The River Isen (Angren) started out in a valley in the southern Misty Mountains just north of the fortress of Isengard.

It flowed south to the Gap of Rohan where the North-South Road crossed it at the Fords of Isen.

As the Isen left the Gap it turned to the west and meandered to the Great Sea.

The Isen acted as the southern boundary of Enedwaith, and of Gondor. At the Gap of Rohan the river also acted as the border between Dunland and Rohan.
Encyclopedia entry originally written by PotbellyHairyfoot