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 In 1998 two fast-growing aquatic plants, hydrilla and water hyacinth, almost choked the Rio Grande along the Texas-Mexico border. The two plants had massed together to form a dam that interfered with deliveries of river water to communities and farmers. So dense was the mass that operators of the river's two main water-supply reservoirs, Amistad and Falcon, had to increase releases by as much as 30 percent to ensure customers received adequate amounts of water.
The Rio Grande is hardly alone in the threat posed by "nuisance plants," the innocuous term for aquatic vegetation choking a growing number of water bodies in Texas, other states and abroad.
These nonnative aquatic plants have been introduced from other parts of the country or world. Without their native, natural enemies to keep them in check, these plants can spread quickly in lakes, ponds and rivers, interfering with the use of water for irrigation and drinking water supplies, power plants, and recreation. The tools available for management and control can be costly and controversial.
Situation will only get worse, experts say
What is the experts' prognosis?
It's only going to get worse, said John Wedig, senior aquatic scientist at the Lower Colorado River Authority, which oversees water quality along the lower portion of the Colorado River of Texas. There's no single approach to controlling nuisance plants that's effective and acceptable to the public.
The lack of a single, effective knockout punch complicated a solution for the Rio Grande, especially dissimilarities in herbicides registered on either side of the U.S.-Mexico border, according to Dr. Earl Chilton, director of the Texas Parks and Wildlife Department's Aquatic Habitat Enhancement Program.
From 1998 to 2003 the three management tools used south of McAllen — mechanical harvesting, herbicides and sterile Asian grass carp — helped water managers get a handle on the problem, Chilton said. Upstream, however, between McAllen and International Falcon Reservoir, hydrilla is an increasing problem.
Lake Austin struggles with hydrilla
The battle against hydrilla in Central Texas' Lake Austin is a microcosm of how multiple water uses can complicate a treatment strategy. Created by damming the Texas Colorado, Lake Austin is popular for boating and fishing. Expensive homes line the lake's shores. And the lake is a primary source of Austin's drinking water.
Hydrilla was discovered in the lake in 1999 and began to spread quickly. The City began discussions with Texas Parks and Wildlife Department, LCRA and a private group of residents, Friends of Lake Austin, on a solution. They considered four options but found that no single approach was satisfactory:
- Mechanical harvesting is slow, cumbersome, extremely expensive and may spread the weed.
- Lowering the lake in winter to expose hydrilla to dry, breezy weather can hinder the operation of lakeside businesses. The exposed parts of the plant die, but the roots do not. So the plant eventually grows back.
- Biological solutions, such as the introduction of sterile grass carp, are opposed by bass fishermen who cite examples of large populations of the carp being introduced into lakes, eating almost all aquatic vegetation, and destroying fish habitat.
- The use of herbicides is costly and unacceptable to residents who depend on the lake for drinking water.
The City, LCRA and Parks and Wildlife eventually agreed on an integrated plan that included harvesting, drawdowns and grass carp in a controlled and measured fashion to eliminate or reduce any drawbacks. The City and LCRA carry out the plan, with the oversight of Parks and Wildlife.
It is a continuing struggle. The hydrilla coverage declined after the force of floodwaters in July 2002 scoured the lakebed, ripping up dense mats and flushing them downstream. Since then, the hydrilla has been growing back steadily, although the city has noted some success in fighting the plant at some locations in the lake. As of the end of the year the City has released about 8,000 grass carp into the lake. Because these fish are sterile, they do not reproduce, thus eliminating one concern about the proliferation of plant-eating fish denuding the lake of aquatic vegetation.
Major aquatic threats to Texas waterways
Hydrilla is the most widespread exotic weed in Texas, found in about 100 reservoirs. It is a submerged plant, growing from the bottom. If fragments break loose they may survive, root and produce new plants. They have potato-like tubers that separate from the plants and may remain dormant for 10 to 12 years before germinating.
Even if you think they're killed, thousands of tubers could sprout at any time, Chilton said.
Water hyacinth is a chronic problem in East Texas water bodies like Caddo Lake, Lake Livingston, Toledo Bend, Lake Houston and Lake Texana. Lake Texana, a coastal reservoir, hosts a smorgasbord of aquatic plants, Wedig said. Hydrilla, water hyacinth and giant salvinia grow in this lake that is used for recreation and water supply. Sterile grass carp, herbicide and a water hyacinth shredder have been used to help control the vegetation.
A new front in the fight against invasive species has opened at Toledo Bend, a 186,000-acre reservoir on the Texas-Louisiana border. The offender is giant salvinia, a floating plant that spreads quickly, forming layers of mats that shut out sunlight and smother other plants. It is also found in private ponds and other water bodies in the Houston area and further west in Lake Texana.
Giant salvinia is the number one thing we need to track and get rid of, said Dian Smith of the Lewisville Aquatic Ecosystem Research Facility, a division of the U.S. Army Corps of Engineers.
Other management approaches being considered
Researchers are looking into other approaches for controlling water weeds, Smith said. Introducing insects that are natural predators is an option only after years of research to ensure the intended solution doesn't create another problem. Weevils and flies have been used but their effectiveness is unproven. They won't completely destroy the plant, but they can slow growth, Smith said.
Smith added that an important part of plant management is to re-establish native species. Killing exotic species opens a niche that needs to be filled with native plants, Smith said. But you have to protect the native plants so they can survive until they are well established.
The sad story is that once you have hydrilla, you always have hydrilla, Wedig said. Some years are good, some are bad. But as lakes age, they stand a better chance of hosting nuisance plants. There's no simple solution, and who knows what the next problem plant will be.
Carol Kimbrell is a writer at LCRA specializing in water topics. You can e-mail her at carol.kimbrell@lcra.org.
Related Web sites with more information about exotic plant and fish species and harmful algae:
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