Lake Data and FAQs


Frequently Asked Questions (FAQs)

Notifications are made when increasing or decreasing discharges through the St. Stephen Hydro and/or the Spillway at Wilsons Landing. Submit an email with the request to OperationsPlanning@santeecooper.com or call 843-761-8000 ext. 5857.

The normal range of the Rule Curve is just above 72.0 ft in winter to 75.5 feet in the summer.  It represents the target elevation for Lake Marion that is recommended by FERC to have more stable downstream water levels.  The Rule Curve allows Santee Cooper to:

  • Adequately manage reservoir resources (aquatic and terrestrial)
  • Allow for a seasonal range of water levels in Lakes Marion and Moultrie
  • Anticipate and provide adequate storage for normal seasonal inflows.   For example, lake elevation targets are lowered during the winter in anticipation of increased inflows during the spring.
  • Provide some degree of flood management associated with high inflows during the winter and early spring.

Lake Marion elevation is managed to the Rule Curve and is dependent upon inflows and outflows of the Santee Cooper Lakes.  Most of our inflows into Lake Marion come from the Wateree and Congaree Rivers.  Most of our outflows come from Lake Moultrie through St. Stephen Hydro.  Water passing through St. Stephen Hydro is then discharged to the Santee River.  Water passing through Jefferies Hydro discharges to the Cooper River.  

Higher elevation levels on Lake Marion may result if the inflows from the Wateree and Congaree Rivers exceed the  outflows through St. Stephen  and Jefferies hydro units and Lake Marion spillway  during the same period.

Increasing or decreasing flows through the Diversion Canal, which connects the upper and lower lakes, is the primary way that Santee Cooper controls the Lake Marion elevation to remain near the rule curve (target elevation) during normal inflows.  In order to increase/decrease the flows through the Diversion Canal, we change the difference in elevation between the two lakes by raising or lowering the water storage in Lake Moultrie. This is done through hydro generation output from St. Stephen.

Local rainfall has a minimal impact on lake elevations at Lake Moultrie and Lake Marion when compared to inflows from upstream rivers and lakes. Most of inflows into our water system come from the Wateree and Congaree Rivers.

The rule curve for Lake Marion regulates the elevation for Lake Marion only. A rule curve for Lake Moultrie in addition to the one for Lake Marion isn’t possible since Lake Moultrie’s elevation is dependent on Lake Marion’s elevation. Under normal conditions, the Lake Marion Rule Curve drives our hydro-related actions such as how much water to run through St. Stephen hydro. These actions ultimately drive changes to the Lake Moultrie elevation as well. As a rule of thumb, when we increase the discharges through St. Stephen Hydro, the Lake Moultrie elevation decreases and pulls further away from Lake Marion’s elevation. When we decrease the discharges through St. Stephen Hydro, the Lake Moultrie elevation will rise and draw closer to Lake Marion’s elevation.

The Army Corp of Engineers regulates the amount of water that can be channeled through the Cooper River.  Their actions mitigate silting in the harbor as well as protect aquatic life. Under a contract prescribed by the U.S. Army Corps of Engineers, these discharges are limited to a weekly average discharge of 4,500 CFS.

Spilling occurs when anticipated inflows would result in exceeding the FERC license requirement that stipulates a maximum elevation of Lake Marion at the Santee Dam (76.8’).  We prefer not to spill because the water represents an economical energy source for our hydroelectric facilities.  Santee Cooper operates the lakes using the exiting Rule Curve to balance the interests of all stakeholders and to optimize the amount of water that is used to generate low cost electricity for its customers.

The St. Stephen Hydro output is increased or decreased to maintain the Lake Marion elevation near the Rule Curve target and to provide generation to meet Santee Cooper’s load. 

When the amount of inflows into Lake Marion combined with the amount of water that we can use to generate hydroelectric power results in projected levels exceeding the maximum elevation at Lake Marion level (76.8’), a spill event may be necessary.