Sediment delivery to headwater stream channels following road construction and timber harvest in the Blue Mountains, Oregon

Graduation date: 1995Presentation date: 1994-05-25Many studies have focused on improving our understanding of the effects of timber harvesting activities on soil, water, and fisheries resources. Much of this work has led to the development and widespread use has often resulted in model applications that are outside the bounds in which the models were developed. There is currently no adequate method for predicting the quantity of sediment delivered to first and second order channels following road construction and harvesting in areas of ash-influenced soils in the Blue Mountains of eastern Oregon. The objectives of this study were: (1) to determine the amount and rate of sediment delivery to ephemeral (first and second order) stream channels following road construction and logging, and (2) to evaluate the WWSED sediment yield predictions. A variety of methods were employed to accomplish these objectives, including: in-channel and on-slope sediment trapping for quantity and rate determination, physical characterization of the area contributing flow and sediment, physical characterization of the soil samples themselves, and statistical analysis for extrapolation of results. No statistically significant relationships between the quantity of sediment yielded versus either inherent or management disturbance factors could be concluded from this data set. While there appears to be no significant relationship between inherent or management induced disturbance factors and sediment yield, there has been a two-fold increase in sediment yield when comparing 1993 to 1991 sediment yields, a ten-fold increase in sediment yield when comparing 1993 to 1992 sediment yields. The R-Squared values for 1993 sediment yield versus inherent values were considerably higher than 1991 or 1992 values. It can be concluded that while there was an increase in annual sediment yield in the Syrup Creek Study Area, there is no statistically significant relationship between this increase and inherent or management factors. This may be due, in part, to the limited data set with only three years of observations. It is likely that there are other inherent and management factors which would help explain the variation in sediment yields. Results indicate that the WWSED Model has drastically over estimated the sediment yield from this area. From this, we can conclude that the variability of natural systems is far more complex than can be simplified into a prediction model. Several additional years of measurement are necessary. The WWSED model predicts sediment yield for a seven year period. At a minimum, measurements should continue for an additional four years and preferably longer. In addition, it is recommended that a pumping sampler be installed at the mouth of the study area to quantify total suspended load yielding the watershed. This may assist additional years of sampling and provide a more robust data set in which to evaluate the WWSED model