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Low Dissolved Oxygen Levels in Summer Powerplant Discharges from Fort Randall Dam, South Dakota
CORPS OF ENGINEERS OMAHA NE
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In Lake Francis Case, water is routed to the dam intake structure through a submerged approach channel that connects to the old Missouri River channel Figure 1. The invert elevation i.e., 1229 ft-msl of the intake for the power tunnels at Fort Randall Dam is 2 feet above the bottom of the approach channel. Thermal stratification that develops in Lake Francis Case during the summer results in a significant water density gradient between the epilimnion and hypolimnion. Due to the water density gradient, restricted approach channel, and bottom withdrawal, the vertical extent that water is withdrawn from Lake Francis Case in the summer is dependent upon the discharge rate of the dam. During periods of lower flows through the powerplant water is pulled from lower elevations along the bottom of the reservoir. Higher powerplant flows are believed to create enough turbulence near the dam intake structure to allow warmer, less dense water to be drawn from higher elevations in Lake Francis Case. Degradation of dissolved oxygen occurs in the quiescent hypolimnion of Lake Francis Case during summer thermal stratification. Dissolved oxygen degradation is greatest near the reservoir bottom and expands upward in the water column as summer thermal stratification continues. Past water quality monitoring indicates that the lower depths of Lake Francis Case typically degrade to hypoxic conditions by early July. At that time, a heightened potential exists for hypoxic water to be drawn into the powerplant and discharged downstream to the Missouri River. The discharge of hypoxic water could result in dissolved oxygen conditions in the Fort Randall tailwaters not meeting South Dakota s water quality standards minimum criteria for dissolved oxygen.
APPROVED FOR PUBLIC RELEASE