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Show PART VII PROCEDURES Determination of Sediment Yields The present yield rate map was made following the method outlined in the report "Factors Affecting Sediment Yield and Measures for the Reduction of Erosion and Sediment Yield" published by the Pacific South- west Interagency Committee in October 1968. The report was prepared by the Water Management Subcommittee of the PSIAC. Nine factors considered in estimating sediment yield rates by this method are surface geology, soils, climate, runoff, topography, ground cover, land use, upland ero- sion, and channel erosion and sediment transport. The method was developed for broad planning purposes rather than for specific project planning. It was intended to serve as a means for delineating boundaries between sediment yield areas and extrapolating existing data to areas where none are available. Several terms which have specific and limited meaning in this appendix are defined as follows: Erosion is the detachment and movement of soil or rock by water, wind, ice, or gravity. Accelerated erosion is erosion much more rapid than normal, natural, or geological erosion, and is primarily a result of the influence of the activities of man or in some cases, of animals. Geological or natural erosion is the wearing away of the earth's surface by water, ice, or other natural agents under natural environmental conditions of climate, vegetation and other factors undisturbed by man. Sediment, the product of erosion, is solid material that is being transported or has been moved from its site of origin. Sediment yield is the quantity of sediment transported out of a drainage area or past a given point within it. Sediment yield rate is the quan- tity of sediment yield per unit of drainage area per unit of time. It is usually expressed in acre-feet per square mile per year, or tons per square mile per year. Sediment, sediment yield, and sediment yield rate may be expressed in terms of suspended sediment, bedload, or total sedi- ment. Suspended sediment is that part which is carried in suspension by a stream while bedload is the part of the total sediment that is moved along the streambed. Stream sediment stations usually measure only suspended sediment. Determination of Average Annual Damages Resulting from Upstream Watershed Problems Existing agency reports and studies were used in developing data on acreage affected by erosion and fire according to land category. |