Variations in Suspended Sediment Yield and Nitrogen Export in Three Small Oregon Coast Range Streams

* Hale, C (cody.hale@oregonstate.edu) , Department of Forest Engineering, Oregon State University, 204 Peavy Hall, Corvallis, OR 97331, United States
Schoenholtz, S H , Virginia Tech, 210 Cheatham Hall, Blacksburg, VA 24061, United States
Ice, G G , National Council for Air and Stream Improvement, PO Box 458, Corvallis, OR 97339, United States
Stednick, J D , Watershed Science Program, Colorado State University, Ft Collins, CO 80523, United States

The Alsea Watershed Study (1959-1973) was a paired watershed study in the Oregon Coast Range testing in- stream physical and biological effects of two timber harvesting techniques (clear-cut without restriction and patch- cut with stream buffers). Results from the study indicate a 205% increase in average annual suspended sediment yield following road construction and clearcut harvest while the patch-cut watershed exhibited a 53% increase, mainly attributed to a road failure. Maximum mean daily suspended sediment concentrations (SSC) for the control watershed were 1580 and 1530 mg/L during the pre-treatment and post-treatment periods, respectively. Maximum mean daily SSC from the clear-cut watershed increased by 960 mg/L (from 300 to 1260) following harvest while the patch-cut watershed increased by 1230 mg/L (1220 to 2450). Nitrate-nitrogen export was found to increase following the clear-cut, but concentrations returned to pre-treatment levels by the second year following harvest. The Alsea Watershed Study Revisited, a project initiated in 2005 to investigate the effects of modern forest practices on water resources, has created an opportunity to assess current suspended sediment concentrations and yields and nitrogen concentrations and export in the three watersheds. The original gauging stations have been re-instrumented, including automated samplers for collecting suspended sediment samples using the Turbidity Threshold Sampling method. Maximum SSC concentrations measured during the 2006 water year were 445, 220, and 380 mg/L for the control, clear-cut, and patch-cut watersheds, respectively. Water quality monitoring in the early 1990s showed that the nitrate-nitrogen concentrations and export were higher in the control watershed than the clearcut watershed. Monthly grab samples have been collected at each gauging station for nitrate-nitrite nitrogen analysis to further assess watershed differences. This presentation will summarize the results for the suspended sediment and nitrogen data collected for the 2006 water year and compare to pre-treatment and post-treatment findings from the original study.