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

Pool-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.