274-7 Groundwater Contributions to the Rio Grande in the Rio Grande Wild and Scenic River

Session: Advances in Ecohydrogeology

Presenting Author:

Authors:

Abstract:

The Lower Canyons of the Rio Grande are located downstream of Big Bend National Park within the Rio Grande Wild and Scenic River (RIGR).  This section spans 134 km (83 miles) from La Linda to Dryden Crossing.  Significant groundwater contributions to the river occur in the area mostly from the Edwards-Trinity Plateau aquifer (ETPA).  We have conducted numerous gain/loss studies in this area and estimate a total increase in flow in the Lower Canyons to be 144 Mm³/yr (116,000 AF/yr) while the overall groundwater contributions to the river that include other springs upstream in Big Bend National Park within the RIGR are 200 Mm³/yr (160,000 AF/yr).  We have also identified two groups of springs based upon water chemistry.  The upstream one is characterized by specific conductivity of 900 uS/cm and the downsteam one is more dilute with a value of 425 uS/cm.  Major structural features including the Bullis monocline and what appears to be a transpressional feature separate these different spring groups.  We have also identified similar chemical differences in a limited group of wells to the north and northwest.  Due to various factors including upstream diversion on both the Rio Grande and the Rio Conchos, it is not uncommon for there to be little or no flow upstream of the spring contributions.  During these times, the springs provide all of the flow in the river.  As a result, we consider this section of the river to be a ‘groundwater dependent ecosystem’ which is capable of supporting species such as the endangered Texas Hornshell (Popenaias pipeii, PP).  Typical aquatic habitat for the PP includes crevices in bedrock with streams of fine sediment and sufficient low TDS water quality (USFWS, 2018).  These features are found in the RIGR.  Using many years of repeat topographic cross section surveys, we have been able to document an influx of sediment at several locations within the Lower Canyons.  This is an unfortunate result of reduced flow due to flow regulations and represents a degraded habitat for the PP.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10362

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