Environmental Assessment for Lake Ashtabula Winter Drawdown, Barnes County, North Dakota
CORPS OF ENGINEERS ST PAUL MN ST PAUL DISTRICT
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The Corps of Engineers, St. Paul District, proposes to amend the Water Control Manual for Baldhill Dam to allow a maximum winter drawdown to elevation 1255.0 feet Datum NGVD 1929. The increased drawdown would provide additional storage for spring flood waters, which could potentially decrease flooding in Valley City, ND. The current maximum allowable drawdown elevation is 1257.0 feet, and the new allowable drawdown would be 1255.0 feet. During some flood events, drawdown to 1255.0 feet could lower peak flood stage in Valley City by up to 0.2 feet. This estimate of flood stage reduction is only an approximation. Each flood is unique, and in some years, additional drawdown may have no benefit to flooding downstream. Large drawdowns can only be implemented in years when the snow cover reaches high levels early in the winter. Many flood events occur during years when the snowpack develops later in the winter, leaving too little time to draw the lake down to the lower target elevation. Drawdown to elevation 1257.0 feet has been attempted only six times in the last thirty years, and an increased drawdown to elevation 1255.0 feet would be implemented even less frequently. For these reasons, it is expected that the actual flood benefits would be relatively infrequent. The current maximum drawdown to 1257.0 feet has some adverse environmental effects, such as exposure of lakebed, water quality impacts, and potential impacts to cultural resources. Increasing the maximum drawdown elevation to 1255.0 feet would increase the adverse effects to natural and cultural resources. As compared to drawdown 1257.0, drawdown to 1255.0 would dewater an additional 518 acres of lakebed, and the lake would lose 5,900 acre-feet, or 19 of its volume. This additional drawdown would increase the risk of a fish kill by lowering dissolved oxygen levels.
- Hydrology, Limnology and Potamology
- Civil Engineering
- Water Pollution and Control
- Environmental Health and Safety