Determining the impacts of two-hundred year old mill dams on in-channel sediment transport and carbon cycling of adjacent floodplains in northern Delaware

Abstract

Dams that allow the water to flow freely over the top of the dam and exert little control over the discharge of the stream are called run-of-river (ROR) dams. I document that ROR dams, unlike reservoir dams, are capable of transporting bedload through their impoundments because of high sand content lowering critical shear stresses for particle entrainment and the formation of a previously undocumented sediment ramp at the dam that allows large particles to roll up and over the dam. Because bed material can be removed from ROR dams in our study area, the accommodation space within the channel of the impoundment of the Barley Mill dam is only 25% filled (Chapter 2). I determine through modeling that ROR structures located on sandy-gravel-bedded streams with lower supply rates of gravel-sized bed material will be able to maintain unfilled impoundments and establish equilibrium transport where the flux of sediment entering the impoundment is equal to the flux of material leaving the impoundment. Furthermore, I determine that ROR dams on coarse gravel-bedded streams with high supply rates of gravel-sized bed material have the potential of filling before establishing equilibrium transport (Chapter 3). Lastly, I document that the long term presence of ROR dams does not statistically enhance carbon or sediment storage within floodplains. Removal of ROR dams does not have a prolonged disturbance signal such as enhanced CO 2 flux to the atmosphere. Impounded floodplains can become a source of CH 4 to the atmosphere for almost two months following a large flood event unlike similar riparian forested floodplain ecosystems that remain a sink of CH4 regardless of flooding (Chapter 4).