The North Stair was the ancient way to travel between the lake of Nen Hithoel and the foot of the Falls of Rauros. When the Fellowship of the Ring was camped above Sarn Gebir Boromir objected to traveling across the lake, pretending that there was no way down past the falls. Aragorn sternly reminded him of the stairway and the possibility of carrying their boats to the lower river. Later, when the Company was awaiting Frodo’s decision at Parth Galen, Aragorn spoke to the others about the great works in the area, including the great Stair “beside the falls”. It was not stated in the book but presumably the North Stair was on the western side of the river where there was firmer land opposite the Nindalf marshes.