Where the Guadalupe River Gains and Loses Water: Floating Geophysics in Texas

Figure 1. The equipment on the Guadalupe River in September 2016: a GPS receiver and data-logging multimeter in the kayak, and a raft carrying electrodes 2 m apart. C. Hartman, USGS.

The question

In south-central Texas, how much water runs in the Guadalupe River matters to towns, farms, wildlife and recreation downstream. To understand the river's water budget and why its flow varies, you need to know where, and how fast, its water trades places with groundwater in the Carrizo-Wilcox aquifer below.

Where the river crosses the aquifer's outcrop, those exchanges were poorly known — and the usual method, comparing flow at different points to infer gains and losses, doesn't work here: upstream dams make the flow swing widely every day.

So the U.S. Geological Survey, with the Guadalupe-Blanco River Authority, tried geophysics in an exploratory study to find stretches where the river might be gaining or losing water.

Two floating methods

In 2016 the team measured about 15 kilometers of river with two floating instruments, towed behind a boat or fixed to a kayak. Both read the voltage between electrodes a fixed distance apart, just under the water surface.

• Electric resistivity tomography (ERT): the voltages relate to the geology under the riverbed and its electrical resistivity — roughly, fine versus coarse material.
• Self-potential (SP) profiling: the voltages relate to water moving through the ground.

What they found

Figure 2. (A) The SP profile and its four subreaches; (B) the ERT section beneath the channel, upstream at left. USGS.

Under the riverbed (ERT). Upstream, the Carrizo-Wilcox aquifer lay under just a few meters of alluvium; downstream, the bed turned to thicker alluvium and channel sands. The change came about where the aquifer goes from outcrop to subcrop — no longer at the surface — and probably where the Reklaw Formation begins.

Water in and out (SP). Numerical modeling of the SP profile showed where exchange was likely:

• a positive signal meant a net gain of water to the river;
• a negative signal meant a net loss — into the alluvium and channel sands, or the deeper aquifer.

The gains and losses probably combine regional exchange, with the aquifer as a whole, and hyporheic exchange — water moving in and out of the streambed and floodplain sands over short distances. Which dominated varied from reach to reach.

The biggest exchanges were over the aquifer outcrop:

Figure 3. The same SP profile mapped along the river, September 2016. Base from USDA National Agriculture Imagery Program imagery, 2016. USGS.

Why it matters

Knowing where a river gains and loses water, and how it connects to its floodplain and aquifers, helps water managers decide how to run the Guadalupe. The study showed that floating geoelectric methods can map aquifer properties and likely gain and loss zones faster and cheaper than traditional methods.

Next steps could measure the actual flow rates between river and groundwater, which shift with season, drought, flood and aquifer conditions, and pin down which exchanges are local and which regional.

Sources

Based on New Insights Into Surface-Water/Groundwater Exchanges in the Guadalupe River, Texas, From Floating Geophysical Methods, USGS Fact Sheet 2018–3057, by Scott J. Ikard, J. Ryan Banta and Gregory P. Stanton, U.S. Geological Survey, itself drawn from Ikard and others (2018), Journal of Environmental and Engineering Geophysics; a work of the United States government in the public domain. The data are in a USGS data release. Figures are taken from the fact sheet's PDF.