Stream temperature affects aquatic system productivity, community composition, and species’ developmental rates and fecundity. Anthropogenic elevation of stream temperature is a common water quality concern for Pacific Northwest states due to its effects on salmonid development and survival. Riparian timber harvest and resulting decreases in stream shade are well-associated with rises in stream temperatures. This relationship among others has prompted states to adopt riparian protection regulations. Oregon’s water quality regulations require the Oregon Department of Forestry to demonstrate the effectiveness of its harvest regulations at protecting streams from temperature increase. To that end, the Department of Forestry developed a manipulative study in 2002 to directly address this requirement. This study involves 33 sites in Oregon’s Coast Range, each with an upstream unharvested control reach and downstream treatment reach, with data collected from two years pre-harvest to five years post-harvest. The study’s objective is to determine the degree to which current timber regulation and management prescriptions meet Oregon water quality regulations. We evaluated stream temperature response to timber harvest with a linear mixed-effects modeling approach, and included associated stream covariates (e.g., stream length, buffer width, stream gradient). The analysis approach allowed us to quantify temperature response variability within and among sites and years.
Results/Conclusions
Pre- and post-harvest results indicate that State Forest site management (n=15) protects streams from temperature increase while an estimated 40% (95% CI = 27% - 54%) of privately-owned site examinations exhibit a harvest-related temperature increase. Stream shade declined along treatment reaches on private land. Our results are likely relevant to forestry practices throughout the coastal Pacific Northwest and may ultimately inform future policy discussion on Oregon’s forest practices regulation.