Benthic Macroinvertebrate Productivity and Community Response to Flow Alteration in the Gila River, New Mexico

Our study evaluates ecological implications of proposed diversion scenarios in the Cliff-Gila Valley reach of the Gila River, New Mexico, pursuant to the 2004 Arizona Water Settlements Act.  The relationship between BMIs and flow were evaluated in terms of 1) BMI productivity and 2) the composition of the BMI community metrics.  BMI productivity over space and time was assessed with the river benthos biomass model (RIVBIO).  We estimated productivity at several locations at a range of flows.  Productivity estimates were binned by mesohabitat within each reach and related to the distinct BMI communities that partition themselves among these mesohabitats.  We created fine-scale habitat maps from hydraulic modeling to refine predictions of biological response.  BMI community metrics were examined in the Gila River and comparable regional streams along a gradient of hydrologic alteration.  Potential causal relationships between hydrologic alteration and biological response were modeled with regression analysis.  The BMI community appeared to vary as a function of both high-flow magnitude and duration and low-flow conditions.  These BMI productivity and community metric responses to hydrologic alteration will factor into the New Mexico Interstate Stream Commission’s decision on whether or not to divert additional Gila River water.