The authorities in Florida said on Sunday that they were making progress in their efforts to drain a leaking reservoir holding more than 300 million gallons of wastewater but warned that were it to breach, it could result in a 20-foot wall of water.

“What we’re looking at now is trying to prevent and respond to, if need be, a real catastrophic flood situation,” Gov. Ron DeSantis said at news conference.

The governor issued an executive order on Saturday declaring a state of emergency for three counties that could be affected by the leaking 79-acre reservoir.

Controlled releases from the reservoir to reduce the chances of a full-fledged breach started on Friday, officials said, resulting in an average of 35 million gallons a day being siphoned.

Still, Mr. DeSantis, a Republican, warned that residents needed to be prepared for “further degradation” of the reservoir, which is part of a system of ponds connected to a former phosphate mine in Piney Point, Fla., south of Tampa.

Scott Hopes, the acting administrator for Manatee County, said the reservoir was down to about 340 million gallons but warned that models suggest that if the reservoir were to give way at that volume, it could result in a “20-foot wall of water” cascading across residential and commercial areas.

“If you are in an evacuation area, and you have not heeded that, you need to think twice and follow the orders,” he said.

On March 26, when the initial leak was reported, the reservoir held about 480 million gallons of water. Before officials started to pump water to reduce the threat of a breach, the reservoir was leaking at a rate of two million to three million gallons per day but conditions deteriorated in recent days, officials said.

More than 300 homes were under a mandatory evacuation order and arrangements had been made to put displaced residents in hotels and shelters. The Florida National Guard was bringing more pumps to augment the 20 pumps already deployed, officials said on Sunday.

The Manatee County Jail, which Mr. Hopes said is a two-story building, is in the evacuation zone. Inmates and staff members had been moved to the second floor — about 10 feet above expected flood levels — and sandbags had been placed at the ground level of the jail, Mr. Hopes said.

The water being discharged from the reservoir is seawater — primarily saltwater from a dredging project — “mixed with legacy process water and storm water runoff/rainfall,” according to a website tracking developments about the reservoir.

“The water meets water quality standards for marine waters with the exception of pH, total phosphorus, total nitrogen and total ammonia nitrogen,” the Florida Department of Environmental Protection said. “It is slightly acidic, but not at a level that is expected to be a concern.”

Officials said the primary concern about the discharged water was the concentration of nitrogen and phosphorus but emphasized that the water was not radioactive.

A sudden, uncontrolled breach could upend stacks of phosphogypsum, a waste product of phosphate mining, that hold the ponds. Phosphogypsum contains “appreciable quantities” of radioactive materials, like uranium and radium, according to the federal Environmental Protection Agency.

The authorities said there were no public water supply wells in the evacuation zone.

Mr. Hopes said it was unlikely that officials would seek to repair a liner in the leaking reservoir. He suggested instead that efforts would be made to deplete the holding ponds and then move to a permanent solution, like filling and capping them.

He expressed hope on Sunday about heading off a catastrophe.

“We have a good plan in place,” he said. “We feel much better than we did three days ago.”

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