The apron you are standing on has been under water more times than the park cares to count. The Potomac crested at 36.5 feet here in March 1936, the record, and at 33.8 feet in October 1942. The lower town floods, the towpath floods, and the park closes both when it happens.
That is not a historical curiosity. It happens on a modern schedule, usually after heavy rain in the Shenandoah valley, and the warning sign is upstream weather rather than what is falling on you. If it has rained hard for two days west of here, check the park's conditions page before you drive.
Practical consequences at the confluence itself. Do not swim here. The water at a confluence of two substantial rivers is doing complicated things, there are ledges and old industrial ironwork on the bed, and the flow is much stronger than the flat surface suggests. There is no lifeguard, no designated swimming area and no easy exit.
Fishing is permitted in the park with the appropriate state licence, and the confluence is popular for smallmouth bass, but be aware you may be standing on the state line and licence rules differ either side of it.