The balancing act is already tricky. Enough water must be released from the Pinopolis Dam to manage Lake Moultrie and operate the passage locks.
If too much is released, the flow down the Cooper River will silt in the billion-dollar Charleston Shipping Channel too fast.
Then the balance gets more complicated.
Seventy miles away in Columbia, Tim Lanier has performed a routine check on the safety of the drinking water for Charleston. After his review, the U.S. Geological Survey hydrologist says to release more from the dam.
Saltwater has crept too close to the Bushy Park Reservoir on a tributary of the Cooper River fed by a canal. It's the source of the main drinking water supply for nearly a half-million people. Lanier's job is to make sure the salt doesn't get there.
While the concern for Bushy Park doesn't get the attention that flooding or sea rise issues do, it potentially could affect far more people more quickly and in their wallets. Lanier is one of three USGS hydrologists in the Columbia office who rotate 24-hour-per-day shifts, doing four checks per day every six hours, day and night.
He gauges the readings of three real-time salinity testers along the Cooper River. He consults a spreadsheet to make sure the readings aren't out of line with each other or previous readings, because any one reading can be inaccurate.
The job is exacting enough that he does it by hand rather than rely on automatic updates.
"You can discern it with your eye," Lanier said.
If there's trouble, he has to compute just how much freshwater from Lake Moultrie will make a difference without jeopardizing the shipping channel unnecessarily.
Then he picks up the phone.
The phone call doesn't come often. When it does, it's usually big trouble — such as hurricane storm surge or drought.
But the alerts go out regularly, too, when there are flood tides, also called king tides or astronomical high tides. Those tides bring up seas 6.6 feet or more, or at least a foot above the usual tidal range.
The flooding is being prodded by rising seas. Charleston, which flooded about four times per year a half century ago, now floods some 40 days per year. Federal researchers say it might flood nearly every day by the turn of the century.
Tides topped 7 feet as recently as early June. They topped 8 feet in February. The highest recorded flood tide so far was more than 12 feet, during Hurricane Hugo in 1989.
It's not widely recognized that until the Marion-Moultrie lakes were dug, the Cooper River was a shallow blackwater run similar to the nearby Ashley River. The headwaters was Wadboo Creek near Moncks Corner, which drains a swamp.
Water released at the dam must travel for the first three miles or so through the man-made Tail Race Canal to reach those headwaters. In other words, without the Pinopolis water release, it doesn't take much to move saltwater upstream.
"When it gets close (to Bushy Park) up there, that makes a lot of people antsy. Once it gets up there, it's really hard to get out," Lanier said.
When the alert goes out, the Army Corps of Engineers gets notified because the Army Corps district office is in charge of the shipping channel, and that means the district is in charge of the water flow.
The channel is worth an estimated $53 billion per year in commercial shipping, and federal law determines how much flow the lake has to release at any given time.
"The Corps has oversight of the process, and has the ability to become actively involved where circumstances merit, such as tropical storm or hurricane storm surge, for example," said Glenn Jeffries, an Army Corps district spokeswoman.
The Charleston Water System also gets notified, and the bright spot in all this is the utility doesn't rely exclusively on the Cooper River for water. A pipe also feeds the system from the Edisto River.
The system's Hanahan Water Plant crews continually monitor the quality of raw water coming in and adjust treatment to handle it. If the river water goes saline, the plant pumps in more Edisto water. Treating that water costs more.
It requires "a variety of techniques and methods," said spokesman Michael Saia.
But it's the fail-safe for Bushy Park. In a worst case scenario, the system can pull all the water it needs from the Edisto pipe, Saia said.
"If salinity (or specific conductivity) in the Bushy Park Reservoir ever exceeds our ability to produce high quality drinking water, we could use more source water from the Edisto River until any salinity event passes," Saia said.
The threat to the system of sea rise isn't a subject anyone is comfortable discussing.
"That's not something we're studying (in the USGS hydrology office)," Lanier said. "We're collecting data."