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These factors, although not all related to the geology, may be used together to partly explain the increase in sediment runoff between sta- tions 09212500 and 09213500. The stream flows through an area of a more erodible material after flowing from the glaciated terrain where the sediment load is small. Where the stream flows through the area of more erodible material, the pattern of the stream changes from a straight channel to a meandering channel, and the stream becomes more sinuous as it flows toward the Big Sandy Reservoir. As the stream flows toward the reservoir, it has a large capacity for sediment, but very little sediment is supplied by the ephemeral tributaries from watershed runoff. There- fore, the easily erodible banks and bed are the main sources of sediment for the stream. The Pacific Creek drainage basin shows a similar occurrence of bank wasting. The area drained by Pacific Creek contains mostly siltstone, sandstone, and mudstone, all of which are easily eroded, but this alone may not be a significant contributing factor. Some return flow from irrigation, which could cause some bank instability, occurs in this drainage basin, but the quantity and occurrence are small enough that irrigation return flows are probably a minor part of the bank wasting. Climate Langbein and Schumm (1958) found that as precipitation increased, annual sediment yield increased until a maximum was reached at a mean annual precipitation of 12 inches. The sediment yield then decreased regardless of additional precipitation. The variation in sediment yield with precipitation can be explained by the interaction of precipitation and vegetation on runoff and erosion. As precipitation increases from zero, sediment yields increase at a rapid rate because more runoff be- comes available to move sediment. However, vegetation becomes more abundant as precipitation increases, holding the soil in place and pre- venting its transport as sediment. More than 70 percent of the Big Sandy River basin has a mean annual precipitation of about 8 inches per year. This includes all the inner region of the basin, which has the largest sediment production. This same area also has sagebrush-type vegetation, which is not very resistant to erosion. Therefore, Langbein and Schumm's relation may be used to explain the erodibility in the basin. For example, Pacific Creek heads in the low mountains, receives moderate precipitation and runoff, and has a large sediment runoff, while the interior part of the basin receives little precipitation and runoff and has little sediment runoff. 48