90 MISC. PUBLICATION 9 0, IT. S. DEPT. OF AGRICULTURE 



Where the run-off spreads over a longer period, as is usually the 

 case where there is adequate plant cover, the flood head also would 

 be extended over a longer time, and its destructive power would 

 be less. 



DAMAGES CAUSED BY LACK OF WATERSHED PROTECTION 



With serious depletion of vegetative protection, streams from 

 watersheds become turbid, more of the water runs off on the surface, 

 and stream flow becomes less regular with unusually high water in 

 the spring and a low flow later in the season, thus making the water 

 less usable for irrigation, water power, and other purposes. Under 

 such conditions floods are more likely to occur during heavy summer 

 rainstorms. These floods often cause heavy financial loss by wash- 

 ing out roads, railroads, ditches, and other improvements, by deposit- 

 ing debris on farm lands, and by flooding towns and farm build- 

 ings. Erosion removes the fertile soil and reduces the carrying 

 capacity of livestock ranges. Sediment is carried down into the reser- 

 voirs, and thus their storage capacity and their value are reduced. 

 The whole economic life of the people living below the watersheds 

 may be affected, or their continued occupation of the land threatened. 

 Sometimes even human lives are lost in the floods. 



It can not be claimed that a normal plant cover will prevent all 

 floods, nor that the destruction of the vegetation is the cause of all 

 floods; for if the precipitation is heavy enough and lasts for a 

 sufficient period, or if it falls on bare rock slopes, heavy run-off is 

 bound to occur regardless of plant cover. However, good vegeta- 

 tion will check the greater floods and prevent many of the smaller 

 ones entirely. 



Even on the foothills and other watersheds under 7,000 feet in 

 elevation a vegetative cover is necessary. These areas are subject 

 to heavy summer rainstorms, often called cloudbursts, and the soil 

 on them is easily eroded. All areas, in fact, with the exception, 

 perhaps, of those which are so nearly level that the water can not 

 attain great velocity, require the maintenance of plant cover as a 

 measure of protection against excessive run-off and erosion. 



CAUSES OF PLANT DEPLETION 



The primary causes ox depletion of the cover on watersheds are 

 overgrazing (fig. 46, A and B), fire, and destructive logging. Conse- 

 quently, watershed protection centers around the prevention of these 

 things. All of them, except fires started by lightning, are attributable 

 to man and are preventable. Not only do they cause injury to the 

 watershed, but they destroy the timber and forage resources, the 

 renewal of which is made difficult because of the depletion of the soil. 

 It has been found, however, that watershed protection does not re- 

 quire retention of all of the timber and forage. A careful system 

 of logging removes the older, mature trees and leaves the young 

 growth on the ground. This is adequate to protect the watershed. 

 Under average range conditions, where the cover has not already been 

 badly depleted and the soil much exposed, the grazing practice that 

 is most profitable to the stockman and that will maintain the maxi- 

 mum carrying capacity of the range will also maintain the plant 

 cover in a condition for satisfactory protection of the watershed. 





