Exploring Relationships Between Flow Variability and Low Flow Occurrence and Fish Assemblages in the Susquehanna River Basin

In 2009, The Nature Conservancy (TNC), the Susquehanna River Basin Commission (SRBC), and the U.S. Army Corps of Engineers (USACE) identified a set of literature-based flow recommendations for maintaining long-term hydrologic variability and ecological integrity in the Susquehanna River Basin (SRB). SRBC attempted to field-test a subset of proposed flow-biology relationships using biannual fish assemblage, water chemistry, land use/habitat, and discharge data collected from 17 minimally-altered streams in the SRB from 2010 through 2014. Synthetic hydrographs were developed for each stream using drainage-area ratio and linear regression techniques. Using estimated streamflow data, we calculated a suite of variables quantifying flow variability and occurrence of low flows prior to and during sampling events. Linear mixed-effects modelling techniques were utilized to model relative abundance of three flow-sensitive groups of fishes as well as macrohabitat generalist species. Optimal models for all response variables contained at least one flow variable, and models containing only flow variables performed better than models comprised of only water chemistry or only land use/habitat variables. Flow variables based on magnitude were better predictors than those based on variability. These results support recommendations made by TNC, SRBC, and USACE, and provide direction for future flow-ecology studies in the SRB.