SEDIMENT DYNAMICS AFTER DAM REMOVAL: INVESTIGATING SHIELDS PARAMETER AND CHANNEL EQUILIBRIUM ON BULL RUN RIVER, VERMONT

Dams impact natural river processes, and dam removals should allow rivers to return to a more natural state. However, it is uncertain how long it takes for rivers to return to equilibrium post dam removal, and it has been unclear exactly how to determine if reaches are affected or not. This project used a dam removal in the Bull Run watershed, located in Northfield, VT, as a natural laboratory of a river with a recent disturbance to investigate the timing and magnitude of geomorphic changes within the former reservoir, downstream of the dam, and in downstream waterways. A 4-meter tall, 30-meter-wide dam was removed on Bull Run in 2020, 1.5 km upstream of the confluence with the larger Dog River. There were moderate flows from 2020-22 and an extreme flood in 2023.

We calculated Shields parameter (τ*) at 24 cross-sections from field-surveys of channel dimensions, slope, and sediment grain-size in 2021 and 2022. Previous work suggests that alluvial rivers are stable when Shields parameter is between 0.02< τ* <0.1. We found that 4 of 6 cross-sections downstream of the dam showed 0.02< τ* <0.1, suggesting the channel is not stable. In the former reservoir, 2 of 5 cross-section showed 0.02< τ* <0.1, suggesting an unstable channel. In addition, the Dog River showed 4 of 5 cross-sections had 0.02< τ* <0.1, suggesting the dam removal is not presently affecting the larger downstream waterways. The effects of the 2023 flood are yet to be determined. This project advances the strategy of using Shields parameter to assess channel stability after disturbances—either natural or human-caused. Furthermore, this project adds to understanding the timing and geographic extent of channel adjustments following dam removals, showing that immediate downstream channel adjustments may occur for several years, but larger receiving waters may show little effect.