Timber Cutting and Water Yields 



597 



happens to water after it falls on the 

 forest and what may be expected when 

 the timber is removed. Ample experi- 

 mental evidence has been obtained and 

 published on the bad effects of unwise 

 timber removal and destructive log- 

 ging, especially in the areas that are 

 sensitive to soil deterioration and to 

 erosion. But what about less delicately 

 balanced areas? Is there any evidence 

 to show that water supplies can be 

 increased by timber harvesting on such 

 lands without damaging the watershed 

 or causing serious erosion and flood 

 production? 



There is evidence on this aspect of 

 watershed problems, though less than 

 on erosion and flood damages caused 

 by excessive timber use or unskillful 

 watershed management. Several inves- 

 tigations have been conducted in the 

 United States for the special purpose 

 of showing how the forests influence 

 stream flow and water yields and how 

 this influence is changed by timber 

 removal. 



The first major study was started 

 by the Forest Service and the United 

 States Weather Bureau on the famous 

 Wagon Wheel Gap watersheds in 

 southern Colorado at about the time 

 Colonel Chittenden published his 

 report. Near the headwaters of the 

 Rio Grande, two small watersheds 

 were controlled by means of rain 

 gages, stream-gaging stations, sediment 

 basins, and other scientific equipment. 

 After they had been studied for 8 years 

 in their original condition covered 

 largely with a forest of conifers and 

 aspen all of the woody vegetation 

 was removed from one of the areas. 

 During the following 7 years, total 

 water yields increased about 1 5 percent 

 under this treatment, and yields during 

 the snow-melt period rose about 22 

 percent. Even the summer and autumn 

 stream flow was built up to some ex- 

 tent. Melting started a little earlier in 

 the spring, but not enough to cause any 

 important change in flood peaks or in 

 the amount of water available during 

 the irrigation season. 



The increased yields seemed to be 



due to decreased losses from evapora- 

 tion and transpiration rather than to 

 reduced soil porosity and storage, as 

 the augmented late-season stream flow 

 indicated an ample supply of water to 

 the ground-water table. Also, no ap- 

 preciable erosion was caused by the 

 complete removal of the forest cover. 



Little sediment was caught, and 

 practically all of it seemed to have 

 come from the minor logging roads 

 that had been built into the watershed. 



Judging from those findings, timber 

 removal obviously had a gentle effect 

 on both water yields and erosion. There 

 are several good reasons. 



First, the climate of this area is mild 

 from the hydrologist's viewpoint cool, 

 with long winters and slow melting 

 rates of snow and with relatively low 

 rates of summer rainfall. In those re- 

 spects it resembles vast areas of forested 

 watershed along the top of the Rockies, 

 including 10 million acres or more from 

 the Rio Grande to Montana. 



Second, the forest cover was rather 

 thin before treatment, so that even the 

 removal of all the woody vegetation 

 did not cause so drastic a change as 

 might be expected. Because a consid- 

 erable part of the forest was aspen, too, 

 winter interception losses must have 

 been: low, and the aspen grew up rap- 

 idly after treatment so that the effects 

 of denudation were relatively short- 

 lived. 



Finally, the soil on those areas was 

 fairly porous and apparently did not 

 deteriorate badly. 



Thus, the quantitative results of the 

 investigation can be applied only in a 

 limited way to other areas. They do 

 indicate the general effects of timber 

 removal in a region like the backbone 

 of the Rockies and similar mountain 

 ranges that timber removal may not 

 cause damage and may even benefit 

 water yields. 



Thorough as it was, too, that experi- 

 ment did not really show the influence 

 of silviculture on water supplies, be- 

 cause every stick of woody vegetation 

 was removed from the treated area. 

 While clear cutting is a perfectly sound 



