Lake Austin and Town Lake are located on the Colorado River in Travis County, central Texas, and serve as a source of water for municipal and industrial water supplies, electrical-power generation, and recreation for more than 500,000 people in the Austin metropolitan area. Lake Austin, located immediately downstream of Lake Travis, extends for more than 20 miles into the western edge of the city of Austin. Town Lake extends through the downtown area of the city of Austin for nearly 6 miles where the Colorado River is impounded by Longhorn Dam.

Many of the detrimental effects of impoundment of water in a lake or reservoir are related to thermal stratification, which generally does not occur in Lake Austin or in Town Lake. The largest detected difference in vertical temperature was 6.5 degrees Celsius in Lake Austin and 3.5 degrees Celsius in Town Lake. The small vertical temperature variations in both lakes can be attributed to shallow depths in the lakes and to the short retention times of water in the lakes during the summer months.

Large vertical dissolved-oxygen gradients were not detected in Lake Austin and Town Lake. Average dissolved-oxygen concentrations for Lake Austin at site Ac, a deep site (about 50 feet) at the dam, differ by about 2.5 milligrams per liter from surface to bottom during the summer. At site Ac on Town Lake, average dissolved-oxygen concentrations differ by about 1 milligram per liter from surface to bottom. The largest areal variations in dissolved oxygen generally occur in Lake Austin during the summer. Water released to Lake Austin during the summer is from below the thermocline in Lake Travis, and consequently, dissolved-oxygen concentrations generally are small. For example, in August 1984, dissolved-oxygen concentrations in Lake Austin increased from 2.8 milligrams per liter in the headwaters to slightly greater than 7.0 milligrams per liter approximately 14 miles downstream. This increase in dissolved oxygen was caused by reaeration from the atmosphere and from photosynthetic production of oxygen by aquatic plants.

Dissolved trace-element data collected from Lake Austin and Town Lake indicate that with the exception of iron, manganese, and mercury, none of the dissolved trace elements analyzed for exceeded either the primary maximum contaminant level or secondary maximum contaminant level set by the U.S. Environmental Protection Agency. Average concentrations of dissolved iron and dissolved manganese in water collected near the bottom of Lake Austin did not exceed 40 and 50 micrograms per liter, respectively.

Little seasonal or areal variation was noted in nitrogen concentrations in Lake Austin or Town Lake. Organic nitrogen is the predominant nitrogen species in both lakes. Stormwater runoff had little effect on nitrogen concentrations in Lake Austin. Nitrogen concentrations in Town Lake were slightly larger following periods of runoff. Total nitrogen concentrations in Town Lake following periods of runoff often exceed 1.0 milligram per liter.

Total phosphorus concentrations are small in Lake Austin and Town Lake. About 95 percent of the total phosphorus concentrations measured in Lake Austin and about 81 percent of the total phosphorus concentrations measured in Town Lake were less than 0.03 milligram per liter. Total phosphorus concentrations are largest in Town Lake following periods of runoff.

Dissolved-solids concentrations ranged from 240 to 340 milligrams per liter in Lake Austin and from 170 to 360 milligrams per liter in Town Lake. The smallest concentrations of dissolved solids in Town Lake occurred following periods of runoff. During periods of no runoff, dissolved-solids concentrations in Town Lake ranged from 240 to 360 milligrams per liter, which was very similar to the range in Lake Austin.

Densities of feca1-coliform bacteria in Lake Austin ranged from less than 1 to 600 colonies per 100 milliliters, and densities of fecal-streptococci bacteria ranged from less than 1 to 340 colonies per 100 milliliters. Densities of fecal-coliform bacteria in Town Lake ranged from 4 to 14,000 colonies per 100 milliliters, and densities of fecal-streptococci bacteria ranged from less than 1 to 15,000 colonies per 100 milliliters. The largest densities of both bacteria in Town Lake occurred following runoff.

Little or no effect of stormwater runoff on temperature, dissolved oxygen, or trace elements was detected in either Lake Austin or Town Lake. Increased concentrations of total nitrogen and phosphorus were detected in Town Lake, but not in Lake Austin following runoff. A decrease in concentrations of dissolved solids and major ions occurred in Town Lake, but not in Lake Austin, following runoff. Densities of fecal-coliform and fecal-streptococci bacteria were larger in Lake Austin and Town Lake following runoff, but significantly larger increases were noted in Town Lake.

Water-quality data collected from Lake Austin and Town Lake, following runoff, generally were not adequate to fully determine the effects of runoff on the lakes. Data collection should not to be limited to fixed-station sampling following runoff, and both lakes need to be sampled simultaneously as soon as possible following significant precipitation.