Using the sediment record in a western Oregon flood-control reservoir to assess the influence of storm history and logging on sediment yield

Dorena Lake, a flood-control reservoir with a 686 km2 watershed in the western Cascade Mountains of Oregon, contains a 50-year record of sediment deposition. By measuring the amount of lake sediment, performing 137Cs dating, and examining the stratigraphy of sediment cores, information was obtained about variations in the sediment yield of the watershed. The average yield for 50 years is 108 tons/km2/year, and rates for shorter time periods vary from 42 to 269 tons/km2/year depending on the time of average and the occurrence of large storm events. A comparison of sediment yield values with timber harvesting rates and with the number of high-discharge days during different time periods implies that sediment yield is controlled primarily by flood magnitude and frequency. The amplitude of variation in yield that might be produced by changes in logging rates or methods is too small to be detected with the methods used in this study because in large watersheds (1) the effects of localized events tend to be averaged out and (2) sediment delivery ratios are generally low. Also, the reservoir was constructed after a significant portion of the watershed had been logged, so the data cannot be compared with pre-disturbance sediment yields. This study shows that reservoirs in the Western Cascades are worthy of further study; but information is needed to constrain reservoir trap efficiency, and problems of scale in large watersheds may hinder the detection of land-use influences on sedimentation.