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Saving the River by Flying the Skies

Environmental Stewardship

The San Marcos River is the beating heart of Texas State University.

From recreation to research, the river is central to our community, so protecting it is a deeply held shared value.

drone shot of the river in Sewell park

In addition to its value to the human community, the river supports the survival of eight threatened or endangered species and serves as an indicator of the health of the Edwards Aquifer, which supplies drinking and irrigation water to millions of Central Texans. One major threat to the river is an innocuous-looking but extremely invasive aquatic plant species with an appropriately monstrous-sounding name: hydrilla. Texas State University researchers are combatting hydrilla’s expansion in the upper San Marcos River using a technology not normally associated with aquatic restoration: drones.

Dr. Thom Hardy, endowed professor of Environmental Flows in the Department of Biology, and chief science officer at the Meadows Center for Water and the Environment, has worked with drones since the mid-1980s, back when the only commercially available drones were military surplus, costing hundreds of thousands of dollars. Hardy and coresearchers envisioned an affordable, field-deployable drone for remote sensing of natural resources and agriculture. He collaborated with a team at Utah State University to design DIY airplane-style drones, built from RadioShack parts, called “Aggie Air” and “Minion.”

texas wild rice

Although Hardy still uses these platforms for certain projects, the Edwards Aquifer restoration project requires low-altitude surveillance in a restricted airspace. That mission is perfectly suited to inexpensive, off-the-shelf “quad-copter” drones that are now widely available. Research associate Tom Heard, an aquatic biologist at the Meadows Center, explains the process: “We fly a reach of the river, maybe 300 meters. Our quad-copters use a pattern to take photos, and then we mosaic them into an image that is geo-rectified, so that it is true to the location.” These drone-mounted cameras can see straight through the San Marcos River’s crystal-clear waters, revealing patches of native and non-native vegetation underneath, including hydrilla.

hydrilla

Researchers believe hydrilla invaded the San Marcos River as a result of local residents dumping their fish tanks, but the species has thrived to such an extent that it threatens the entire ecosystem. A contract with the city of San Marcos, pursuant to the Edwards Aquifer Habitat Conservation Plan, tasks Hardy’s team with eradicating hydrilla from the upper San Marcos River and replacing it with restored beds of the endangered and endemic species Zizania texana, commonly known as Texas wild rice.

Texas wild rice grows in only one place on earth: the upper San Marcos River. It forms essential habitat for other endangered species such as the fountain darter (a small perch-like fish) and the Texas blind salamander, among others, but hydrilla’s spread risks crowding Texas wild rice out of existence.

2027 – Goal for when hydrilla will be completely eradicated from the upper San Marcos river

Using drones to identify hydrilla beds in the river is only the first step in eradication. The standard hydrilla eradication strategies used in lakes – killing off everything in the lake and reseeding it with native species – won’t work here. “Because of the endangered species, we can’t use mechanical or chemical strategies,” Heard explains. Instead, they remove the hydrilla and replant Texas wild rice by hand.

Pulling up the hydrilla is a painstaking, laborious process. On a near-daily basis, teams of faculty, staff, and volunteers from departments across the university jump into the river to pull up the hydrilla, strand by strand. Some volunteers wade in the river, while others use snorkeling or SCUBA gear to remove the invasive plants from the roots up.

The team has had to adapt, and is now “working on an upstream to downstream path,” because, according to Heard, if they leave any plants intact upstream while clearing areas downstream, “it’s so invasive it’ll take over the area you just cleared.” The massive quantities of vegetation they remove are composted.

drone shot of the river and meadows center

The team is roughly one third of the way through the near-Sisyphean task of clearing the targeted length of the upper San Marcos River, which stretches from Spring Lake to the Cape’s Dam, an area roughly 5 kilometers in length and covering 150,000 square meters. Hydrilla is such a robust foe that eradication will require persistence and continued innovation. Even with an aggressive removal strategy the team estimates that total eradication will take until 2027.

Undaunted by this challenge, Hardy and his team look forward to extending their eradication efforts to Spring Lake, as well as expanding new research applications for their drones. “The breadth of application for drone research is geometrically increasing,” Hardy says, “We’ve used drones for everything from agriculture to natural resources, from monitoring bird colonies to fish to lizards, from riparian vegetation to geomorphology, from aquatic restoration to conservation under drought conditions. The only limitation is what remote sensing allows you to do.”

Given Hardy’s dedication to technological innovation and environmental conservation, when it comes to Texas State’s drone research, the only real limit is the sky itself.

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