UNDERSTANDING MIXING AND ANNUAL STRATIFICATION OF A LARGE RESERVOIR: GEOCHEMISTRY OF LAKE TEXOMA, TEXAS-OKLAHOMA

Lake Texoma is a large reservoir along the Texas-Oklahoma border, impounded in 1943, with a surface area of 360 km². The lake is a dynamic system, receiving input from two major river systems, the Red River and the Washita River. The Red River drains Permian evaporite and clastic sediments in west Texas and Oklahoma and, as a result, typically has elevated salinity (1200–2000 mg/L total dissolved solids, TDS, where it enters the lake and > seawater in some headwater reaches), making the Red River one of the saltiest major rivers in the world. In contrast, the Washita River drains west-central Oklahoma and typically has lower salinity than the Red (< 500 mg/L TDS). Water from Lake Texoma has been collected during the spring, summer and fall over two years as part of an undergraduate class in geochemistry. We have sampled the lake at multiple sites and depths along the two influent arms, as well as at the deepest portion of the lake, proximal to the dam. Our data show that mixing between these two rivers results in horizontal chemical gradients. Near the Red River, Na concentrations are around 275-300 ppm but near the Washita River, Na concentrations are only 70-100 ppm. Near the dam, Na concentrations are at 200-240 ppm. There are also strong vertical chemical gradients generated in the main lake region during the development of a summer thermocline. At the deepest part of the lake (26 meter depth), near the dam, there is a boundary between the surface waters and the anoxic bottom waters at about 15-18 meter depth. The lake becomes well mixed some time during the fall, dependent on annual variation in rainfall and air temperature. During stratification, Dissolved Oxygen (DO) varies from 4 to 7 mg/L in surface waters, decreasing dramatically across the anoxia boundary to close to zero below 20 meter depth. Once the lake becomes mixed again, DO levels vary from 7 mg/L in the surface waters to 5 mg/L at 26 meters depth approaching zero only at the sediment-water interface. Similarly, during stratification Fe concentrations vary from very low values to 5 ppb at the surface, and can increase to up to 70 ppb in anoxia waters while, once mixed, Fe concentrations are consistently 1-2 ppb at all levels. When stratified, Mn concentrations vary from < 5 ppb at the surface to as much as 800 ppb in the deepest water, becoming around 1 ppb throughout once the lake becomes mixed.