Date of Award

1-1-2010

Document Type

Campus Access Thesis

Department

Geography

First Advisor

Jean T. Ellis

Abstract

Eighty-five percent of dams in the United States will reach the end of their operational design lives by 2020, thus driving the recent shift towards planning dam removals as an alternative to repair and maintenance. Like the dams themselves, dam removal impacts stream morphology and chemistry, and these impacts are hard to predict without a comprehensive (spatially and temporally) analysis of the pre-dam removal fluvial system. This thesis investigates the linkages between the physical and chemical systems upstream and downstream of two small dams in eastern Virginia planned for removal. Data were collected during twelve sampling trips between 9/2007-10/2009 at one upstream control site (Site 1) and three progressively downstream impact sites (Sites 2-4) of the dams on an adjacent stream as part of a BACI analysis. Channel survey measurements indicate variability in channel morphology, which is most likely attributed to tidal influences and measurement error. Student's t-test results suggest that Sites 1-4 are not statistically different regarding suspended sediment (D50 and concentration). Sites 1, 2, and 4 are not statistically different regarding total phosphorus, but Site 3 is statistically different from the other sites. No statistical relationship exists between suspended sediment and total phosphorus. Two sediment cores were collected, one from each reservoir created by the dams, for 210Pb analysis using a slope regression (SR) model and the constant rate of supply (CRS) model. Estimated average sediment accumulation rates for Core 1 are 0.1665 (SR) and 0.1882 g cm-2yr-1 (CRS), and for Core 2 are 0.1335 (SR) and 0.1725 g cm-2yr-1 (CRS). Linear regression of excess 210Pb and cumulative mass depth using SR data and change in CRS and SR-derived sediment accumulation rates were used to estimate dam construction dates. Topographic maps and aerial photographs validated the CRS-derived change in sediment accumulation rate method and suggest dam construction between 1953 and 1957. Approximately 364,500 kg and 49,184 kg of sediment has accumulated in the lower and upper reservoirs, respectively, since dam construction. Understanding pre-dam removal conditions is required for quantifying the impacts of dam removal. In the next phase of this project, data presented in this thesis will be compared to data collected after the dam removal.

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