Managing reservoir levels has its ups-&-downs

Water levels in the three Corps of Engineers reservoirs on the upper Savannah River change constantly – and with good reasons, according to the experts who manage the water in the basin. Changes in precipitation patterns, evaporation levels, and downstream flow requirements occur every year in an ecosystem like the Savannah River Basin. Even in years of abundant rain, the reservoirs will fluctuate up and down.

“It takes constant monitoring of data and adjustment to fulfill as many of the purposes of the reservoirs as we can,” said Stan Simpson, a Corps water manager in Savannah, Ga., who helps oversee water flows out of the dams and onward downstream. “We must balance water supply, water quality, flood risk reduction, navigation, recreation, hydropower production, and environmental stewardship. It is our mission to meet these project purposes in an equitable manner within our authorities”.

The “guide curve” represents the target water surface level for any particular day on the three reservoirs operated by the Savannah District – Hartwell Lake, Russell Lake, and Thurmond Lake. Many factors impact the Corps’ ability to maintain the pools on guide curve including the natural inflow to the reservoirs from tributaries and the release of water from upstream reservoirs. The reservoir levels also vary depending on evaporation, transpiration, and groundwater interaction. Yes, even evaporation will cause the pools to fluctuate.

On the Corps’ Savannah River reservoirs, guide curves vary by season. “The water level targets are lower in the winter to protect the shoreline and to prepare for typical spring flooding. The guide curve was designed to enable the reservoirs to successfully operate for a range of annual circumstances,” said Jason Ward, another Corps water manager. Seasonal high winds in the winter and spring if combined with high surface levels would lead to shoreline erosion and add silt to the reservoirs, Ward said. Lower winter levels also give the Corps added storage capacity for normal winter and spring rains. This extra storage allows the Corps to be more successful in reducing flood damage downstream. “We can’t keep the reservoirs level year round without causing some damage somewhere in the basin.”

The three reservoirs sit on the border between Georgia and South Carolina – the two states that share the water. Lake Hartwell, the uppermost reservoir, has a guide curve that runs from 656 feet above mean sea level (ft-msl) in December then slowly increases to 660 ft-msl from April 1 through Oct. 15. Lake Thurmond, the oldest and most downstream reservoir on the Savannah, follows a guide curve from 326 ft-msl in December to 330 ft-msl for most of the year. Lake Russell, between the other two, has a constant guide curve of 475 ft-msl due to its design and smaller size. The water managers like to begin each summer with the pools at the summer guide curve level which helps them meet both in-lake and downstream needs throughout the typically dry periods in Georgia and South Carolina.

“Water management is dynamic, as is nature, always changing,” Simpson said. “The guide curves were developed using many years of data and future updates will reflect changes in the hydrology of the basin. Our record of meeting the needs of the entire basin – upstream and downstream – is very good, in spite of its ‘ups-and-downs.'” Reservoir information can be found on the Water Management web site at http://water.sas.usace.army.mil/home/indexDU.htm.

About US Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multimillion dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on Facebook.com/SavannahCorps
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