The river begins in Seekonk, Massachusetts in the swamps east of Prospect Street, near Walker Street. From there, the river flows in a southwesterly direction towards East Providence, Rhode Island, where it then forms the boundary between Massachusetts and Rhode Island. It continues flowing along the state line to its end at Mobil Dam. Below Mobil Dam (the boundary between fresh and salt water), the river broadens out and empties into Hundred Acre Cove and becomes the Barrington River.
Below is a list of all crossings over the Runnins River. The list starts at the headwaters and goes downstream.
Aitken Brook and Luthers Brook are the only named tributaries of Runnins River, though it has many unnamed streams that also feed it.