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I I ultimately pass through Island Park and affect downstream conditions, but the time period required could be on the order of decades or more. I 6.3 Sediment Source Areas (after Gregory. 1989) I 6.3.1 General. Section 2.2, above, briefly reviewed the conclusions of Andrews (1978) on sediment source areas in the Yampa River basin. Perhaps the most significant conclusion was that while the drainage areas of the Little Snake and the Yampa River above the Little Snake are about equal, the Little Snake River provides only 27 percent of the water to the Deerlodge Park reach, but 69 percent of the sediment, with most of this sediment being derived from the lower portion of the basin. In other words, the lower Little Snake watershed is a significant sediment source area for the lower Yampa River. Furthermore, given the steepness of the Yampa Canyon reach and the likelihood that aU sand-sized sediment at Deerlodge Park eventually passes through the canyon, the Little Snake is also a significant sediment source area for the Green River. I I I To develop a better understanding of the Little Snake watershed as a sediment source area, additional investigation was completed. Aerial reconnaissance was conducted to identify major sources of sediment delivered by tributaries between Baggs, Wyoming and the mouth of the river. This reconnaissance also included tributaries to the Yampa River between Williams Fork River and Cross Mountain. In addition to aerial reconnaissance, a review was made of existing geologic maps for the area in order to estimate soil conditions and the potential for erosion and sediment delivery from tributary basins. I I I Detailed discussion of each major tributary and its significance as a sediment source area is presented in Appendix D. The following information summarizes the results and presents a ranking by estimated contribution of the major sediment source areas in the lower Little Snake basin. I I, I I I I I I I 6.3.2 Rankin~ of Little Snake River Sediment Source Areas. Sediment yield is related to many factors including geology, soils, vegetation type and density, topography, and precipitation. The aerial reconnaissance of Little Snake River provided qualitative information on geology/soils, vegetation density, and topography. Through observations of vegetation density an indirect measure of precipitation was obtained which suggests that precipitation generally decreases from east to west across the basin with the lowest precipitation occurring in Sand Creek drainage basin and the west side of Muddy Creek drainage basin. A map of annual precipitation over the Little Snake River basin presented by Andrews (1978) confmns these observations (Figure 6.1). Langbein and Schumm (1958) show that sediment yield is highest in regions with about 10 to 14 inches of annual precipitation (semiarid). In wetter regions, vegetation holds the soil together and intercepts precipitation resulting in decreased sediment yields while in dryer arid regions there is simply not enough rainfaU to move significant volumes of sediment from a basin. The Little Snake River Basin between Dixon, Wyoming and the mouth is generally underlain by geologically young, erodible rock formations. By considering precipitation it is possible to determine which tributary basins are likely to produce the most sediment. Muddy Creek and Sand Wash lie in areas with precipitation in the range of 10 to 14 inches (at least in some portions of the headwaters) while Sand Creek lies almost entirely in an area receiving less than 9 inches of annual precipitation. This suggests that although Sand Creek has a ready supply of sediment from the uplands and the channel it may produce less sediment than either Muddy Creek or Sand Wash. Precipitation over Powder Wash is roughly intermediate to Sand Wash and Sand Creek, however the small drainage area suggests that it will produce less sediment than Sand Creek. 6-5