Lake Austin's hydrilla stopped spreading this year. The city now wants to add carp faster

Hydrilla, the invasive water plant that has choked parts of Lake Austin, did not spread further this growing season, according to an Oct. 22 memo from Austin Watershed Protection Director Jorge Morales. That is a change for boaters, lakefront owners and the businesses along the lake, after coverage climbed to near record levels.

This follows our Oct. 20 story on the canceled fall drawdown of the lake.

What the memo says

Texas Parks and Wildlife surveys the lake's plants each spring and fall. Its September survey found:

• No increase in hydrilla since February. Staff credit the plateau mostly to grass carp stocking and easing drought.
• A modest spread of Eurasian watermilfoil, another invasive plant, about five miles below Mansfield Dam. The memo says it does not appear to be blocking boats.
• Local shifts. Plants increased around Bee Creek, Bull Creek and Ski Shores, and decreased above the Pennybacker Bridge.

Most hydrilla now grows deeper below the surface, the memo says, but dense patches that reach the top still cause navigation problems at Bull Creek, Bee Creek and near the shore. July floodgate releases at Mansfield Dam did little, because the flow was low and spread over a long period.

KUT reported in August that coverage was near its all-time high as of June. The city posts its updates on its Lake Austin page.

The plan: more carp, sooner

Sterile grass carp, which prefer hydrilla to other plants, remain the city's main long-term tool. Watershed scientists built a model for Lake Austin that weighs past and current data, including the carp population and the latest survey, to pick a stocking rate. Based on that model, staff will ask Texas Parks and Wildlife to allow a higher stocking rate than it normally permits. Fish would be released in the upper, middle and lower lake where growth is densest.

The model is also meant to avoid overshooting. Staff want to keep plant cover above 2% of the lake's surface, below which they say aquatic life would suffer. A full Lake Austin Vegetation Management Plan is due by the end of the year.

The memo also says the city is writing rules for bottom barriers, mats that property owners lay on the lakebed to stop plants from rooting. Staff warn that poorly installed or neglected barriers can break loose and become hazardous trash. Watershed Protection and Development Services plan a simple, low-cost site plan exemption with size, location and maintenance limits, aligned with the state's own permit.

What our records add

Why there was no drawdown. The city's Sept. 24 memo said it would not lower the lake in 2026. The original Oct. 12 to Nov. 30 window conflicted with fall weddings, UT football and Formula 1, and a Dec. 1 to Jan. 15 option raised holiday concerns. The Lower Colorado River Authority has not done winter drawdowns since Winter Storm Uri in 2021, so it could keep hydroelectric generators available for the grid, the memo said.

What staff told the council. At the Aug. 26 meeting of the Climate, Water, Environment, and Parks Committee, Environmental Officer Liz Johnston said a drawdown would not kill hydrilla, though it might help control milfoil. She said scientists had spent the summer building a model and would adjust carp stocking depending on whether a drawdown happened. The new memo is the result: no drawdown, and a request to stock more aggressively.

Speakers keep raising it. At the council's Oct. 22 meeting, a public speaker again asked for a winter drawdown to repair erosion around docks.

What's next

The Climate, Water, Environment, and Parks Committee meets Wednesday, Oct. 28, at City Hall; its agenda had not been posted in our records at publication. The vegetation management plan is due by Dec. 31. Lake Austin is in District 10, represented by Council Member Marc Duchen.