Sediment storage in a headwater valley of the Oregon Coast Range : erosion rates and styles and valley-floor capacitance Public Deposited

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  • This study is part of the effort to quantify sediment budgets and understand the geomorphic evolution of steep mountains where debris flows are the dominant agent of upland erosion. Observations indicate that headwater basins in the Oregon Coast Range (OCR) can store a large amount of sediment, mostly from debris flows, in wide valley floors. In Bear Creek, a 2.23 km2 tributary basin of Knowles Creek, surveyed sediment volume in the main-stem valley was 6.95 x 104 m3. We hypothesize that, by storing sediment, headwater valleys in the OCR act as a sediment reservoir that buffers larger-basin reaches from episodic debris flows. These reservoirs are "well-mixed": they release sediment "cohorts" in proportion to their relative volumes in storage and, therefore, have an exponential distribution of sediment ages or residence times. This exponential distribution of residence times results from these valleys receiving pulse inputs of sediment at discrete times and producing gradual outputs of sediment representing a wide range of input times. We determined two separate residence time distributions in Bear Creek by dividing the main-stem valley into two reservoirs: lower and upper reaches. In each of these reaches, wood and/or charcoal from randomly selected sampling points in the valley sediment were dated by radiocarbon methods. The mean radiocarbon age from sediments in the upper-reach was 4.43 x 102 years. The mean radiocarbon age from sediments in the lower-reach was 1.22 x 103 years. The volume-weighted mean age of sediments in the entire main-stem valley was 9.96 x 102 years. Erosion rates of 0.013 - 0.038 mm/yr and 0.011 - 0.033 mm/yr for each of the upper 1.32 km2 and full 2.23 km2 basins, respectively, were calculated by dividing the volume of sediment by the mean age and the contributing area. A density correction for conversion of rock to sediment defines the lower limit with no density change defining the upper limit. These erosion rates are generally lower than those determined by other methods in the OCR possibly reflecting inherited age in radiocarbon dates plus denudation by processes that do not form deposits datable by radiocarbon. Such denudation processes include dissolution of bedrock and sediments and the direct discharging of soil from the hilislopes to the channel, transported as suspended load. In order to account for inherited radiocarbon ages and assuming that all denudation processes cycle sediment through the reservoirs datable by radiocarbon, we rescaled the residence time distributions using independently derived mean ages for the sediment. These mean residence times are 5.5 x 101 years for the upper-reach and 1.18 x 102 years for the lower-reach, respectively. These mean residence times are derived from an assumed basin average erosion rate of 0.1 mm/yr. A two-stage 8-corrected Kolmogorov-Smirnov statistical goodness-of-fit test was used to find whether the radiocarbon age distributions from the upper and lower reaches are statistically consistent with exponential distributions defined by the mean sediment ages in each reach and, thus the hypothesis that these valley floors act as well mixed reservoirs. At the 5% significance level the lower-reach ages are consistent with an exponential distribution while the upper-reach ages are not. Combined with geomorphic and stratigraphic data, these statistical results support the conclusion that the lower-reach acts as a well mixed reservoir for episodic debris flows that form deposits that are then randomly and incrementally removed by fluvial reworking. Deviations in the shape of upper-reach's distribution from exponential may reflect a lesser degree of mixing as determined from geomorphic and stratigraphic data and also limitations in the determination of residence times. The upper-reach has less sediment than the lower-reach and a greater proportion of that are too young to reliably date by radiocarbon methods.
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