NATURAL POOL-RIFFLE FORMATION AND MAINTENANCE IN A WATERSHED IMPACTED BY DEFORESTATION AND HISTORIC CHANNEL MODIFICATIONS

Numerous studies have documented channel-maintenance processes in existing pool-riffle couplets formed by natural processes. However, it has been almost impossible to characterize detailed pool-riffle formation mechanisms because of the difficulty of accurately predicting the location where a new pool-riffle couplet will form. The Blackledge River in Connecticut contains a reach within Veterans Fishing Area that has been heavily modified by stream-improvement devices installed by the Civilian Conservation Corps in the 1930s and by the Connecticut Department of Transportation in the 1950s. The area was also impacted by historic deforestation. The combination of these factors has limited large-wood (LW) loading to the channel, with potential impacts on channel morphology. Previous research done at the site in 1999-2001 documented pool-riffle conditions associated with historic stream-improvement devices, including a paired set of triangular deflectors. In 2001, a pool with a residual depth of only 0.27 m was measured just downstream of the deflectors, no riffle was observed and erosion of the left bank was noted with severe undercutting of a large hemlock tree. Years later when the site was revisited, it was noted that the hemlock tree had fallen completely across the channel forming a spanning LW jam. The site provides a unique opportunity to document how pool-riffle couplets form from a relatively flat-bedded channel. Topographic surveys indicate a new residual depth of 1.4 m. Repeat surveys and a bed characterization reveal a complex sediment-sorting pattern and sediment storage system operates in the pool.