Suspended sediment, dissolved organic carbon, and dissolved nitrogen export during the dam removal process

Abstract

Total suspended solids (TSS), dissolved organic carbon (DOC), and total dissolved nitrogen (TDN) loads were calculated for all stages of the dam removal process (dewatering, breaching, and removal) at various points upstream, within, and downstream of Lowell Mill Impoundment on the Little River, North Carolina. The impoundment dewatering exported loads of TSS, DOC, and TDN which were all 1–2 orders of magnitude less than loads associated with historic floods. Conversely, floods exiting the former impoundment following dam removal produced TSS, DOC, and TDN loads comparable to, but slightly greater (1.2–1.75 times) than historic floods. Exported loads were greatest following the complete removal of the dam, most likely because of increased channel gradient. We assert that the disturbances (i.e., concentrations and loads) associated with dam removal should be compared to those generated by floods within the same system rather than comparing the impacts of dam removal with base flow conditions. During the dewatering, impounded floodplain wetlands were found to contribute the following percentages to total impoundment loads: 44% of stored water, 12.6 % of TSS, 49% of DOC, and 33% of TDN. Moreover, the dewatering flood wave was sampled at various points along a 19.2‐km reach below the dam to characterize the routing of TSS, DOC, and TDN. TSS released by the impoundment was retained within 10 km of the dam, while TDN and DOC loads increased slightly. Finally, we used our results with those from other removals to provide insight into regional and morphologic controls on exports of impounded materials following dam removal.