8 BULLETIN 675, I*. S. DEPARTMENT OF AGRICULTURE. 



the drainage of water from these high basins that, in rushing through 

 to the steep canyon-, relatively little of the rain is absorbed by the 

 soil: most of it plunges into the valleys below. Obviously, therefore, 

 if the run-off from such areas is to be properly controlled, steps must 

 be taken to maintain the soil cover. Unfortunately, it is these very 

 basins that have been damaged most from overgrazing, consequently 

 the soil and the ground cover are in the worse condition. Originally 

 they supported a superb lot of feed especially suited to sheep, and 

 being less broken in topography, less brushy, and more easily acces- 

 sible than other parts of the range, generally they were the areas most 

 sought for. 



As is shown by the accompanying topographic and type map of 

 the selected areas, A and B (fig. 1), seven rather distinct plant 

 associations occur, viz : Yarrow-needlegrass-cinquef oil ; yarrow- 

 Douglas knotweed ; adder's tongue-larkspur-sweet sage ; currant- 

 gooseberry-yarrow ; bluegrass-wheatgrass-needlegrass (semi-scab- 

 land) ; giant larkspur-blue foxglove-Douglas knotweed; and yellow 

 bush-sweet sage-pea vine. The density of each of these associations is 

 shown on the map. Most of the species are valuable as forage and as 

 soil binders. 



The soil is of limestone and sandstone origin, though chiefly the 

 former, and varies in depth from a few inches to several feet. While 

 there is some outcrop on both areas, the soil for the most part is 

 fairly well decomposed. The principal drainage channels vary from 

 2 to 9 feet deep. In many places wherever a vegetative cover is 

 lacking rills occur, though most of these are less than a foot in depth. 



Although the two areas, as stated, are as nearly comparable as 

 could be selected locally, yet several dissimilarities as to soil, slope, 

 drainage, and vegetative cover occur, which occasion a much greater 

 run-off from area A than from area B, other factors being equal. 

 First, as shown in figure 1, area B has a vegetative cover exceeding 

 that on area A by a density of 20 per cent. Accordingly, the soil on 

 area B is bound together much more firmly by the plant roots than 

 that on area A; the erosion is in a less advanced stage: and the 

 greater amount of organic matter makes possible a greater absorption 

 and retention of the rainfall. Area A, on the other hand, with its 

 steeper slope lacks vegetation most where the greatest slope occurs 

 and this tends greatly to increase run-off. Finally, area A has a 

 differed type of drainage system from area B. In cross section area 

 A i.-^ broadly V-shaped and the main drainage is confined to one large 

 channel running lengthwise through the area. Area B, on the other 

 hand, is relatively flat in cross section and the drainage is divided 

 among three principal channels. Naturally, there is more resistance 

 to run-off which is distributed over the drainage than to run-off 



