Water at Palo Alto Battlefield: Resacas Cut Off From Their River

The resacas at Palo Alto are usually dry except after heavy rain. National Park Service.

Palo Alto Battlefield National Historical Park lies in the Rio Grande Delta, a landscape of low ridges and short, curving former channels of the Rio Grande. These meanders, called resacas, were once part of a vast network of channels that spread the river's floodwaters across the delta.

The resacas used to flow seasonally with enough force to keep their channels open and carry sediment. Today levees, ditches and the Brownsville Ship Channel cut them off from the Rio Grande, and large dams upstream tame the floods that once carved new channels and created new resacas. Across the Lower Rio Grande Valley they now simply hold water. The City of Brownsville uses the larger ones to move irrigation water, catch stormwater runoff, and give the city a scenic backdrop. In the Park Service's words, the resacas at Palo Alto and in the lower delta are what is left of a river system cut off entirely from its natural function.

The park's two units

• Palo Alto Battlefield Unit, 3,400 acres. Its resaca remnants are relatively untouched, apart from livestock tanks and feeder ditches dug into their beds in the 20th century. They are usually dry but hold water after heavy rain.
• Resaca de la Palma Battlefield Unit, 34 acres. It sits within a resaca meander that the City of Brownsville maintains as an active waterway.

Heavy rain, such as that from tropical storms, can flood the resaca beds. National Park Service.

Trying to monitor the water

GULN hydrologist Joe Meiman made several trips to the park to judge whether a surface water quality monitoring program was needed and possible.

• Surface water. The first effort sampled the resacas after a large rainfall, repeating as the water fell over the following weeks. Every measurement varied too much to reveal any change over time, so the results were of little use to the park.
• Groundwater. A quick survey of existing wells found the larger, deeper wells held water year-round while the shallow ones were dry most of the year. The Millennium Engineers Group drilled a series of deep wells through the clay layer at the surface into a sand aquifer about three meters down; when they were drilled, two of the ten had water. Measurements in October 2010 showed the deeper water was salty. With deep wells holding water that barely affects surface resources and shallow wells holding none, groundwater monitoring would give the park no useful long-term data, though the measurements could help if the park decides to properly plug the wells.

National Park Service.

A partnership instead

Palo Alto Battlefield has no year-round surface water or natural shallow groundwater. Water does border much of the Resaca de la Palma unit, but the City of Brownsville owns and manages it, even though it forms an important cultural and scenic backdrop for the park.

In 2015 the University of Texas Rio Grande Valley (UTRGV) began a three-year project, Resaca Rangers, to monitor the health of the resaca ecosystem and measure the effects of the city's dredging. Teachers and students from several schools collected the data, gaining real-world science experience and a closer tie to their local environment.

The Park Service sees a partnership with UTRGV as the most practical route to long-term monitoring. An agreement would call for using GULN's water quality monitoring protocol and standard operating procedures and for sharing data. Students would get field experience, the park would gain a natural resource presence at Resaca de la Palma, and GULN would help draft the agreement and keep advising the park on water issues.

Sources

• National Park Service, "Hydrology at Palo Alto Battlefield." https://www.nps.gov/articles/000/paalhydrology.htm
• GULN water quality monitoring: https://www.nps.gov/im/guln/water-quality.htm