IS POST-DAM REMOVAL STREAM CHANNEL ADJUSTMENT A SCALE INDEPENDENT PROCESS?

Dam removal has recently gained attention as a watershed and ecological restoration strategy in the United States. Post dam removal geomorphological adjustment of the stream channel has been documented in scientific literature at the watershed scale and lab scale, but insufficient data exist at the hillslope scale. The objective of this study is to examine whether stream channel geomorphological adjustment is independent of scale. The study was conducted over an 11-month period in 2014 on an eroding hillslope at the Valleybrook research facility, East Tennessee State University, TN. Accumulated sediments from hillslope gully erosion were trapped behind a 0.8 m X 12 m silt fence to mimic a reservoir-siltation scenario. Once full, the silt-fence dam was removed in three phases and the channel morphology was monitored as the channel adjusted to the dam removal. Channel morphology in terms of headcut erosion, and incision (width and depth) was monitored weekly at eleven transects perpendicular to the channel using a stadia and tape measure, and changes in channel sinuosity were observed. After dam removal, channel development was initiated by headcut formation which migrated at a rate of 0.24 cm/day upstream for about three months and then gradually reached attenuation. The channel progressed through four distinct stages. The wide, shallow, meandering channel (1^st stage: initial conditions and dewatering) became incised to a maximum depth of 0.51 m, and sinuosity decreased as flow was concentrated in a deep, narrow channel (2^nd stage: downcutting). Upon reaching base level, surface runoff began to meander within the channel, widening it through slumping and erosion of the channel banks (3^rd stage: floodplain development). The maximum width of the incised channel reached 0.46 m. Currently, the channel is in a state of widening and aggradation as some deposition has been observed (4^thstage: quasi-equilibrium). The stages of channel development and the headcut retreat pattern are consistent with other findings at the watershed scale.