A mechanistic understanding of channel evolution following dam removal

Studies of post-disturbance recovery in alluvial channels have considered form- and/or process-based paradigms to describe pathways to dynamic equilibrium. Yet, an integrated understanding of the covariation between form and process as well as the mechanisms that drive the recovery of each remains incomplete. We used a small (4 m tall) upland dam removal in New England as a natural experiment to investigate the timing and arc of a channel's return to dynamic equilibrium conditions, using both form-and process-based perspectives. We observed rapid post-removal adjustments in the former reservoir and downstream channels, then relative stability after a 2-year flow that occurred 6 months post-removal. This pattern, previously described as the two-phase model of channel recovery, was the result of temporal variations in the ratio of Shields values at the 'bankfull' flow (theta(Q2)) to critical Shields (theta(cr)). Immediately post-dam removal, elevated sediment flux was driven by the high ratio of theta(Q2) to theta(cr) that resulted from decreased theta(cr) values and elevated theta(Q2) values. As fine-grained sediments were winnowed, theta(cr) quickly recovered back to typical values within weeks to months. Subsequent adjustments to channel geometry by higher flows (i.e., the 2-year flow) redistributed bed shear stress and facilitated the relaxation of theta(Q2) so that theta(Q2): theta(cr) approximate to 1, the threshold channel criterion. Despite the rapid return to process-based equilibrium, nearly 3 years post-removal the former reservoir reach remains morphologically immature with respect to reach-scale characteristics of natural channels. Our results indicate that process-based recovery is rapid, facilitated by relatively frequent, modest flows, and dependent on the adjustment of the characteristic bed grain size (d(50)) to reflect prevailing hydraulics. In contrast, the slower attainment of form-based equilibrium depends on the timing of less frequent flows equal to and/or larger than the bankfull discharge as well as extrinsic channel controls such as local valley topography and the availability of recruitable large wood. (C) 2021 Published by Elsevier B.V.

Automated summary

Studies on post-disturbance recovery in alluvial channels have shown that the relationship between form and process, as well as the mechanisms driving recovery, are not fully understood. After the removal of a small upland dam in New England, rapid adjustments were observed in the former reservoir and downstream channels followed by relative stability after a 2-year flow, indicating a pattern of two-phase model of channel recovery.