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001401 - 9 - Detention or multi-purpose reservoirs retain all sizes of sediment behind the structure and sediment yield downstream is dependent on the trap effi- ciency of the reservoir. Reservoirs may be placed above a valley trench to reduce stress on the eroding channel by a reduction in flood peaks. However, long duration low flow releases may render a channel more vulnerable to erosion. Such a condition can exist when fine, lightly cemented or cohesive soils lose their resistance to erosion with the extended wetting. A system of drop structures and bank revetments can ,reduce sediment yield when channel erosion is a major source. However, it is unlikely that one or a small number of measures installed in channels will result in a sub- stantial reduction unless a particularly favorable situation occurs. This might include a drop structure located to stop a headcut from trenching an extensive valley. Evaluation of Land Treatment and/or Structural Measures Considered here are the potential off-site benefits from treatment under high or moderate yield potential for both Upland Erosion and Channel Erbsion and Sediment Transport, Columns H and I in the report "Factors Affecting Sediment Yield". When both are in the high classification, treatment ,of uplands but not channels is less likely to result in a significant reduction in total yield. The reason is that material is readily available in the channel and the stream may become loaded to capacity from this source with- out regard to contributions from hill slopes. Measures applied to one of the other combinations of upland and channel erosion conditions should have a greater impact on sediment yield with the possible exception of the treatment of high channel erosion but not that on the upland. The topography, cover and precipitation patterns determine to a large degree what sediment load the upland eroding areas furnish when the flow reaches the channel. Procedure for Evaluating Effect of Application of Measures on Erosion and Sediment Yield Table 2 presents numerical values for estimating the effect of measures on sediment yield. As in Table 1, climatic environment is subdivided into four types to facilitate classification in accord with more or less favor- able vegetative response to varying moisture conditions. (See Figure 1).' The factors which can be affected by treatment are Ground Cover, Land Use, Upland Erosion and Channel Erosion. Table 2 reflects changes in the numer- ical ratings on the chart attached to the report on Factors Affecting Sedi- ment Yield in the Pacific Southwest. Based on the treatment to be applied, the new rating uses the same numbers as given on the chart for factors A through E and new values in accord with Table 2 for columns F through I.