**Executive Summary: **The Big Bend reach of the Rio Grande is an important region for binational collaboration on ecosystem management and restoration. The river is well protected from further development as a national wild and scenic river and by national and state parks on both sides of the border, but has been degraded over time primarily by altered hydrology, water quality degradation, and non-native species impacts. The success of future restoration and management efforts depend on a fuller understanding of the linkages between physical and chemical processes and biotic communities. A robust monitoring program in the region and targeted research are needed to supply this understanding. In this report, we synthesize information on the river ecosystem to describe changes to physical and biological properties and highlight key uncertainties in responses of biotic communities to dynamic physical and chemical conditions. We also provide recommendations for a monitoring program to address these uncertainties in the future and suggestions for targeted food web and fish research projects.
We reviewed existing literature relevant to the Big Bend region, including reports and published research conducted in the region and elsewhere on the Rio Grande. We also discussed ongoing research with local experts in the region, and solicited their opinion on important data gaps through an online survey. Current understanding of the river ecosystem was summarized in a conceptual model emphasizing links between physical, e.g., habitat and water quality, and biotic components, e.g., fish and invertebrate communities. Existing data on fish communities, active channel width, hydrology, and water quality at several sites were collected and analyzed using a structural equation model to gain insight into the relative importance of different controlling factors on the fish community and to demonstrate the potential utility of a coordinated research effort.
