The longer arc of channel recovery post-dam removal

Abstract

Dam removals are opportunistic experiments to address fundamental questions about river recovery to disturbance. Previous studies have shown that gravel-bedded rivers are resilient with covariate adjustments to channel dimensions occurring rapidly in the wake of disturbance. Yet, beyond the cross section, at the reach or watershed-scale, adjustment appears to take much longer. Understanding of the longer arc of reach and watershed recovery is limited by the relatively few dam removals studies that include long-term monitoring. Here, we present results from a five-year dam removal study punctuated by an extreme flood and show that the initial, rapid response of a channel is driven by the prevailing hydrology whereas longer-term adjustments to morphology at the reach scale are driven by external forces imposed on the channel. We summarize these results by classifying various channel features as either ‘intrinsic channel properties’ that are rapidly adjustable by the prevailing hydrology or ‘extrinsic channel properties’ that respond over various time scales to external boundary conditions imposed on the channel by climate, vegetation, geology, and valley dimensions (extrinsic controls). We show that this framework applies to channel recovery beyond the former reservoir and thus may prove applicable to channel disturbances beyond dam removal, such as extreme floods.