| Using dams as natural experiments that present a discrete disturbance in sediment flux, I quantify the state of fluvial equilibrium below six run-of-river dams and one impoundment dam ranging in age from 59 to 94 years old. Previous studies of run-of-river dams show inconsistent results concerning changes in downstream channel morphology. Here, I augment these investigations by analyzing the response of covariant channel parameters to regulation following dam emplacement. Equilibrium channel form is defined as one where, absent of tributary inputs, sediment flux is constant in the downstream direction and hence the channel is neither aggrading nor degrading. Equilibrium form for bedload-dominated channels is evaluated using the bankfull channel Shields parameter. Upstream and downstream bankfull Shields values were calculated at 8 to 14 cross-sections extending 200--450 m upstream and downstream of each dam. While statistically significant differences in channel slope (p = 0.015) and width (p = 0.038) were found at two run-of-river dam sites, no statistical difference was found between any upstream and downstream bankfull Shields values, suggesting that all run-of-river sites evaluated here are in a state of equilibrium. Evaluation of the impoundment dam found the downstream reach in equilibrium, with no statistical difference between the upstream and downstream bankfull Shields value; however, three downstream site parameters were statistically different from upstream: bankfull channel slope (p = 0.022), width (p = 0.016), and flow depth (p = 0.008). This difference emphasizes the importance of considering covariant changes in channel parameters in response to disturbance. Despite elevated sediment trapping behind the impoundment dam, the downstream reach has established an equilibrium form in the 59 years since dam emplacement. While this unique method for quantifying equilibrium channel form can be applied to a wide range of geomorphic disturbances, the dominant mode of transport must be unchanging along a reach or accounted for by comparing only transects under similar transport regimes. As the dominant transport mechanism shifts, so too do the parameters used to identify equilibrium, including the bankfull and critical Shields values. Here, a shift in transport regime from bedload to mixed-load dominated transects resulted in elevated Shields values. |
