Graduate Thesis Or Dissertation
 

Summer Low Flow Response to Timber Harvest and Riparian Treatments in Forested Headwater Streams of Coastal Northern California

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/37720m72j

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  • Headwater streams represent the majority of the drainage area of a river network and provide many critical functions supporting aquatic and terrestrial ecosystem health. The riparian zones that surround these streams are transitional areas between terrestrial and stream ecosystems, which provide key habitat for aquatic and terrestrial organisms, especially during the dry season and severe weather events. As such, forest management activities in headwater areas can have substantial impacts on the riparian zones and adjacent streams. To protect the vital ecosystem services provided by riparian areas and headwater streams, most regions regulate riparian buffers as part of timber harvesting practices. Some jurisdictions in the western United States have modified riparian buffers in recent years to increase protections for aquatic ecosystem health, including increasing the requirements for buffer width and/or retention of tree density during forest operations. However, our understanding of the effectiveness of riparian buffers at protecting stream ecosystems remains limited due to a lack of research on the rapidly changing forest management practices. In our study, we assessed the effects of timber harvests and different riparian buffer treatments on total daily streamflow and diel streamflow during the summer low flow period in coastal Northern California. We quantified streamflow in three catchments that each received a different riparian buffer treatment with different tree density retention. Our objectives were to compare the effects of these different riparian buffer treatments on summer low flows with observations from two reference catchments. We measured streamflow and weather conditions for one pre-harvest year and two post-harvest years. During the two post-harvest summers, we observed substantial increases in summer streamflow in all treated streams, ranging from 0.07 to 1.13 mm day-1 (1.3- to 4.4-times greater than the pre-harvest streamflow). However, we did not find a relationship between the different riparian buffer treatments and the changes in streamflow. Rather, the increases in streamflow were most strongly correlated with the area of the catchment that was harvested, which ranged between 3–25% in our three treated catchments. We also observed 1.1- to 2.5-times greater diel streamflow after forest harvesting, ranging from 0 to 0.35 L s-1. These increases in the daily fluctuation of summer low flows after harvesting were most strongly related to the riparian treatment, area of catchment harvested, and the aspect of the harvested slope. Our research highlights the need for additional studies to further explore if similar patterns persist across the study region or appear evident in other hydro-climatic settings. Ultimately, a better understanding of the efficacy of best management practices such as riparian buffers, will contribute to continued improvements in practices for watershed protection.
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  • Region 18
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  • Pending Publication
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  • 2022-12-08 to 2024-01-11

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