Spatial and Temporal Stream Temperature Response to Contemporary Forest Harvesting in the Oregon Coast Range

Abstract:

Historical research at the Alsea Watershed Study (1958-1973), in the Oregon Coast Range, demonstrated that clear-cut harvesting, with complete removal of riparian vegetation, can result in large and dramatic changes in mean daily, maximum daily, diurnal variation, and annual patterns in stream temperature (T_s). This previous research was instrumental in the addition of regulations in the Oregon Forest Practices Act of 1971, necessitating retention of streamside vegetation (riparian management zones) in harvest units to maintain water quality and aquatic habitat. Due to the ecological importance of T_s, preventing or mitigating changes in the thermal regime following land use activities, such as forest harvesting, is a primary focus of contemporary forest watershed management. The Alsea Watershed Study Revisited (2006-Present) has provided a unique opportunity to investigate the T_s responses to contemporary forest harvesting practices and compare these with the impacts from the 1960’s harvest. In general, T_s increases from late spring through about mid-July, with peak T_s occuring between about mid-July and mid-August, after which T_s decreases into the fall. During the pre-harvest period (2006-2008; n=244) the June to September mean daily maximum T_s was 13.0°C in Flynn Creek (control) and 12.0°C in Needle Branch (harvested). In the post-harvest period (2010-2012; n=240) the June to September mean daily maximum T_s was 12.4°C in Flynn Creek (control) and 12.0°C in Needle Branch (harvested). Similarly, the difference (Flynn Ck – Needle Branch) in 7 day moving mean of daily maximum T_s decreased from the pre-harvest (1.0°C) to the post-harvest (0.3°C) period, which was principally driven by a decrease in T_s in the control catchment. Longitudinal sampling of T_s within Needle Branch indicated a cooling trend such that the slight increases in post-harvest T_s weren’t detectable in downstream, unharvested stream reaches.