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Sediment-yield data for 121 sites in eastern Montana were compiled from previous local and regional studies for the purpose of establishing a regional data base. Methods used in the previous studies for determination of mean annual sediment yields include reservoir sedimentation surveys, stream sampling of suspended sediment at streamflow-gaging stations, and indirect estimation based on physical characteristics of the basin. The data summarized in this report can serve to illustrate regional variation of sediment yields or to identify areas where additional studies might be warranted. Multiple-regression equations were developed and evaluated for their adequacy in estimating mean annual sediment yields from small, ungaged watersheds in eastern Montana. Sediment yields determined from reservoir surveys were used in regression analysis as the dependent variable, because they were considered to be the most representative of long-term yields. Independent variables consisted of basin characteristics that could be readily measured from maps or obtained from published sources. Consequently, the equations developed in this study can be used without onsite data. Evaluation of equations indicated that the best prediction capability was obtained when reservoirs were segregated by the vegetation/soil complex of the basin. The predictive capability of regression equations developed for each of three classes of vegetation/soil complex, as indicated by the coefficient of determination (R ), ranged from 0.59 to 0.96. Corresponding standard errors ranged from 95 to 20 percent. The equations are applicable for small drainage basins of 2 square miles or less within the study area and possibly in adjacent areas having similar physiographic and hydrologic characteristics. Considering the inherent variability of sediment yields, the equations may be better-suited to providing comparisons of relative magnitudes between basins rather than absolute values. INTRODUCTION Much of eastern Montana consists of sparsely vegetated, semiarid rangeland, which is subject to extremely variable and sometimes severe soil erosion. In addition, soil disturbances associated with various land-use practices may accelerate natural erosion rates and sediment delivery from affected watersheds. Numerous studies of sediment yield in selected basins of eastern Montana have been conducted by various Federal, State, and private agencies. These studies provide data for watersheds that vary substantially in size and are distributed over a large geographic area. Summarization of the data from previous investigations would permit effective illustration of areal variation in sediment yields and enable identification of areas where additional studies might be warranted. A concern of land-management agencies is to minimize soil losses from watersheds and to prevent sedimentation problems in streams and reservoirs. Therefore, policy decisions regarding land use commonly take into account the impacts of various management options on sediment yields. To accurately assess such impacts, a means to predict potential sediment losses from specific sites is essential. Using existing data to define relationships between mean annual sediment yield and selected basin characteristics enables the development of equations to estimate sediment yield from small watersheds where onsite data on erosion rates and runoff are lacking. Such equations would be a useful management tool for agencies responsible for implementing soil conservation measures. Purpose and scope The purpose of this report is to (1) summarize existing sediment-yield data and (2) describe proposed techniques for estimating mean annual sediment yields from small, ungaged watersheds in eastern Montana. An extensive literature search was made in compiling available sediment-yield data. Because sediment yield is a function of many physical, climatic, and hydrologic factors, multiple-regression analysis was used to identify which basin characteristics exhibited statistically strong correlations to sediment yield. Equations were developed and evaluated for their adequacy in estimating mean annual sediment yields. To alleviate the need for collection of onsite data, basin characteristics were selected that could be measured from maps or other published sources. This report was prepared in cooperation with the U.S. Bureau of Land Management. i Description of study area The area described in this report (fig. 1) extends, in general, from the Wyoming border north to the Missouri River, and from the Musselshell River east to the North and South Dakota borders. The area encompasses about 33,500 square miles. Most of the area is within the Fort Union and Powder River coal regions, where extensive development of coal resources is in progress in southeastern Montana and is expected to increase in areas of east-central Montana. Currently, the principal land use within the study area is livestock grazing. Dryland crops are produced in some upland areas, whereas irrigation farming generally occurs along the major streams and their larger tributaries. Climate The climate of eastern Montana is semiarid continental and is characterized by cold dry winters, cool moist springs, and warm dry summers and autumns. Large ranges in daily and annual temperatures are typical of the area. January is the coldest month, with average temperatures ranging from about 10°F in the northern parts of the study area to about 20°F in the southern parts. July is the warmest month, averaging about 70° to 72°F throughout the region (U.S. Department of Commerce, issued annually).

Sediment yields in eastern Montana; summary of data and proposed techniques for estimating sediment yields from small, ungaged watersheds