The Isen, called Sîr Angren in Sindarin, was a river that formed the western border of Rohan from its junction with the Adorn to the outer fences of the fortress of Isengard.
Course
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 river, the Adorn. The North-South Road ended on the western shore of the Fords of Isen. There, the Isen passed in two arms around a large eyot. The Fords of Isen were the only place south of Isengard where large armies, escpecially if they were mounted or heavily armed, could cross over the Isen. About a mile east of the gates of Isengard, a bridge had been built over the Isen where it ran through a very deep channel.
In the Third Age, the Isen and the Adorn formed the western 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 during certain periods of the Third Age.
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.
The mouth of Isen was the northernmost landing point of the fleets of the Corsairs from Umbar and Harad that assailed the coasts of Gondor in T.A. 2758 and aided the Dunlendings against Rohan.
When the Ents attacked Isengard, they dammed the Isen and other springs and streams in Nan Curunír, and dug trenches to divert the river's flow, before breaking the dams and temporarily drowning all of Isengard.