A 20‐year life history of a pool‐riffle couplet from formation to demise

Abstract

AbstractPool‐riffle couplet maintenance results from an unknown balance between hydraulic influences and sedimentological processes. Periodic monitoring of a study reach on the Blackledge River in Connecticut from 1999 to 2022 provides a unique data set that includes the full evolutionary history of a pool‐riffle couplet formed by a large‐wood (LW) jam. Displacement of the key LW forcing element during a 2021 event provides an opportunity to evaluate the relative importance of hydraulic versus sedimentological pool‐riffle maintenance mechanisms. A comparison of channel‐bed topographic and sedimentological data shows that hydraulic processes associated with the LW constriction dominated pool maintenance along this 90‐m reach. The existing pool lost more than half its residual‐pool depth and width and nearly 80% of the residual‐pool volume in response to changes in hydraulic conditions. The reduction in pool‐riffle amplitude was associated with a decrease in sediment sorting and homogenization of channel‐bed embeddedness. The results show that critical processes maintaining the forced pool‐riffle couplet for more than a decade ceased to operate when the key LW piece was removed by high flows.