High water closes the lower town. When the rivers come up, the park shuts the lower town and the C&O towpath, and that closure is the whole of the advice. This is a town at the meeting of two rivers with a flood record of 36.5 feet in 1936 and 33.8 feet in 1942, and the 1936 flood ended people living on Virginius Island altogether.
Rain takes the outfitters off the water. Not always for the obvious reason. A trip can be cancelled for a rise that happened yesterday sixty miles upstream, under a blue sky here. Outfitters read a gauge rather than a window, and when they call it off, take it seriously.
Wet rock is the real hazard on the heights. Maryland Heights is rated moderate to strenuous, climbs up to 1,788 feet, and ends at an unfenced ledge above a long drop. In the wet, the descent on rock and root is where injuries happen. The 1810 stone steps are the same problem in miniature, and in winter they hold ice long after the streets have cleared.
Thunderstorms in summer build fast over the Blue Ridge and arrive with little warning. Being on an exposed ridge when one does is worse than being wet.
What does not close: the park itself, which opens every day except Thanksgiving, 25 December and 1 January, and the shuttle, which runs on every day the park is open.