Graduate Thesis Or Dissertation
 

Characterizing Sediment Flux in Headwaters of the Oregon Coast Range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/1n79hc404

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  • Abstract Valleys of headwater channels in the Oregon Coast Range impound significant amounts of sediment, with the majority of deposits originating as debris flows. Headwater valleys function as transitional sedimentary reservoirs linking hillslope sources in active orogens to sinks in sedimentary basins, yet the residence time of deposits in the channel is poorly constrained. While hillslope erosion rates constrain flux on a scale of millenia, and stream transport measures provide feedback on flux in real time, how long deposits reside in the valley between these two stages remains highly uncertain. This research uses measures of valley sediment in storage and radiocarbon dating of deposits from the 1999–2011 Lancaster creek surveys, covering five headwater reaches draining \approx11 km^{2} in contributing area, to estimate flux based on the turnover period of sediment in storage. Over the past 100 ky, hillslope uplift and erosion rates have been roughly equivalent in the central portion of the range, and estimates of flux over the turnover period have the potential to set a historic baseline against which we can measure anthropogenic impact, in land use and remediation. Improved estimates of deposit transit time through valley channels can help to explain the morphology and landforms evident in headwater reaches, an ongoing challenge in landscape evolution modeling, while impacts from flux events on downstream aquatic habitat and human water infrastructure make accurate estimates of flux from headwater reaches relevant for land managers, even in areas well downstream of steep terrain. Abstract Steeply dissected ridge lines and heterogenous topography prevent the probability of debris-flow deposition along the channel from being uniform, and the rate of flux implied by the presence of debris-flow deposits in surveys depends upon a prior expectation of delivery probability that varies along the channel. We use the observed record of deposits to optimize a predictor of delivery probability based on a debris-flow routing model developed by Miller & Burnett in 2008. The optimized predictor allows us to weight debris-flow deposit presence, such that volume in an unlikely location is indicative of a higher flux rate than an equivalent volume in a more likely location, other conditions being equal. Field observations confirm that debris-flow deposit likelihood increases with delivery probability, but indicate the relation is non-linear, with the density of deposits increasing sharply over a transition zone of probabilities from 0.009–0.019, going from half as common as average to more than three times as common. Abstract While radiocarbon dating is the most commonly-applied method to estimate the age of channel deposits, time that elapses between charcoal formation and deposition in the channel introduces a significant source of uncertainty. Among the 370 charcoal samples used for radiocarbon dating in the Lancaster creek surveys, 128 include multiple samples from single deposits, in order to capture the range of inherited ages in deposit charcoal. We use a reservoir model to estimate the mean transit time of deposits, given inherited age as a constraint. Fitting model results to observed charcoal ages, we characterize inputs and outputs as independent Poisson processes, reducing uncertainty from inherited age through decomposition of variance. Modeled mean residence time of debris-flow deposits ranges from 221–373 years, resulting in a flux estimate ranging from 0.108–0.231 mm yr^{-1}, which is sufficient to account for the majority of incoming sediment predicted by hillslope erosion rates, suggesting most denuding hillslope sediment spends a measurable amount of time in valley storage. Abstract The final portion of our research focuses on estimating the probability that gravels or fines in storage will exit from the valley within a turnover period. During transport events, sorting that occurs between gravels and fines results in different mean travel times through the valley for each fraction. We characterize traversal time of gravels and fines using their exit probability as the rate in a Poisson distribution. Model results indicate that debris-flow deposit sources are sufficient to account for observed ages of gravels and fines, provided an additional term to account for traversal time along the reach. Gravels in storage accrue an average of 1,200–12,500 years additional traversal time compared to debris-flow deposits, while fines in storage accrue an average of 80–7000 years in additional traversal time. Consistent with being the most mobile fraction of sediment, fines have a higher chance of capture into storage and shorter storage times, compared to gravels. Because gravel deposits produce new fines through comminution during traversal, fines in storage are often as old or older than gravels, and among the oldest deposits in the valley. By demonstrating that debris-flow deposits are sufficient to account for observed deposits of gravels and fines, and that debris flows are sufficient to account for the majority of hillslope denudation, this research provides independent confirmation that valley sediment storage exhibits a long term equilibrium between hillslope erosion and stream transport rates.
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