20260701-002, Reservoir Update for Fiscal Year 2025-2026 Presentation — original pdf

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Reservoir Update FY 2026 Reservoir Update FY 2026 Brent Bellinger, Ph.D. Conservation Program Supervisor Austin Watershed Protection July 1, 2026 Overview  Austin Lakes Index  Aquatic vegetation  Harmful algal proliferations  Turbid water in Lake Austin this winter 2 Austin Lakes Index • Purpose – understand conditions and drivers impacting Lake Austin, Lady Bird Lake, and Lake Walter E Long • Policy Relation – inform management recommendations to protect and/or improve the health of our lakes • Goal – maintain scores as high as possible reflecting “healthy” ecological integrity 3 Austin Lakes Index – Scores Scores fair-to-good • Water and sediment chemistry, biological communities, habitat • Drivers: land-use, pollutant inputs, climate, flow Good Fair Marginal Abundance of vegetation • Positive for water quality, aquatic invertebrates • Non-native species or excess cover negatively impacts scores 4 Aquatic Vegetation – Lake Austin Hydrilla continued to be abundant through winter  Upper reservoir: some die-back  Lower reservoir: similar extent TPWD recently completed survey Feb 27, 2026 5 Aquatic Vegetation – History hydrilla total vegetation grass carp population 25 years of data indicate that: • Climate (drought/flood) is a driver of hydrilla growth • Grass carp system-wide control measure ) s e r c a ( n o i t a t e g e V c i t a u q A 700 600 500 400 300 200 100 0 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Year 35000 30000 25000 20000 15000 10000 5000 0 G r a s s C a r p ( # ) 6 Aquatic Vegetation – Grass Carp Implementing in a measured way to avoid catastrophic crash in lake ecology FY2026 Grass carp stocking  Nov 2025 – added 1,955  May 2026 – added 2,790 May ‘26 Carp Stocking Current population as of July 1 is estimated to be: 4,467  8 fish/acre (based on February 2026 veg survey) 7 Aquatic Vegetation – Lake Austin drawdown Pros  Ease of access for shoreline maintenance  temporary reduction of exposed Eurasian watermilfoil, hydrilla Cons  May increase hydrilla long-term  Impacts to water storage & energy  Impacts to recreation and businesses Guidance  AW, AE, LCRA collaborating on logistics 8 Aquatic Vegetation – Management Developing a long-term guidance document  Focus on hydrilla, but also includes, watermilfoil, fanwort, and water lettuce Example Simulation Results Goal: Balance  Functionality  Ecology  Stakeholders Key facet: retrospective of past management actions and responses  Modeling the complicated relationship between grass carp population and vegetation density 9 Harmful Algal Proliferations (blue-green algae) S i x y ea r s of m o n i t or in g • Final year of current monitoring program • Density and toxin contents of mats are variable within and between years and sites 10 Harmful Algal Proliferations – Bentonite Pilot P u r p o s e : m o d i f y s e d i m e n t p h o s p h o r u s a v a i l a b i l i t y • Key limiting nutrient to cyanobacteria growth M o n i t o r i n g p h o s p h o r u s s e d i m e n t c h a n g e s • 2021 testing prior to first application • 2021 – 2025 applications (3x/yr) • 2026 post-treatment monitoring 11 ) g k / P g m ( t t n e n o C s u r o h p s o h P t n e m d e S i 800 700 600 500 400 300 200 Results through 2025 P l a t o T f o % 100 80 60 40 20 0 Pmobile Pimmobile 2021 2022 Date 2024 2025 Total P Jan Jul 2021 Jul Jan 2022 Jan Jul Date Jan Jul 2024 Jan Jul 2025 Jan L a r g e v a r i a b i l i t y i n t o t a l p h o s p h o r u s a r o u n d R e d B u d I s l e A m o u n t “ m o b i l e ” p h o s p h o r u s h a s d e c l i n e d f r o m ~ 4 5 % t o 2 0 % To t a l P i n c r e a s i n g t h r o u g h t i m e • Not surprising w/constant fresh inputs from upriver and plant/algae biomass around island H o w w i l l t h i s p a t t e r n c h a n g e a f t e r a y e a r o f n o n e w a p p l i c a t i o n s ? 12 Turbid water in Lake Austin this winter W h a t w a s t h e i s s u e ? • Very turbid water in upper Lake Austin in Jan/Feb 2026 • Lot of hypotheses – some blamed hydrilla D a t a s h a r i n g w i t h L C R A • Strong partnership between COA and LCRA • Also, LCRA is deploying some cool new equipment! 13 Lake Travis (bottom) data S i g n i f i c a n t t u r b i d i t y e v e n t i n b o t t o m w a t e r s o f L a k e Tr a v i s M a n s f i e l d D a m r e l e a s e s b o t t o m w a t e r t o L a k e A u s t i n B u t w h y t h e t u r b i d i t y ? 14 Turbid water in the winter – lake turnover Te m p e r a t u r e p r o f i l e Very Warm, Stratified Cool Mixed • Time-series of surface – bottom data from above Mansfield Dam Te m p e r a t u r e • Temperature stratification until winter cold snap • Texas reservoirs “flip” 1x/yr Ve r t i c a l m i x i n g o f w a t e r r e s u l t e d i n r e s u s p e n s i o n o f f i n e s e d i m e n t s -10 -20 -30 -40 C 0 5 10 15 20 25 30 35 40 Oct 25 Nov Dec Jan 26 Feb 15 15 Current Conditions 16 Summary Austin’s reservoirs continue to thrive but are challenged  Dense growth of hydrilla in Lake Austin  Toxic mats of blue green algae at Red Bud Island  Floods transporting sediments/nutrients Continuing to engage stakeholders  Communication  What are we doing, why, and how to manage and mitigate stressors Long-term vision in a world of short-term expectations 17 17 Next Steps… What can citizens do? Continue to monitor hydrilla and impact of grass carp  Will also track cabomba growth dynamics Nutrient, runoff management  Reduce use of fertilizers  Plant native, drought-tolerant plants  Install rain gardens, cisterns Finish toxic algae and bentonite data collection  Evaluate data, make recommendations, pursue new approaches Green Lawns  Green Lakes  Plants and algae benefit from excessive nutrient inputs 18 Questions? 19