The Potomac River at Great Falls Park kills more than half the people who get into trouble in it.

That statistic from the National Park Service should give any McLean resident pause, especially as the remnants of Hurricane Isaias were forecast to bring up to 2.5 inches of rain to Northern Virginia on Sunday, Oct. 11. Forecasters also warned of tidal flooding along the Potomac as strong onshore winds pushed water upstream, coinciding with high tides and a new moon.

Fifty-one percent of all river-related injuries in the Potomac River Gorge are fatal, according to the Park Service. Seventy-two percent of those incidents involve swimmers, waders, or people who slipped off rocks.

What lurks below the surface

The river's danger is its deception. The Park Service warns that the Potomac looks calm on top but hides invisible underwater currents and whirlpools that pull downward. Those currents can trap a person 15 to 30 feet below the surface or pin them under submerged rock ledges.

The riverbed compounds the problem. Gaps between rocks, called sieves, can hold a person underwater. Recirculating hydraulics tumble someone in place. Deep potholes create additional entrapment points. These are specific physical mechanisms, not simply fast current carrying a swimmer downstream.

WorldAtlas ranked the Potomac at Great Falls and Mather Gorge second on its list of American rivers with the strongest undertow, behind only the Niagara River.

On average, the Potomac at Great Falls runs 27 feet deep with a flow rate of 10,000 cubic feet per second, according to the Park Service. Mather Gorge, the narrow canyon the river cuts through the park, contains Class 3 to 5 rapids and is classified as one of the most dangerous sections on the Potomac.

Swimming and wading are illegal

The Park Service prohibits all water contact at Great Falls Park. That includes dipping feet in the water. Violations carry a fine of $200 or more and a required appearance in federal court. Offenders can be formally arrested.

The prohibition exists because even standing at the water's edge risks a slip that ends in the current.

A pattern of drownings

The danger is not theoretical. Montgomery County Fire and Rescue Service's Swift Water Team patrolled the Potomac near Great Falls on Saturday, Aug. 1, after several drownings in local waterways that summer. Days before that patrol, search crews had recovered the body of a 25-year-old man near the park, 7News reported.

Pete Piringer, chief spokesperson for Montgomery County Fire and Rescue, described the river's deception during that August patrol: "It looks very peaceful. It's quiet. It looks very … calm on the surface, but I assure you that the currents are very swift and dangerous."

After Isaias, extra caution

The Patch forecast for Northern Virginia projected 1.5 to 2.5 inches of rain from Isaias remnants on Sunday, with isolated totals of 3 to 4 inches possible. Whether those totals materialized at Great Falls has not been confirmed.

Residents with questions about current conditions at Great Falls Park can call the visitor information line at 703-757-3101.