How Duke Energy decides when to open floodgates in Carolinas
5 of the spill gates had to be opened after Hurricane Helene
CHARLOTTE, N.C. (WBTV) - This week marks six months since Hurricane Helene slammed into western North Carolina, triggering historic flooding and damage across the region.
For more than a week after the storm, the Catawba River system was overwhelmed by storm waters, leading to the opening of spill gates and the flooding of neighborhoods along Mountain Island Lake.
While the damage left lasting scars, new insight from Duke Energy is shedding light on the behind-the-scenes decisions that led to the water rising into people’s homes — and why some flooding, officials say, may have been unavoidable.
“This system, the Catawba-Wateree, was not built for flood management,” said Bryan Walsh, Duke Energy’s VP of Carolina’s Regulated Renewables and Lake Services. That’s one of the main things that Duke Energy wants people to understand.
The Catawba-Wateree River system — which includes 11 interconnected reservoirs stretching from Lake James to Lake Wateree — was designed primarily for hydroelectric generation, drinking water supply, and infrastructure .
“It was built for power generation and for our infrastructure,” Walsh said. “So the margins that we have are a lot less [than flood management]. We are more towards full pond a lot of the time.”
That means there’s very limited wiggle room when a major storm like Helene moves through. Unlike flood-control reservoirs, which have large buffer zones designed to absorb surging water levels, the Catawba system is often operating just below capacity, leaving engineers with tough decisions.
Why releasing water upstream matters downstream
One of the biggest factors in last year’s flooding came down to math and geography.
Lake Norman, the largest reservoir in the Catawba system, sits directly upstream from Mountain Island Lake. And while a one-foot drop in Lake Norman may not sound like much, the impact downstream can be dramatic.
“If we dropped Lake Norman by one foot,” Walsh explained, “it would have a ten-foot rise on Mountain Island Lake.”
They compared it to a bathtub being emptied into a cooler. Lake Norman stretches about 32.449 acres in size while Mountain Island Lake, which is next on the Catawba system, measures less than a 10th of the size at 3,117 acres.
That’s exactly what happened after Helene. With rainwater pouring into the system from the storm’s aftermath, Duke Energy had to open floodgates at Cowans Ford Dam to prevent water from overtopping the dam and threatening the integrity of the structure.
“Every time we move water, open a gate, or make any changes in lake level, we have to be cognizant of the downstream effects,” Walsh said.
Balancing drinking water, power needs, and safety
Duke officials say there are legal and operational constraints tied to their management of the water system — including a federal license from the Federal Energy Regulatory Commission (FERC) that mandates certain lake levels.
“This is the dam that protects the drinking water supply for the city of Charlotte,” Walsh said from atop Cowans Ford Dam. “It’s also the water supply for cooling power plants on Lake Norman.”
Trevor Turner, General Manager of Hydro East at Duke, walked us through the dam’s inner workings — from the spinning turbines that generate electricity, to the massive gates used to release floodwater during emergencies.
“This station operates with 92 feet of hydraulic head,” Turner said, referencing the nearly 100-foot height difference between the water on one side of the dam and the other. “You can actually see a water line on the island from when we had Helene — all of this rock was underwater. It was up to the fence line.”
Despite advanced efforts to lower reservoir levels ahead of the storm, the sheer volume of rainfall from Helene overwhelmed the system.
“Where we are with lake levels or drought conditions at the time really plays into how we respond to a storm,” Walsh added.
With 225 miles of river and 1,800 miles of lake shoreline to manage, Duke Energy says storm forecasting and risk modeling are ongoing efforts, particularly as weather patterns shift and storms become more unpredictable.
For residents living along the lakes, the reality is that the system has to serve multiple — sometimes competing — purposes. Duke officials say they’ll continue to walk that line carefully, balancing energy needs, environmental responsibility, and public safety.
They encouraged anyone wanting more information on lake levels to check their live resources, including a live lake level site.
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