The Wallkill River, a tributary of the Hudson, drains Lake Mohawk in Sparta, New Jersey, flowing from there generally northeasterly 88.3 mi to Rondout Creek in New York, near Rosendale, with the combined flows reaching the Hudson at Kingston.

The river is unusual because it flows north between two major south-flowing rivers, the Hudson and the Delaware River. It also has the unusual distinction of being a river that drains into a creek, due to being impounded shortly before the Rondout confluence into a small body of water called Sturgeon Pool near Rifton, and what reaches the Rondout from there is the lesser flow.