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Flaxman (1975) described watersheds in relation to the value of b. However, when b became small, a nearly independent relationship between discharge and sediment concentration resulted. An unlimited supply of fine sediment in the streambed and banks might result in low-flow and high-flow sediment con- centrations of equal magnitude. Flaxman developed a significant predictive equation for a using average annual runoff and the b coefficient. He then predicted undisturbed suspended sediment rating equations for three Western United States streams by assuming a value for b for undisturbed condi- tions. These curves were then compared with rating curves developed from sample data on these. watersheds, which have undergone development. The difference between the predicted curves and the actual curves was attributed to development impacts. Rosgen (1980) presents one method for predicting relative changes in potential sediment discharge from silvicultural activities using sediment rating curves. This procedure assumes that sediment rating curves are constants. But when used in conjunction with predicted streamflow increases following distur- bance, the procedure can predict relative changes in sediment following silvicultural activities. The fact that shifts do occur in sediment rating curves due to disturbances and floods is recognized, but this proce- dure does not predict these changes. The role of sediment supply in determining the sediment rating curve has gained increasing atten- tion. Shen (1972) noted that for large rivers carrying mostly suspended sediment and little bedload, sedi- ment transport of the river is more dependent upon supply from the watershed than on transport capac- ity. The amount of fine sediment in streamflow is more dependent on the intensity and location of rainfall, cover conditions, and other watershed fac- tors than on water discharge. Bedload sediment der- ived from channel erosion should be more closely correlated with streamflow (Holeman 1975). Van Sickle and Beschta (1983) propose a supply-based model for small streams that uses the suspended sediment rating curve and a supply depletion func- tion. The supply function expresses a declining sus- pended sediment concentration to reflect storm hys- teresis and seasonal decline of sediment supply. Simultaneous measurements of sediment and streamflow are required to develop a sediment rating equation. The intended use of the rating equation dictates how these measurements are taken. If an average annual sediment yield estimate is desired, the sediment and discharge should be taken over several years (if time permits) and include a wide range of flows. The resulting composite sediment rating curve can be used with a long-term flow dura- tion curve to estimate the expected annual sediment for a typical year. Streamflow is not always availa- ble for the entire year or period of sediment meas- urement. If it isn't available, the flow duration curve must be developed by correlating the instantaneous streamflow measured in conjunction with sediment sampling with streamflow records of a nearby gaug- ing station. Extreme hydrologic events (floods and droughts) and management impacts can significantly affect sediment discharge. Sediment rating equations developed from data collected during these events may not reflect average conditions. This should be remembered when analyzing data for sediment rating equation development. If the intended use of the rating curve is to esti- mate yearly sediment yields, or to monitor the effects of watershed disturbances on sediment yield, or both, annual rating curves should be developed. Actual or estimated streamflow for each year is used to estimate annual sediment yield. Estimates of streamflow could again be obtained by correlation with nearby gauging station records. In this case, mean daily flows are used in the sediment rating curve to calculate daily sediment. The daily sedi- ment values are added to estimate annual sediment yield. OSCO IDAHO BATHOLITH CLEARWATER NATIONAL FOREST & M O N T A N A n 4 BOISE NATIONAL FOREST / SILVER CREEK STUDY AREA F ? e °fas A \ ? j d + O OISE jj I D A H O Sna4• Figure 1-Locations of the Idaho batholith, Clearwater and Boise National Forests, and Silver Creek study area in Idaho.