Stream bank legacy sediment contributions to suspended sediment and nutrient exports from a Mid-Atlantic, Piedmont watershed
Date
2019
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Publisher
University of Delaware
Abstract
While stream bank erosion has been increasingly recognized as a major source of fine sediment to watersheds, little research has been done on stream bank nutrient concentrations and their potential contributions to watershed nutrient budgets. This uncertainty is exacerbated by the ubiquitous presence of legacy-sediment impacted stream banks in the Mid-Atlantic region of the United States. This study applies the established sediment fingerprinting technique to quantify sediment source contributions to storm suspended and stream bed sediments in the Big Elk Creek, an agricultural Mid-Atlantic tributary of the Chesapeake Bay. Source sediments were collected from agricultural, developed, and forested soils as well as stream banks within the watershed, and ten storms were sampled at three locations within the watershed over the study period of August 1, 2017 to July 31, 2018. Stream bed samples were also collected once a month from each of the three field sites, to investigate if and how stream bed sediments interact with storm-mobilized sediments. We then applied the sediment fingerprinting technique to apportion sediment-bound nutrient yields into their constituent sources using Sed_SAT, a sediment fingerprinting toolbox developed by and available through the United States Geological Survey. ☐ Our key findings are first: stream bank sediments constitute a significant source of sediment to storm suspended and stream bed sediments; second: although stream bank contributions are generally higher during the Winter and Spring months, source sediment contributions are largely uncoupled from streamflow conditions; and third: we find that stream bank erosion accounts for 44% of watershed sediment yields by percent contribution, and 50% of sediment yields by mass. We further find that stream banks have significantly lower nutrient (carbon, nitrogen, and phosphorus) concentrations than the sampled upland sediments, and thus only contribute 32% of sediment-bound carbon, 26% of sediment-bound nitrogen, and 32% of sediment-bound phosphorus to watershed nutrient yields. Our findings of 50% stream bank contribution to sediment yields and 32% contribution to phosphorus yields are comparable to Phase 6 Chesapeake Bay Model results for the Big Elk Creek. ☐ This thesis expands upon our current understanding by quantifying stream bank contributions to watershed sediment and nutrient yields. In-channel pollutant generation from stream banks and beds is currently neglected or overlooked in most watershed management and restoration practices. The results of this thesis indicate that stream bank sediments and nutrients must be accounted for in watershed models, best management practices, and total maximum daily loads to ensure that pollutant mitigation and watershed restoration efforts are not mistargeted.