A TWENTY-YEAR LIFE HISTORY OF A LARGE-WOOD FORCED, POOL-RIFFLE COUPLET FROM INCEPTION TO ERASURE

Pool-riffle couplet maintenance results from an unknown balance of hydraulic and sedimentological processes. Most studies of pool-riffle morphologies focus on maintenance of existing bedforms because it has been almost impossible to characterize detailed natural pool-riffle formation because of the lack of baseline data prior to undulation establishment. For example, Edward Keller (1971) famously proposed a reversal in the relative location of maximum bed velocities from riffles to pools as stage increases to explain existing pool-riffle maintenance. Periodic monitoring of the Blackledge River in Connecticut from 1999 to 2022 provides a unique data set that includes the full evolutionary history of a forced pool-riffle couplet formed by a large-wood (LW) jam from inception (formed between 2001 and 2004) to eventual demise of the pool in 2022. It was hypothesized that flow constriction by the LW with localized increases in velocity at high flow, a velocity reversal, provided the dominant pool-riffle formation and maintenance process. The pool had a residual depth of 1.36 ±0.03 to 1.48 ±0.03 m from 2013-2019, which resulted from a combination of 95.6% or less of incision below the old channel bed and up to 13.2% or less of depth increase due to downstream deposition and associated backwater formation. During the 2021 water year, a series of four high-flow events pivoted and then displaced the key LW piece. Although the pool-riffle couplet had been dynamically stable for more than a decade, massive aggradation less than a year after displacement of the LW resulted in a reduction of residual-pool volume of almost 80%, a loss of half its residual-pool depth and width, and a reclassification of the feature from a pool to a smaller scour hole. The reduction in pool-riffle amplitude and associated changes in hydraulic conditions was associated with a decrease in sediment sorting and homogenization of channel-bed embeddedness. The sequence of changes highlights the dominance of constriction-dominated, hydraulic processes in forming and maintaining the pool-riffle couplet, and the inability of sedimentological differences or persistent hydraulic conditions in pools versus riffles to maintain the feature once the forcing LW piece was removed.