Integration of a Technical Fishpass and Eroded Natural Channel to Restore Upstream Fish Passage, Somes Brook, Somesville, Maine

A pool-and-weir fishpass was constructed in 2007 at the second dam on Somes Brook in Somesville, Maine to replace a deteriorated pool-and-weir fishpass that was no longer upstream-passable for the target species (river herring [Alosa spp.]).  Water discharging from a failed-section of a former fishpass approximately 30 feet (ft) downstream from the adjacent dam had eroded an approximately 70-ft-long channel adjacent to the fishpass. Observations indicated that river herring regularly swam upstream along this channel to the failed section of the former fishpass, but were largely prevented from moving further upstream due the presence of a 3- to 4-ft hydraulic drop.

Restoration of fish passage at this site included construction of a labyrinth pool-and-weir fishpass within the footprint of the upper 30-ft-long segment of the former fishpass and use of the naturally eroded channel for upstream passage to the hydraulic outlet of the new fishpass.  The total hydraulic drop through this fishpass system is approximately 11 ft, including 6.4 ft over the 70-foot-long natural channel section and 4.7 ft over the 30-ft-long technical fishpass section.  The streamwise slope of the new fish passage system is in excess of 0.1%, and therefore necessitated a means to provide for reasonable upstream passage characteristics for the target fish species; however, the proximity of regulated historical and natural resources precluded expanding the footprint of the fishpass.  The use of a labyrinth weir in lieu of an in-line fishpass was necessitate by typical fishpass design criteria, including energy dissipation, maximum hydraulic drops (0.5 ft), and the desire to provide for upstream passage of non-target species, including brook trout (Salvelinus fontinalis), at low flows.  The natural channel section of the fishpass has a relatively steep slope (0.16%), but the high degree of observed heterogeneity, and previous observations of upstream passage by the target fish species, alleviated concerns related to the suitability of this channel for use as part of the upstream fish passage system.  Integration of a technical fishpass with the eroded channel allowed for restoration of upstream fish passage while working within significant site constraints dictated by project proximity to historical and