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It is apparent from figures 28-30 that Pacific Creek contributes a significant part of the sediment that enters the Big Sandy River down- stream from Big Sandy Reservoir; however, there is a substantial increase in sediment transport between the mouth of Pacific Creek and the mouth of Little Sandy Creek that is unaccounted for. Part of this increase in transport rate may be from Dry Sandy Creek, which has basin characteris- tics similar to Pacific Creek. Part also may be coming from a flushing of the main channel of Little Sandy Creek. This would be the case when sediment is deposited in the wider main channel of Little Sandy Creek by low flows from narrower Pacific and Dry Sandy Creeks. This type of behavior would most likely occur when irrigation return flows are enter- ing the channels, picking up sediment from the banks, but not having enough flow to transport the material to the Big Sandy River. FACTORS AFFECTING SEDIMENT YIELDS The quantity of sediment eroded from a watershed is affected by several factors. Bedrock geology, soil type, vegetation, climate (mainly precipitation and temperature), topography, and land use are the most important, and many of these are interrelated. Therefore, the sediment yields vary considerably throughout the basin. More than 50 percent of the sediment runoff in the basin is supplied by 34 percent of the drain- age area downstream from Big Sandy Reservoir, while less than 8 percent of the sediment is supplied by 25 percent of the drainage area. It is useful to consider which of the above-named factors are responsible for the variability of sediment yields within the basin. Geology Bedrock of the Big Sandy River basin is composed mostly of Tertiary and Cretaceous siltstone, mudstone, sandstone, and shale, all of which are easily eroded. They crop out widely throughout the basin in all areas where sediment production is large. This indicates that the geo- logy of the region is a major factor contributing to the large sediment production. The erodibility and weathering of the bedrock is noticeable along the Big Sandy River and its major tributaries, so the sediment transport rate in the Big Sandy River and Pacific Creek may be explained in part by the geology. A reconnaissance trip was made along the Big Sandy River upstream from Big Sandy Reservoir during the summer of 1978. It was observed that: (1) There was a downstream increase in sediment transport rate once the stream left the glacial deposits of the mountains and flowed across the semiarid plains into the Big Sandy Reservoir (fig. 8); (2) the stream was carrying almost no sediment in its upper reach; very few point bars existed, indicating there was large stream capacity for more sediment than was being supplied; (3) many locations along the stream had mass wasting of the banks (fig. 31), and (4) tributaries of the plains area supplied very little inflow to the river, probably due to the lack of precipitation, rapid infiltration rates, significant evaporation, and sublimation of much of the snow by winds. The small amount of inflow is apparent by the similar mean annual discharges measured at stations 092125005 Big Sandy River at Leckie Ranch, near Big Sandy, and 09213500, Big Sandy River near Farson (table 4). 46