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OO?5'C,tj SEDIMENT YIELDS IN THE YAMPA RIVER BASIN Source Areas of Sediment and Water Frequently, the sediment load of the stream is not supplied equally from all areas of the drainage basin. Some areas of a drainage basin contribute a relatively large part of the annual sediment load; whereas, other areas of the drainage basin contribute relatively minor quantities of sediment. Similarly, runoff seldom is supplied evenly -from throughout the drainage basin. Thus, sedlment- and runoff-source areas often can be Identified for a drainage basin provided the sediment loads and runoff are measured or esti- mated at several points within the drainage basin. The term "source area" Is used In a relative sense to describe those parts of the drainage basin which supply a large percentage of the sediment load or runoff compared to their areal extent. The mean-annual sediment load and runoff at the gaging stations in the Yampa River basin are shown on figure 4 as a percentage of the total sediment and runoff of the Yampa River at Deerlodge Park (fig. 1) . A comparison of the values of the individual stations shows that sediment and runoff are not contributed to the streams equally throughout the basin. Furthermore, the principal source areas of sediment and runoff are different. One of the most striking differences exists between values at sites on main-stem rivers draining the two major subbaslns--the Little Snake River near Lily (site 1) and theYampa River near Maybell (site 17). Although the drainage areas contributing to these two gaging stations are about equal, 3,730 ml2 (9,660 km2) for the Little Snake River subbasin at site 1 versus 3,410 ml2 (8,830 km2) for the Yampa River subbasin at site 17, the respective sediment loads and runoff are markedly different. The Little Snake River subbasin supplies 2} percent of the annual runoff to the Yampa River at Deerlodge Park but nearly 69 percent of the sediment load. Conversely, the Yampa River subbasin contributes 73 percent of the runoff and only 27 percent of the estimated total~sediment load for the entire Yampa River basin (fig. 4). Comparisons for other areas are equally striking. Most of the large sediment load of the Little Snake River subbasin enters the main-stem Little Snake River between Dixon (site 8) and Lily (site 1) (fig. 4). About 60 percent of the entire sediment load of the Yampa River at Deerlodge Park is contributed from the drainage area between the Little Snake River near Dixon and the Little Snake River near Lily gaging stations. Thus, the lower part of the Little Snake River subbasin is the major sediment source area within the Yempa River basin, This area Is less than 35 percent of the entire basin area and supplies less than 3 percent of the runoff. In contrast, the eastern part of the basin upstream from site 8 on the Little Snake River and site 47 on the Yampa River supplied approximately 76 percent of the total basinwide runoff and only 14 percent of the sediment load. 14