ENVIRONMENTAL ASSESSMENT OF THE RIO GRANDE IN BIG BEND NATIONAL PARK AND THE RIO GRANDE WILD AND SCENIC RIVER

The Rio Grande Wild and Scenic (RIGR) reach, located in the Big Bend region of far west Texas, is one of the most significant groundwater-dependent aquatic ecosystems in North America. However, diminishing flows and altered sediment dynamics have resulted in decreased conveyance capacity, increased flooding, decreased aquatic habitat and decreased water quality. These trends are especially evident in the upper portion of the reach. A particularly important aspect of the aquatic habitat of the RIGR is its ability to support the reintroduction of the Rio Grande silvery minnow (RGSM). The RGSM was extirpated from the region in the 1960s, and has recently been reintroduced at several locations in the RIGR. The recent Upper Rio Grande Basin to Bay Expert Science team noted the lack of biological data related to flow and other habitat conditions in the RIGR. With a multi-institutional and multi-disciplinary team, we have assessed various aspects of the river in the RIGR reach, including the interactions between flow, water quality, geomorphology and sediment mobility, and aquatic habitat. Results include the quantification of the gaining reach between the stream gages at Johnson Ranch and Foster Ranch (~6 cms/212 cfs) concomitant with an improvement in water quality. These results come from numerous synoptic gain/loss studies, water quality studies, and from analysis of stream gage data. These improvements to the overall condition of the river (more flow and less salinity) are the result of base flow contributions to the river from the Edwards Trinity Plateau aquifer and a related aquifer in adjacent Mexico. The aquatic habitat of the RIGR has been assessed through numerous down-river trips to sample for fish and invertebrates and associated parameters (water depth and velocity and basic field water quality parameters). Results include the recent discovery of the RGSM 57 river miles downstream from a reintroduction site near Dryden and its affinity for silt and sand substrate and backwater conditions. The invertebrate sampling confirms the general observation of downstream improvement of water quality indicated by an increase in the Index of Biological Integrity median scores.