The Rio Grande Wild and Scenic River

Authors: Kevin Urbanczyk, Jeffery Bennett

Year: 2019

Abstract

The Rio Grande Wild and Scenic River (RIGR) starts in Big Bend National Park (BBNP) and extends 315 km to the Terrell/Val Verde county line. This segment of the Rio Grande benefits from a base flow increase provided by the Edwards Trinity Plateau Aquifer (ETPA), which is part of a binational aquifer system. The base flow increases start in BBNP, and continue through the Lower Canyons and on to Amistad reservoir. We initiated monitoring of this system in 2005 and continue to conduct annual monitoring trips to assess water quality, flow, geomorphology and aquatic habitat. Comprehensive seepage runs completed in 2006 and 2011 suggest a flow increase of ~8 and ~9 m3/s for the entire segment. All of this can be attributed to ground water input as there are no perennial tributaries and the field work was completed during low flow conditions with no local atmospheric input to the river. We have also analyzed existing stream gage data between Johnson Ranch and Foster Ranch using only data from low flow conditions for a similar groundwater inflow assessment. This 1962 to 2011 estimate yielded an increase of 6.4 m3/s. This estimate is lower than the seepage run data because it includes a shorter segment and excluded some of the downstream springs. For comparison, 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 ~8.5 m3/s (217,000 ac-ft/yr; an average of the two seepage runs). The water quality in the segment improves as a result of the groundwater inflows. The specific conductivity (SC) of the springs varies from ~350 uS/cm up to a maximum of 1000 uS/cm while the SC of the Rio Grande above the segment is rarely lower than 1000 uS/cm. The groundwater inflows result in a dilution effect with an overall increase in water quality. This increase in flow and water quality support the conclusion made in the Upper Rio Grande Basin and Bay Expert Science Team report that the area represents a Sound Ecological Environment with a functioning aquatic habitat.