INTRODUCTION 



Earlier in this century, the deteriorated condition of numerous high, mountain 

 watersheds in the Western United States resulted in devastating mud-rock flows that 

 flooded valuable lowlands, claimed several lives, and caused considerable property 

 damage (Berwick 1962). These floods followed high-intensity summer rainstorms on the 

 badly denuded areas. Overgrazing and burning of the protective vegetation were con- 

 sidered to be the primary causes of this deterioration (Cannon 1931). 



To restore the watersheds, a rehabilitation program was undertaken in the early 

 1930 's (Copeland 1960). Contour trenching, one of numerous practices applied, was so 

 successful that it has become widely accepted (Bailey et al. 1947). By 1969, approxi- 

 mately 30,000 acres had been contour trenched in the States of Utah, Idaho, Nevada, 

 Montana, and Wyoming. Through the years, contour trenches have evolved from small, 

 handmade furrows, 1 or 2 feet deep, to large, bulldozed trenches, 3 or 4 feet deep. 



It has been contended that annual streamflow is reduced by trenching. This conten- 

 tion is supported by studies of contour terracing and water-spreading techniques on 

 agricultural land (Branson et al. 1966; Mickelson 1968; Zingg and Hauser 1959). 



However, little research has been conducted to determine what effects trenching 

 has on streamflow from high, mountain watersheds. Bailey and Copeland (1960) compared 

 streamflow records from a trenched and an untrenched watershed in Utah. The trenches, 

 which were dug in 1935, were spaced about 25 feet apart. Each had a capacity of 1.5 

 area inches of water. A gradual decrease of 2.7 inches (23 percent) in average annual 

 streamflow from the trenched watershed developed over a 22-year period. Most of this 

 decrease occurred during the high-flow months, March, April, and May. This decrease in 

 annual flow apparently was due to revegetation, resulting from the stabilizing effect 

 of trenches and the prohibition of grazing by domestic livestock. 



Contour trenches in the Western United States are designed to regulate the peak 

 streamflow from the high-intensity summer rainstorm by intercepting overland flow and 

 allowing it to infiltrate into the soil mantle. Total streamflow from these storms 

 represents less than 1 percent of the total annual yields. Therefore, the effect of 

 contour trenching on annual yields would be minimal even if all runoff from these 

 storms were trapped on the mountainside and lost to evapotranspiration . However, con- 

 tour trenches may have influences that extend beyond control of summer torrents. The 

 effects of trenching on snow catch and areal distribution, on snowmelt and runoff, on 

 soil moisture and vegetation, and on runoff from long- lasting , low-intensity rains are 

 integrated and reflected in annual water yields, in spring snowmelt runoff, and in base 

 streamflow. 



Comparisons of the effects of contour furrowing, pitting, and ripping on rangelands 

 from Montana to New Mexico were made by Branson et al. (1966). These treatments added 

 to soil moisture and forage production by increasing infiltration and delaying runoff. 

 These rangelands have a low annual precipitation, most of which occurs during summer 

 rainstorms . 



The effect that trenches might have on snowpack accumulation was suggested by 

 Martinelli (1965), who showed that natural barriers contribute significantly to snow 

 accumulation in the alpine zone. I followed this up with two winters' measurements of 

 snow accumulation, distribution, and water content in the contour-trenched area of 

 Halfway Creek (Doty 1970). The effect of trenches on wind movement of snow was to 

 increase snow accumulation slightly, which probably affected revegetation more than 

 water yields. 



