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'./' 1".>-, that shales are more comrron in the Crct2cec~s units and mudstones are ~ore comr7',on in the Tertiary units. The Tert;ar~.; and Cretaceous sedimentary rocks 1 ie in a broad syncl inal basin, the axis of "hich strikes northvlest. In the extreme western part of the ja~in. Paleozoic sedimentary rocks, prinari 1'1' 1 ime~tone, sandstone, and siltstcne, are exposed on the land surface. These rocks a:-e \'./ell induratec a,d are relatively resistant to ero- s ion com par e d tot he v 0 u n 9 e r sed i men t s . P ~ e c am b ria n g n e i s San d s chi 5, t 0 u t _ crop along the eastern fringe of the \'a~D2 R:ver basin (fig. 6).,_These r'xks also are relatively resistant to erosicn GO~.Jc.red,-'to the Terda.:Vy~ and Creta- ceous sedjmentar~ rocks. The interbedded 5c.na~tones, mudsts:nes, and -sllales shovln in figure 6 and described above are relat:.lv,ely eroaible. Thev crop out vJidely throughcut the basin, in~".ce,a-s of both relatively large and small sediment yield. Therefore, the observed distribution of sedirent yields cannot be entirely due to ~imil~rities or differences in the bedrock geology. Mean-Ahnual Precipitation In many areas, sediment yields are c16sely correlated with mean-annual precipitation. Although mean-annual preci~itation alone is but one of the important factors controll ing sedi~ent yields, many of the other factors, such as vegetation, soil-type and clirate, are related to precipitation. Langbein and Schumm (1958) developed a ceneral relation between sedi~ent "yield and mean-annual precipitation (fig. 7). The most significant feature \ of this relation for the present discussion fs that maximum sediment yields \ may be expected from vlatersheds vlith a Mean-annual precipitation of about \,12 inches (305 mm) per year. The peak in the sediment-yield curve at an L-intermediate level of precipitation is partly explained by the generalized vegetation profi Ie shown at the top of the graph (fig. 7). With increases in mean-annual precipitation, the vegetative cover becomes progressively thicker and more diverse. As a result, the potenti~l erodibi 1 ity decreases bec2use the soil is protected from inte:Jsc:: rainfall, the soil particles are bound together more firmly, and the soil profile is generally more permeable. Thus, the decrease in sediment yield for a ~atershed which receives greater than 12 inches (305 mm) of mean-an:Jual r;-ecipitation is prii:lar-ily due to increased vegetative cover and developme~t of a soil profi le. If mean-dnnual precipitation is less t':an 12 inches (305 mm), sedir;-ent yields are 1 imited by the avai lable runoff. Thus, although potential erodibil ity probably increases contin~aj ly as precipitation decreases, the runoff is insufficient to transport the avai lable supply of sediment. The areal distribution of mean-annual precipitation in the Yampa River basin is shown on figure 8. The 12-inch (305-nm) per year 1 ine is of 8arti- cular interest, because the greatest sediment yields might be expected from areas near this J ine. About 40 percent of the Yampa River basin receives 13