Measuring Floodplain Hydraulics of the Frio River Where it Overlies the Edwards Aquifer

Author RT Green, FP Bertetti, and R McGinnis
Year 2009
Description Structural and hydrogeological survey of the Frio River to determine if water table underflow contributes to the water budget of the Edwards Aquifer Recharge Zone
Report Number 20-14152
Publisher Southwest Research Institute®
Location Frio River, Dry Frio River, Edwards Aquifer Recharge Zone
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The purpose of this study was to reduce uncertainty in recharge calculations involving the Frio and Dry Frio rivers by investigating their hydraulic relationship with the Edwards Aquifer. Geophysical imaging of the subsurface of the floodplains of the Dry Frio River and the Frio River did not detect evidence of significant subsurface flow through either paleo-channel deposits or preferential flow pathways developed in the bedrock. In addition, nearby waterholes and wells contain water from infiltrating rainfall and river flow events or upward leakage of relatively saline water.  The source of the water is either the Edwards Aquifer or the Austin Chalk rather than subsurface water flowing in the floodplain. Consequently, surface water gauging of river flows in the Frio and Dry Frio Rivers accurately represents the true volume of recharge of the Edwards and related aquifers by recharge in the river channels. The study was completed in May 2009 by Southwest Research Institute, San Antonio, Texas.

[Excerpted from the report’s Conclusions]
…[T]he Frio and Dry Frio Rivers are assumed (Hamilton et al., 2008) to be hydraulically connected with the Edwards Aquifer in the Edwards Aquifer recharge zone. Both rivers exit the recharge zone about six miles north of the City of Knippa. There is no evidence of underflow in the Frio and Dry Frio River floodplains. This indicates that surface water flow gauging, if performed immediately before and after the rivers enter and exit the recharge zone, should accurately represent the amount of water recharged to the Edwards Aquifer. Supporting this conceptualization is that the source water for all four waterholes is interpreted to be from depth as springs and that there is no indication of floodplain underflow.