Graduate Thesis Or Dissertation
 

Quantifying Impacts of Timber Harvesting Operations on Suspended Sediment Inputs to Oregon Coast Range Headwater Streams

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/h702qc383

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  • Timber harvesting practices can increase fine sediment inputs to streams due to increased hillslope soil erosion and mass wasting of roads, hillslopes, and stream channels. Excessive fine sediment depositions have been shown to impact aquatic ecosystems, fish habitat, and downstream community water supply. Despite these deleterious impacts, the influence of timber harvesting, using contemporary best management practices (BMPs), on sediment inputs to streams remains poorly understood. In this study, I quantified proportional sediment source contributions in forested and harvested Oregon Coast Range watersheds to determine if timber harvesting altered local erosional processes. Additionally, I tested the efficacy of riparian buffers at mitigating sediment movement off recently harvested areas into streams. To do this, I quantified the amounts of sediment moving off harvested hillslopes, through riparian buffers, and along forested, reference hillslopes. Specifically, in Chapter 2, I present research results from a study using sediment fingerprinting analyses to investigate the primary sources of suspended sediment in streams adjacent to recent forest harvesting activity. To do this, I instrumented two catchments, Enos Creek (harvested summer 2016) and Scheele Creek (reference), in fall 2016. Phillips samplers (5–6 per catchment) were deployed longitudinally down each of the streams to enable robust characterization of suspended sediment. I collected samples monthly over two wet seasons and compared the chemical properties of stream sediment samples with those of potential source areas including roads, streambanks, and hillslopes. I analyzed all samples for total carbon, total nitrogen, stable isotopes (δ¹⁵N, δ¹³C), and geochemistry (Fe, K, and Ca). Then, I used a mixing model and Monte-Carlo simulation to quantify the contributions of each potential source area to the suspended sediment. My results indicated that the suspended sediment in the stream draining the harvested watershed was, on average, comprised of 90.2 ± 3.4 % streambank sediment, 7.1 ± 3.1 % hillslope sediment, and 3.6 ± 3.6 % road sediment. Similarly, the proportional contributions of suspended sediment in the stream draining the reference watershed were 93.1 ± 1.8 % streambank sediment, 6.9 ± 1.8 % hillslope sediment, and 0.0 ± 0.0 % road sediment. These results, in conjunction with field observations of few erosional features and low suspended sediment concentrations (SSCs) during high flow events at the harvested watershed outlet, suggest that contemporary timber harvesting BMPs at the site did not substantially alter erosional mechanisms or increase sediment deposition to the stream. Chapter 3 reports the efficacy of the 50-foot fixed-width riparian buffer in the same timber harvested watershed (Enos Creek). To do this, I installed 36 silt fences in early summer 2017 to capture sediment movement from the general harvest area, within the riparian buffer, and from a forested, reference hillslope. Specifically, I installed 12 fences above the riparian buffer at the buffer/harvest edge, 12 fences within the riparian buffer near the stream edge, and 12 fences at the base of an adjacent forested hillslope, also near the stream edge. I collected sediment from each of the silt fences monthly during winter 2017–18 and returned them to the laboratory for drying and weighing. Results provided strong evidence that sediment movement from the general harvest area into the riparian buffer was greater than both sediment movement within the buffer (10.6-times higher; p = 0.0008) and on the forested hillslope (4.6-times higher; p = 0.005). I constructed a linear model to test if ground cover, canopy cover, and localized slope could account for differences in collected sediment mass. This model accounted for 46 % of the variability in the sediment masses in the silt fences. While there was substantial uncertainty in the relative importance of each of my site characteristic variables, my model indicated that ground cover was the most important explanatory variable (48 %), followed by local slope (31 %), and canopy cover (21 %). In sum, these results suggest that the 50-foot fixed-width riparian buffer was sufficient to mitigate sediment movement from the general harvest area to the adjacent stream during the second rainy season after forest harvesting.
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  • Ongoing Research
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  • 2018-11-14 to 2019-12-15

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