An analysis of the downstream propagation of stream temperature changes following headwater forest management in western Oregon

This study presents a unique examination of changes in stream temperature from three paired-watershed studies in the Pacific Northwest: a) Trask Paired Watershed Study b) Alsea Watershed Study Revisited, and c) Hinkle Paired Watershed Study. All three are high elevation, steep, forested headwater catchments that were either harvested or remained as unharvested, reference catchments. Relative location ranged from 280 m upstream and within a harvested area to 1420 m downstream of the harvest or reference sites. There were a total of 27 sites—9 reference and 18 harvested—with data spanning 14 years (2002-2015). Generalized least squares regression methods were utilized in analysis of changes in the seven-day moving average of daily maximum stream temperature (7DAYMAX). This analysis highlighted potential relationships between forest management practices and physiographic setting. Streams in erosion-resistant geology (e.g., diabase) that were extensively clear-cut (> 95%) with either no riparian management area (RMA) or a narrow RMA exhibited the largest and most persistent stream temperature exceedances (> 2 oC) above the mean model predicted value in the post-harvest years. Extensively clear-cut catchments with RMA utilization for the provision of shade showed the smallest deviations in stream temperature from the model predictions. Moreover, streams located in erosive geology (e.g., sedimentary rock, landslide topography), which likely exhibit greater groundwater contributions to streamflow, also exhibited smaller exceedances (< 2 oC) that were less persistent. The 7DAYMAX stream temperature exceeded the model 95% prediction interval at only 3 of the 8 harvested, downstream sites—these sites were some of the most proximate ( 210 to 780 m downstream) to harvested upstream sites. Among the downstream sites, these three sites also had some of the highest percentage of upstream area harvested—33% to 60%, compared to less than 30% for the remaining 5 downstream sites.