It comes after a mother and daughter are believed to have drowned after falling into the river Monday.
Park Superintendent Kevin Brandt said that while the Potomac may look inviting on a hot day, don't go in it.
"If you slip into the river, you will be sucked under its roiling waves," he said. "In one minute underwater, you will suffer irreversible brain damage. In less than three minutes, you will be dead."
The U.S. Park Police said it will step up enforcement of the no swimming rule. Those caught will not receive a warning. Instead they will be cited, and face a fine of as much as $500.
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