Rangers of the North
The source of the river was in the southern Misty Mountains, first flowing south through Nan Curunír and across the Gap of Rohan to the edge of the White Mountains, where it abruptly bent west and ran to the sea of Belegaer. At a distance of about 50 lári west of the Gap of Rohan, the Isen was joined by its tributary, the river Adorn. The North-South Road crossed the river at the Fords of Isen where there was a small eyot.
In the Third Age Isen and Adorn formed the boundary of the Kingdom of Rohan, but the triangle of land between Isen, Adorn, and the White Mountains was a contested land, claimed by the Rohirrim as well as the Dunlendings.
The Isen formed a natural boundary in the Gap of Rohan, and was only crossable at Isengard or at the Fords of Isen, where the Rohirrim fought a number of great battles against the Dunlendings and Saruman’s Orcs in the late Third Age.
Saruman has diverted the Isen away from Isengard for his own purposes, and it is dammed at the northern wall.
(Source: Tolkien Gateway)