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OO?63u that shales are more common in the Cretaceous units and mudstones are more common in the Tertiary units. The Tertiary and Cretaceous sedimentary rocks 1 ie in a broad synclinal basin, the axis of which strikes northwest. In the extreme western part of the basin, Paleozoic sedimentary rocks, primarily 1 imestone, sandstone, and siltstone, are exposed on the land surface. These rocks are well Indurated and are relatively resistant to ero- sion compared to the younger sediments. Precambrian gneiss and schist out- crop along the eastern fringe of the Yampa River basin (fig. 6). These rocks also are relatively resistant to erosion compared to the Tertiary and Creta- ceous sedimentary rocks. The interbedded sandstones, mudstones, and shales shown in figure 6 and described above are relatively erodible. They crop out widely throughout the basin, in areas of both relatively large and small sediment yield. Therefore, the observed distribution of sediment yields cannot be entirely due to similarities or differences in the bedrock geology. Mean-Annual Precipitation In many areas, sediment yields are closely correlated with mean-annual precipitation. Although mean"annual precipitation alone is but one of the important factors controlling sediment yields, many of the other factors, such as vegetation, soil-type and cl imate, are related to precipitation. Langbein and Schumm (1958) developed a general relation between sediment yield and mean-annual precipitation (fig. 7). The most significant feature of this relation for the present discussion is that maximum sediment yields may be expected from watersheds with a mean-annual precipitation of about 12 inches (305 mm) per year. The peak in the sediment-yield curve at an Intermediate level of precipitation is partly explained by the generalized vegetation profile shown at the top of the graph (fig. 7). With increases in mean-annual precipitation, tne vegetative cover becomes progressively thicker and more diverse. As a result, the potential erodibll ity decreases because the soil Is protected from intense 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 watershed which receives greater than 12 inches (305 mm) of mean-annual precipitation is primarily due to increased vegetative cover and development of a soil profile. If mean-annual precipitation Is less than 12 Inches (305 mm), sediment yields are limited by the available runoff. Thus, although potential erodibility probably Increases continually as precipitation decreases, the runoff is insufficient to transport the available supply of sediment. The areal distribution of mean-annual precipitation in the Yampa River basin is shown on figure 8. The 12-lnch (305-mm) per year line is of parti- cular interest, because the greatest sediment yields might be expected from areas near this line. About 40 percent of the Yampa River basin receives 19