596 



Yearboo^ of Agriculture 1949 



may do no harm ; the remaining capac- 

 ity may be enough to take in all the 

 water that is applied. 



As a variation of this principle, sur- 

 face runoff may sometimes occur after 

 the soil column has been saturated 

 down to some less permeable layer, if 

 the rate of water application exceeds 

 the rate at which this denser layer can 

 transmit it. Then, of course, infiltration 

 is limited by this layer rather than by 

 the soil above it. In such cases, the 

 presence or absence of forest cover may 

 have little effect on the magnitude of 

 floods, as the peak flows may come from 

 water that moves off the watershed 

 rapidly by surface and subsurface flow, 

 relatively unaffected at that stage by 

 the infiltration and storage capacity 

 of the forest soil. 



Even under such circumstances, 

 however, forest vegetation ordinarily 

 does a great service in preventing soil 

 erosion and sediment movement 

 through the stabilizing action of its 

 litter, humus, and roots. Those who 

 know the high, cold spruce forests of 

 the Rockies and elsewhere will recall 

 the wet, boggy mountainsides toward 

 the end of the snow-melt season. Water 

 runs freely and rapidly down the steep 

 slopes, but causes no erosion at all 

 because of the thick, spongy layer of 

 organic material on the ground. As 

 one man remarked, "That isn't surface 

 runoff" you're just standing up to your 

 ankles in ground water." 



These discussions on the behavior of 

 water in the forest can be interpreted 

 in terms of conditions that are com- 

 monly encountered. Where growing 

 conditions for the forest are poor; 

 where precipitation rates are high and 

 the soil is unstable; where the effects 

 of timber cutting are aggravated by 

 fire, destructive logging methods, or 

 overgrazing ; or under any combination 

 of such conditions, the removal of for- 

 est cover may be followed by severe re- 

 ductions in organic matter and in the 

 ability of the soil to take in and trans- 

 mit water. The consequence is usually 

 an accelerating cycle of flash floods 

 and erosion. Surface runoff starts soil 



movement; the eroded particles help 

 clog soil pores, which in turn increases 

 runoff; the increased runoff intensifies 

 the soil movement again, and so on. 

 Finally runoff and erosion become 

 stabilized at a high rate, cutting away 

 the soil and carrying large volumes of 

 sediment to the rivers. The result is 

 lasting injury to the watershed, low- 

 ered production of timber and other 

 vegetation, and probably disastrous 

 damage to cities, farms, reservoirs, and 

 other installations. 



Under gentle climatic conditions, 

 however, it is possible to harvest timber 

 by sound silvicultural methods so as to 

 keep the forest producing wood and 

 other products, and at the same time 

 benefit the water supplies. In some 

 cases conservative management is ad- 

 visable, where too severe cutting might 

 start the exposure of relatively un- 

 stable soil. There the forester plans the 

 removal of individual trees or small 

 groups of trees in light or moderate 

 selection cuttings. On watersheds that 

 are inherently more stable, it is possible 

 to apply the heavier cutting methods. 

 Where good silviculture prescribes it, 

 even the removal of all merchantable 

 trees is sometimes a sound practice, as 

 in the overmature, even-aged stands of 

 lodgepole pine at high altitudes and 

 on stable soil. 



Where watershed conditions are un- 

 usually safe, even forest fire has failed 

 in some places to cause any substantial 

 or permanent damage. Where the soil 

 is stable and the climate cool, with slow 

 rates of snow melt and gentle summer 

 rains, no appreciable excesses of water 

 have resulted and therefore the cycle 

 of erosion and floods has not occurred. 

 In the Colorado Rockies, for instance, 

 many old burned-over areas provide 

 satisfactory and stable conditions for 

 water production and there is little 

 evidence of damaging past erosion. Ex- 

 ceptions to this rule are found where 

 repeated fires, overgrazing, or peren- 

 nial wood cutting and hauling have 

 perpetuated and intensified any de- 

 terioration caused by the first fire. 



Thus far we have discussed what 



