Natural pool-riffle formation and maintenance by large wood in a gravel-bed river impacted by instream structures installed in the 1930s and 1950s

Pool-riffle maintenance has been document in numerous studies, but it has been almost impossible to characterize detailed natural pool-riffle formation mechanisms because of the lack of baseline data prior to pool formation. In 2013, a study was conducted on the Blackledge River in Connecticut to document the formation of a new pool-riffle couplet on a section of river that had previously been studied from 1999-2001. The Blackledge River is a south flowing tributary of both the Salmon River and Connecticut River with a drainage area of 59 km2, channel slope of 0.0038, bankfull width of 17.8 m, a sinuosity of 1.05 and d50 values ranges between 37 mm and 194 mm. In the 1930s and 1950s, instream structures were installed along the river as part of first habitat improvement and later channel relocation effort, which limited riparian tree growth and channel migration. In 2001, the study reach contained a shallow scour hole with a residual depth of 0.27 m at a 1930s paired deflector with no identifiable downstream riffle. At this time, a large, severely undercut, hemlock tree was also noted along the left bank. Sometime between fall 2001 and April 2009, the hemlock fell perpendicular to flow across the channel and formed a LW jam and new pool-riffle couplet slightly downstream of the old scour hole. Pool spacing along the reach decreased from 4.47 BFW in 1999 to 3.83 bankfull widths (BFW). The pool has a residual depth of 1.40 m, which resulted from a combination of approximately 0.96 m of incision below the old scour hole (85% of the depth increase) and 0.18 m of downstream deposition and associated backwater formation (15% of the depth increase). The 23-m-long pool stores 25.7% of the sand-sized sediment and 15.4% of organic material along a 214-m long reach that includes one additional artificially created pool. An adjacent 50-m-long secondary channel impacted by the LW jam stores 65.3% of the sand-sized sediment and 54.8% of organic material along the full reach.