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In cooperation with the Lower Colorado River Authority
and the City of Austin

Characterization and Simulation of the Quantity and Quality of Water in the Highland Lakes, Texas, 1983–92

By Timothy H. Raines and Walter Rast

U.S. Geological Survey
Water-Resources Investigations Report 99–4087


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Contents

Abstract

Introduction

Purpose and Scope

Description of Study Area

Description of Mass-Balance Model

Acknowledgments

Characterization of the Quantity and Quality of the Highland Lakes

Simulation of the Quantity and Quality of the Highland Lakes

Model Development

Model Testing

Model Simulation

Summary

References Cited

Figures

1.   Map showing location of study area
2.   Graphs showing measured monthly streamflow and concentrations of dissolved solids, chloride, and sulfate at Colorado River near San Saba, Texas, 1983–92
3–9.   Graphs showing measured monthly reservoir storage and concentrations of dissolved solids, chloride, and sulfate at:
  3.   Lake Buchanan, Texas, 1983–92
  4.   Inks Lake, Texas, 1983–92
  5.   Lake Lyndon B. Johnson, Texas, 1983–92
  6.   Lake Marble Falls, Texas, 1983–92
  7.   Lake Travis, Texas, 1983–92
  8.   Lake Austin, Texas, 1983–92
  9.   Town Lake, Texas, 1983–92
10.   Map showing location of selected streamflow-gaging stations near the Highland Lakes, Texas
11–13.   Graphs showing daily mean streamflow in relation to dissolved solids, chloride, and sulfate loads at:
  11.   Colorado River near San Saba, Texas
  12.   Llano River at Llano, Texas
  13.   Pedernales River near Johnson City, Texas
14–15.   Graphs showing measured monthly streamflow and estimated monthly concentrations of dissolved solids, chloride, and sulfate at:
  14.   Llano River at Llano, Texas, 1983–92
  15.   Pedernales River near Johnson City, Texas, 1983–92
16–22.   Graphs showing measured and simulated monthly reservoir storage and concentrations of dissolved solids, chloride, and sulfate at:
  16.   Lake Buchanan, Texas, 1983–92
  17.   Inks Lake, Texas, 1983–92
  18.   Lake Lyndon B. Johnson, Texas, 1983–92
  19.   Lake Marble Falls, Texas, 1983–92
  20.   Lake Travis, Texas, 1983–92
  21.   Lake Austin, Texas, 1983–92
  22.   Town Lake, Texas, 1983–92
23.   Graphs showing measured and simulated monthly streamflow and concentrations of dissolved solids, chloride, and sulfate at Colorado River at Austin, Texas, 1983–92
24–26.   Graphs showing simulated monthly streamflow and concentrations of dissolved solids, chloride, and sulfate for wet, average, and dry hydrologic conditions at:
  24.   Colorado River above Lake Buchanan, Texas, 1987–96
  25.   Llano River above Lake Lyndon B. Johnson, Texas, 1987–96
  26.   Pedernales River above Lake Travis, Texas, 1987–96
27–33.   Graphs showing simulated monthly reservoir storage and concentrations of dissolved solids, chloride, and sulfate for wet, average, and dry hydrologic conditions at:
  27.   Lake Buchanan, Texas, 1987–96
  28.   Inks Lake, Texas, 1987–96
  29.   Lake Lyndon B. Johnson, Texas, 1987–96
  30.   Lake Marble Falls, Texas, 1987–96
  31.   Lake Travis, Texas, 1987–96
  32.   Lake Austin, Texas, 1987–96
  33.   Town Lake, Texas, 1987–96
34.   Graphs showing simulated monthly streamflow and concentrations of dissolved solids, chloride, and sulfate for wet, average, and dry hydrologic conditions at Colorado River at Austin, Texas, 1987–96

Tables

1.   Selected physical and hydrologic characteristics of the Highland Lakes, Texas
2.   Applicable water-quality standards for each tributary and reservoir of the Highland Lakes, Texas
3.   Measured concentrations of dissolved solids, chloride, and sulfate in the Highland Lakes, Texas, 1983–92
4.   Characteristics of selected tributary and outlet streams of the Highland Lakes, Texas
5.   Measured and simulated water gains and losses of the Highland Lakes, Texas
6.   Regression equations for estimating concentrations of dissolved solids, chloride, and sulfate for selected tributaries of the Highland Lakes, Texas
7.   Absolute error and error between measured and simulated monthly reservoir storage and concentrations of dissolved solids, chloride, and sulfate in the Highland Lakes, Texas, 1983–92
8.   Monthly demand distribution for Lake Travis
9.   Selected simulated monthly reservoir storage and concentrations of dissolved solids, chloride, and sulfate in the Highland Lakes, Texas, for wet, average, and dry hydrologic conditions, 1987–96

Abstract

The Highland Lakes, located in central Texas, are a series of seven reservoirs on the Colorado River (Lake Buchanan, Inks Lake, Lake Lyndon B. Johnson, Lake Marble Falls, Lake Travis, Lake Austin, and Town Lake). The reservoirs provide hydroelectric power for the area. In addition, Lake Austin and Town Lake also provide the public water supply for the Austin metropolitan area. Saline water released from Natural Dam Salt Lake during 1987–89 caused increased concern among water managers that high-salinity water entering the Highland Lakes could result in water-quality problems, necessitating additional treatment of the water.

The maximum dissolved solids concentrations for the reservoirs after the saline inflow were about two to three times the average concentrations before the inflow. The maximum concentrations of chloride and sulfate after the inflow were about three to five times the average concentrations before the inflow. The concentrations of dissolved solids, chloride, and sulfate in Lake Buchanan, Inks Lake, Lake Lyndon B. Johnson, and Lake Marble Falls were less than the concentrations of the applicable water-quality standards by the end of 1990. Concentrations of these constituents in Lake Travis, Lake Austin, and Town Lake did not decrease to previous levels, which were less than the concentrations of the applicable water-quality standards, until the end of 1991. Constituent concentrations for Lake Buchanan and Inks Lake; for Lake Lyndon B. Johnson and Lake Marble Falls; and for Lake Travis, Lake Austin, and Town Lake were similar because of the relative storage capacities and location of tributary inflows. From the initial increase in constituent concentrations in Lake Buchanan (summer 1987) in response to the saline inflow, the high-salinity water passed through the entire Highland Lakes in about 3.5 years.

A mathematical mass-balance model was used to simulate the input and movement of high-salinity water through the Highland Lakes and to estimate monthly mean concentrations of dissolved solids, chloride, and sulfate for wet, average, and dry hydrologic conditions. The simulated median monthly concentrations during the 10-year simulation period for each reservoir generally are larger for the average condition than for the wet condition and generally are larger for the dry condition than for the average condition. The simulated concentrations of dissolved solids, chloride, and sulfate decreased to levels less than the concentrations of the applicable water-quality standards in about 2 to 5 years after the saline water inflow of 1987–89 was simulated for the three hydrologic conditions.

Results from the simulations indicate that saline inflows to the Highland Lakes similar to those of the releases from Natural Dam Salt Lake during 1987–89 are unlikely to cause large increases in future concentrations of dissolved solids, chloride, and sulfate in the Highland Lakes. The results also indicate that high-salinity water will continue to be diluted as it is transported downstream through the Highland Lakes, even during extended dry periods.

 


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