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THE DATA The sediment rating curves discussed in this paper are derived from data collected on the Boise and Clearwater National Forests and from watershed research in the Silver Creek area (fig. 1). The Idaho batholith is within the Northern Rocky Mountain Geomorphic Province. The streams on the Clearwater National Forest are within the Lochsa Uplands sec- tion and are characterized by moderate slopes (30 to 50 percent) with steeper lower canyon areas draining the rejuvenated uplands. The streams on the Boise National Forest, which includes those in the Silver Creek area, are in the southern batholith section where steep, strongly dissected slopes (50 to 70 per- cent) dominate. Streamflow and sediment transport reach a, peak in response to melting winter snow- packs during April and May. Intense summer thun- derstorms occasionally cause significant flow increases, but these and the associated sediment dis- charge are small relative to the spring snowmelt peaks. For information on precipitation and stream- flow for these areas, see Idaho Water Resources Board (1968). The data include instantaneous suspended and bedload sediment and concurrent streamflow. Sus- pended sediment is sampled with DH-48 depth- integrating samplers. Helley-Smith samplers are used for sampling bedload sediment. Streamflow is measured each time with current meters. Forest Data Sampling on the Boise and Clearwater National Forests was concentrated around the spring snow- melt peak with samples taken infrequently during the summer and fall months. The number of samples per year varied from five to 20 with a mean of 10. Sediment rating curves were developed for sus- pended, bedload, and total sediment for each year of record and for the period of record. Total sediment rating curves were developed by regressing dis- charge on the sum of the DH-48 and Helley-Smith sediment values. The 10 streams on the Boise National Forest had 3 to 5 years of record. Seven streams included in this report from the Clearwater National Forest had 3 to 8 years of record. All streams had various levels and ages of land manage- ment. The watersheds varied in size from 244 to 2,298 ha. Two methods were used to calculate sediment yield. The rating equation approach required con- tinuous streamflow records for the monitored stream, or a correlation between instantaneous flows measured on the monitored stream with mean daily flows from a nearby stream gauge. If the correlation was good (R2>_0.60), the gauged stream was used to generate streamflow values for the sediment rating equation. The rating equation method integrates based on streamflow fluctuations. Stream gauge records did not correlate well with the streamflow measurements on the Clearwater National Forest, so sediment yields are not presented for these streams. The second method, in this paper called the time- integration method, ignores streamflow fluctuations and integrates over time. Sediment discharge meas- ured at two consecutive sample dates was averaged and multiplied by the time elapsed between the two hand samples. When a sample taken during high sediment discharge covered a long period (> 10 days), the time was adjusted based on what the ana- lyst knew about the position of the sample on the hydrograph and the shape of the hydrograph. This adjustment reflected the tendency for peak flows and high sediment discharges to be short lived. Research Data Data were collected in the Silver Creek study area in central Idaho. Streamflow and sediment were measured as described above except that measure- ments in Silver Creek were taken in a rectangular flume to provide a more uniform cross-section and flow distribution. Sampling was limited to the spring snowmelt period with an average of 15 sam- ples of suspended and bedload sediment each year. The period of record for Silver Creek spring sedi- ment monitoring ranged from 3 to 5 years. These third-order streams drain watersheds of 109 to 186 ha. Sediment rating equations were developed for sus- pended, bedload, and total sediment by regression as described for the Forest monitoring data. Sediment yield for Silver Creek streams was computed as above, but in this case mean daily flows were availa- ble for each stream. In addition to the hand sediment samples taken during spring snowmelt, automatic pumping sam- plers were in operation during most of the year in Silver Creek. Discrete samples were pumped from a hydraulic jump in the flume on a timed basis. The sampler intakes were located 1 to 2 cm from the bot- tom of the flume to provide an estimate of total sediment load rather than just suspended sediment load. The interval for sampling varied from 1 to 24 hours. The interval was changed manually in response to streamflow changes or precipitation or both. From 26 to 460 samples were taken per stream each year during 4 years. Sediment rating equations were developed from these data as well. The large number of samples made it feasible to group the data by rising and falling limbs of the hydrograph, by storm, and by time, in an attempt to reduce the variance in the data. Sediment yields were estimated using detailed storm-by-storm rating equations, from more general seasonal equations, and by time integrating. To validate sediment yield estimates from hand samples and pumping samplers, all values were com- pared with sediment yields from sediment dams located just downstream from the flume in each watershed. The volume of sediment in each sediment dam was converted to mass using volume-weight estimates from core samples of the trapped sedi- ment. This was then corrected for the trap efficiency of the reservoirs. The results are believed to be the