THE RIO GRANDE WILD AND SCENIC RIVER - A GROUNDWATER DEPENDENT ECOSYSTEM

The Rio Grande Wild and Scenic River (RIGR) is located in far west Texas. The RIGR is 315 km in length and starts in Big Bend National Park (BBNP). It is one of the most significant groundwater dependent ecosystems in North America because it benefits from a base flow increase from the Edwards Trinity Plateau Aquifer (ETPA; part of a binational aquifer system). The base flow increases start in BBNP and continue down-river through a remote region known as the Lower Canyons and on to Amistad reservoir. We began monitoring this system in 2005 and conduct annual monitoring trips to assess water quality, flow, geomorphology and aquatic habitat. The baseflow contributions from the ETPA have been estimated using synoptic seepage runs covering the reach above Amistad reservoir in 2006 and 2011 (7.95 and 9.17 m³/s, respectively). The seepage runs were completed under low flow conditions and during times of little or no precipitation. There are no perennial tributary streams in the reach. Additional estimates have been made by analyzing stream gages above and near the end of the reach (Johnson Ranch to Foster Ranch; a shorter reach than covered by the seepage runs). A long-term estimate (1962 to 2011) using only low flow conditions (to eliminate effects of upstream variations) yields an average of 6.4 m³/s while individually selected, shorter term, low flow estimates reveal the same average. These estimates are less than the seepage run estimates because they represent a shorter segment of the overall gaining reach. A 1944 water treaty between the US and Mexico stipulates 13.7 m³/s (350,000 ac-ft/yr) in the Rio Grande. The ETPA in the RIGR contributes an additional 8.4 m³/s (215,000 ac-ft/yr).

The increase in flow is accompanied by an improvement in water quality. The springs have a specific conductivity (SC) of ~300 to 1000 uS/cm while the SC of the Rio Grande above the reach is variable, but rarely is lower than ~1000 uS/cm. This results in a dilution effect that represents a reduction in salinity. The increase in water quality contributes to an improved aquatic habitat as indicated by an increase in the Index of Biological Integrity median scores. An important aspect of the water quality increase is its ability to support the reintroduction of the Rio Grande Silvery Minnow which was extirpated in the 1960s.