2017 State of Our Environment Report
Lakes and Rivers
Austin’s reservoirs - Lake Austin, Lady Bird Lake, and Lake Walter E. Long - are essential providers of ecosystem benefits necessary and desired for a successful city. While drinking water remains one of the most important resources provided by the reservoirs, they also provide flood control, recreational opportunities, and aesthetics that enrich the region at large. Watershed Protection Department (WPD) staff monitor and study the City’s reservoirs to gauge ecosystem health. The results of these studies drive policy changes related to development and land use, as well as remediation and restoration projects to protect and enhance these valuable resources.
Status and Trends
Staff utilize the Austin Lake Index (ALI) as a tool to convey the general condition of the reservoirs through the routine sampling of biology, habitat, water quality, and sediment chemistry. In 2017, WPD staff observed a general increase in the condition scores from poor-marginal to marginal-fair across all reservoirs relative to 2016. One important measure of water quality is the diversity of aquatic life found within the reservoir, especially benthic invertebrates such as insect larvae, aquatic worms, mussels, and other small lifeforms. Another important measure of water quality is the “eutrophication” score, which examines the amount of excess nutrients in the water. In both Lake Austin and Lady Bird, both of these scores were higher this year, indicating improving water quality. Part of the reason for this improvement may have been a result of increased flows due to higher rainfall amounts. Water quality scores improved in Lady Bird Lake and Lake Walter E. Long. Habitat and vegetation continue to depress the Lake Austin and Lady Bird Lake scores; the eutrophication score had the largest negative influence on Lake Walter E. Long.
Figure 1. Austin Lake Index components monitored include nutrients, benthic invertebrates, aquatic plants, sediment toxins, shoreline habitat, and phytoplankton communities. The target score is 64 or “good” condition, which was almost realized in Lake Walter E. Long in 2017 (62). More information on scoring can be found at www.austintexas/gov/lakesindex.
The Lower Colorado River Authority, at the City’s request, lowered Lake Austin 10 feet in January 2017 for the first time since 2011. The lower water level was fortuitous for some lakeshore residents, who took the opportunity to repair docks and bulkheads. The exposed lakebed also allowed WPD staff to conduct a survey of native mussels.
Figure 2. Lake Austin downriver of the Pennybacker Bridge (Loop 360) during the January 2017 drawdown. The exposed sand flat is covered by the non-native Asian clam, Corbicula.
In 2017, WPD staff found native mussels at 100 percent of all survey sites (compared to 58 percent in 2011), a total of six species (compared to only two species in 2011), and a greater number of shells in 2017 (3,118 shells) relative to 2011 (153 shells).
Figure 3. Mussel species collected in the 2017 survey included: giant floater, paper pondshell, fragile papershell, Tampico pearlymussel, southern mapleleaf, and Texas Lilliput. In 2011, only the giant floater and paper pondshell were observed.
Mussel abundance increased moving from the upper to the lower reservoir. Findings suggest that mussel populations had been negatively impacted by frequent winter drawdowns of the lake, but the six-year hiatus allowed populations to rebound.
Figure 4. Mussel density (individuals/acre) by location within the mainstem of Lake Austin from upriver to downriver
Although the abundant native mussels found during the drawdown was encouraging, in late summer the Texas Parks and Wildlife Department confirmed the presence of the non-native, highly invasive zebra mussel.
Figure 5. Live zebra mussels growing on a rock from Lake Belton (left). Dead zebra mussel shells remain attached to a ladder that had been submerged in Lake Belton (right). Zebra mussels in Lake Austin could result in negative impacts to recreation, water intake pipes, native mussel populations, and food web/nutrient cycling dynamics. Photos provided by the Army Corp. of Engineers Lake Belton office.
While initially few zebra mussels were found near the Tom Miller Dam, they were subsequently discovered at multiple sites and Lake Austin is now considered to be infested by the Texas Parks and Wildlife Department. As of February, Lady Bird Lake is also suspected of containing zebra mussels. WPD staff, with state and local partners, are developing monitoring plans to ensure zebra mussel population changes can be documented and associated with any broader aquatic ecosystem changes.
WPD’s Environmental Resource Management and Field Operations Divisions collaborated with the City of Austin’s Parks Department to address the crumbling bulkhead wall at Emma Long Metropolitan Park. The old wall reflected waves, which exacerbated wave action and caused erosion. The new shoreline has been designed to absorb wave energy, restore ecological function, and provide some water quality treatment for the parking lot.
Figure 6. The decaying wall at Emma Long Park had degraded the shoreline and was not compliant with current City code.
Figure 7. The new shoreline shows that modern methods can provide stability and ecological function in harmony with recreation.
The Lake Austin shoreline has historically been smothered by vertical walls, which reduced water quality by limiting plant growth and increasing sediment suspended in the water. These walls also degraded habitat by disconnecting the aquatic and riparian shoreline habitat. In 2010, City regulations were improved in an effort to reverse this trend and react to the conclusions of the EPA National Lakes Assessment. This report found that poor lakeshore habitat is the most significant stressor in lakes and that poor biological health is three times more likely in lakes with poor lakeshore habitat.
Figure 8. The progressive vision for the shoreline materialized as crews from the Field Operations Division demolished the old wall and installed the new structure during the winter of 2017.
Figure 9. The new shoreline maximizes diversity and density of plants.
A diversity of native plants along the shoreline is important for many reasons. Dense roots stabilize the soil and thick vegetation protects the soil from erosion. The leaves create shade and the plant parts that fall in the water become both habitat and food for animals. Wetland plants create habitat for juvenile fish, provide food for turtles and waterfowl, and offer habitat for aquatic life such as dragonflies and mayflies.
Figure 10. The restoration project includes a rocky slope that reduces wave return. Vegetated terraces enhance natural habitat and provide water quality treatment for parking lot rain runoff.
Figure 11. The new shoreline requires less maintenance because mowing is no longer necessary. The young shrubs and trees will grow past the grasses to provide habitat, shade, and scenic beauty.
Some of the benefits of the project are already apparent. Birds, lizards, and butterflies are frequently seen in the vegetated terraces, and small fish that had been absent from the old shoreline are now commonly found seeking shelter in the rocky shallows. In addition, boulders replace the wooden fence. This reduces the annual maintenance burden and makes the shoreline safer because of no vertical bulkhead from which to fall.
WPD’s environmental scientists hope that this progressive shoreline design will serve as an example for landowners who desire protection from erosion while maintaining ecological function.