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Hundreds of Acres of Invasive Plants Plague Lake Austin

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Posted: Tuesday, August 31, 2010 3:28 pm

The city of Austin is working on ways to manage around 300 hundred acres of invasive weeds in Lake Austin.

"It is a nuisance," said Mary Gilroy, City of Austin Environmental Scientist.

Hydrilla and milfoil are aggressive aquatic plants that create a dense matt that is hard to swim in, gets tangled in boat motors and intakes, and is not aesthetically pleasing.

"(Hydrilla) is certainly a serious public safety concern, as swimmers can easily become tangled in the dense mats," Gilroy said.

Prior to 2003, Gilroy heard of one drowning death on Lake Austin where onlookers reported the individual tried to swim a short distance to shore at Emma Long Park through dense hydrilla.

Austin Lake Patrol has had to assist several boaters and jet skiiers trapped in hydrilla mats in the past, Gilroy said.

The weeds also block water infrastructure intakes used for power and drinking water.

Currently the densest amount is at the Steiner Ranch boat dock and Mary Quinlan Park and upstream to Mansfield Dam, and to a lesser degree from Mary Quinlan to Emma Long Park.

A survey in March by the Texas Parks and Wildlife showed 130 acres of milfoil and hydrilla mixed and 140 acres of pure milfoil in Lake Austin, Gilroy said.

Another survey scheduled for earlier this week is anticipated to show that vegetation has increased since March. "We won’t know how much until the results are compiled by TPWD," she said.

Hydrilla can grow to the surface in water up to 30 feet deep. Once at the surface, hydrilla can form a mat five feet deep. The mat can be as wide as the water depth allows, even growing all the way across the lake.

Milfoil typically only grows in 12 feet of water and mats are not as thick, dense or dangerous as hydrilla.

There are several effective tools to manage these weeds including drawing down the lake exposing the plants to air, and sterile grass carp.

On October 1 the City of Austin will submit a request to the Lower Colorado River Authority for a drawdown to happen in January 2011.

LCRA then drafts an agreement with the details of the drawdown and both agencies sign it, Gilroy said.

"We anticipate LCRA will approve the request. Information is showing enough water in Lake Travis to draw down," Gilroy said.

The drought of 2009 prevented a drawdown in the winter of 2010. For a drawdown to happen, Lake Travis has to be full enough. The lake was last lowered in January 2008.

If approved next month, Lake Austin would be lowered 12 feet in early January and then by mid February would return to normal pool elevation. It takes six weeks from start to finish, she said.

Carp is another proven management tool to combat invasive weeds.

In October of 2009, over 4,400 sterile grass carp were released in Lake Austin to eat the weeds. Grass carp is effective but the fish only eat the top part of the plants and not the roots. A tuber bank forms eventually running out of energy for the plant to resprout.

"We feel like the carp is doing a pretty good job but not eradicating the problem," Gilroy said.

The carp has had a lot of work to do lately. A year ago in September 2009, there was over 316 acres of pure hydrilla and another 25 acres of it mixed with other plants, Gilroy said.

The last time the hydrilla came close to the current peak levels was in early 2000 when there was over 300 acres.

Aggressive management to lower Lake Austin every winter from 2000 to 2005 helped keep the weed levels at around 50 acres.

But businesses, homeowners and lake users voiced concerns and a compromise was made for draw downs to happen every other year. That happens depending on drought conditions.

Welcome to the discussion.