The Isen and Adorn formed the western boundary of the Kingdom of Rohan and was defended against the Dunlendings, 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 diverted the Isen away from Isengard as he turned the fortress into a war machine, and it was dammed at the northern wall. When the Ents attacked Isengard, they broke the dam and restored the original flow of the river, temporarily drowning all of Isengard.
The Isen seems not to have been bridged, but it could be crossed at the Fords of Isen about forty miles south of Isengard.
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