Print PageRiverine flow modifications (e.g., dams and managed flows), extirpations of native host fish, and water quality declines have resulted in freshwater mussels becoming a highly imperiled group with many species listed as rare, threatened, or endangered. Increasing water demands for human use is adding further stress because many mussels reside in waterways managed for withdrawal. To conserve freshwater mussels a better understanding of the habitat requirements and effects anthropogenic practices are having on these requirements is needed. We examined the ecological flow and bed stability needs for a metapopulation of federally endangered dwarf wedgemussel (Alasmidonta heterodon) in the Upper Delaware River, USA; a river regulated by two dams in its headwaters. Mussel surveys were conducted repeatedly between 2002 and 2010. Bathymetric surveys were conducted in 2005 and 2010 at three reaches containing mussel populations. Hydrologic variables (e.g., depth, velocity, shear) at a suite of flow events (e.g., low, average, flood) were quantified at a local scale (~1m) using a 2-D hydrodynamic model and summarized at two scales (mussel locations, mussel bed). Bed stability was measured as the difference in the 2005 and 2010 surveys; bed positions were georeferenced on this map in ArcGIS.
Results/Conclusions
Long term hydrograph records show average daily discharge of 43 cm3/s, frequent low events (<20 m3/s), short term management practices that result in extreme low flows (<15 m3/s), and several large floods (>1000 cms) between 2005 and 2010. Habitat conditions at mussel locations over all flow scenarios ranged from -0.45m (indicating exposed bed) to 10.12m (extreme flood) and averaged 1.12m. Shear velocity at these sites averaged 0.11m/s and ranged 0.0 to 0.51 m/s. Average column velocity averaged 0.44 m/s and ranged from 0.0 to 3.8 m/s. At low events numerous mussels experienced exposure. Persistent mussel beds averaged depths >0.50m even for extreme low flow events, while marginal beds at low events experienced depths <0.10m and large extents of dry, exposed areas. Average shear velocity in beds was 0.06 m/s for marginal and 0.04 m/s for persistent beds and never surpassed 0.27 m/s. Average bed velocity was 0.52 m/s for marginal and 0.41 m/s for persistent beds. Bed change analysis indicated a lost sub-population at an area that experienced severe scouring (0.25 to 0.75m) from 2005 to 2010. Results suggest dwarf wedge populations are sensitive to local habitat affected by low-flow events and require stable habitat; an effective management plan for this species must account for these requirements.
