The influence of contemporary forest harvesting on summer stream temperatures in headwater streams of Hinkle Creek, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/02870z851

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  • Stream temperature is a water quality parameter that directly influences the quality of aquatic habitat, particularly for cold-water species such as Pacific salmonids. Forest harvesting adjacent to a stream can increase the amount of solar radiation the stream receives, which can elevate stream temperatures and impair aquatic habitat. Oregon Forest Practice Rules mandate that forest operators leave Riparian Management Areas (RMAs) adjacent to streams in order to minimize the water quality impacts from forest harvesting. However, RMAs that contain overstory merchantable conifers are not required for small non-fish-bearing streams in Oregon, thus there is potential for increases in stream temperature to occur in headwater streams after harvesting. There is concern that increases in stream temperatures and changes to onsite processes in non-fish-bearing, headwater streams may propagate downstream and impair habitat in fish-bearing streams. The objectives of the following work are to assess the effects of contemporary forest management practices on stream temperatures of small non-fish-bearing headwater streams and to develop new knowledge regarding the physical processes that control reach-level stream temperature patterns. Summer stream temperatures were measured for five years in six headwater streams in the Hinkle Creek basin in southern Oregon. After four years, four of the streams were harvested and vegetated RMAs were not left between the streams and harvest units. The watersheds of the two remaining streams were not disturbed. Post-harvest stream temperatures were monitored for one year in all six streams. Each harvested stream was paired with one unharvested stream and regression relationships for maximum, minimum and mean daily stream temperatures were developed. Changes to temperatures of harvested streams were detected by comparing the mean pre-harvest regression relationship to the mean post-harvest relationship. Change detection analyses that considered the mean response among all four harvested streams indicated that maximum daily stream temperatures did not increase after harvesting, but that minimum and mean daily temperatures decreased significantly after harvesting. Additionally, diel stream temperature fluctuations were significantly greater one year after harvesting. Pre- and post-harvest surveys of canopy closure in the harvested and unharvested streams were completed in order to compare levels of stream shading before and after harvest. The post-harvest survey quantified canopy closure from remaining overstory vegetation as well as from logging slash that partially covered the harvested streams. The surveys indicated that mean overstory canopy closure in the harvested streams decreased by 84% as a result of the harvest, but as the logging slash provided considerable cover, total canopy closure decreased by only 20%. It is possible that the logging slash effectively attenuated solar radiation and prevented extreme temperature increases in the harvested streams. However, it is likely that streamflow increased after harvesting and that the increased streamflow also prevented increases to maximum temperatures and contributed to lower minimum and mean stream temperatures.
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