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Austin staff detail reservoir trends: hydrilla surge, clay pilot reduces sediment phosphorus
Summary
Austin Watershed Protection reported rising hydrilla in Lake Austin, updated grass‑carp stocking responses and a five‑year lanthanum‑bentonite clay pilot that reduced the mobile sediment phosphorus fraction from ~60% to ~20% at treated sites; staff stressed continued monitoring and modeling.
Brent Bellinger, reservoir ecologist with Austin Watershed Protection, delivered the commission’s annual Austin Lakes Index briefing on July 1, outlining current reservoir conditions, vegetation pressures and toxic‑algae monitoring.
Bellinger said the Austin Lakes Index (a City‑mandated, 0–100 score included in the State of the Environment report) helps identify system drivers — drought, floods, vegetation shifts and nutrient inputs. He flagged a resurgence of non‑native hydrilla and noted patches of Eurasian watermilfoil. “You can have too much of a good thing,” Bellinger said, describing how abundant non‑native vegetation impairs recreation and can degrade ecological scores.
Staff described the longstanding use of sterile triploid grass carp to manage hydrilla, increasing stocking rates in winter 2025 and again in May after Texas Parks and Wildlife (TPWD) surveys showed rapid hydrilla growth. Bellinger cautioned against overstocking, which can cause a vegetation crash and unintended ecological consequences.
On cyanobacteria, Bellinger summarized a five‑year pilot applying lanthanum‑modified bentonite (LMB) clay to sequester sediment phosphorus. He said treated sites showed a sharp drop in the mobile phosphorus fraction (from about 60% to around 20%) and that the department paused further applications this year to observe system response amid large rain events that deliver fresh sediment.
Bellinger also explained a winter turbidity event in which a Lake Travis stratification “flip” and bottom‑water releases resuspended fine sediments that flowed into Upper Lake Austin; LCRA sonde data helped identify the mechanism. He emphasized that watershed inputs, climate variability and operational constraints (LCRA and ERCOT rules affecting drawdown timing) interact to affect reservoir outcomes.
Commissioners asked about monitoring cadence, which is largely collected by LCRA for many sites with bimonthly sampling for active locations, and about the ecological and recreational trade‑offs of drawdown, which LCRA now favors in fall rather than winter to preserve hydroelectric head pressure in emergencies. Bellinger said the city is preparing an aquatic plant management plan to guide future tools and thresholds.
What this means: City staff say the reservoirs are responding to complex climate and watershed drivers; the clay pilot shows measurable sediment‑chemistry effects, but staff caution that long‑term outcomes require continued sampling, modeling and coordinated management with LCRA and TPWD.
Source: Presentation and Q&A recorded during the July 1, 2026 Environmental Commission meeting (Brent Bellinger, Austin Watershed Protection).
