 tion, but with a high head while it lasts. The rapid discharge of 

 water from the highly efficient drainage systems that have been 

 formed on the denuded hillsides may cause the most destructive 

 phase of the flood, the rapid accumulation of water into a high head. 



