The Anduin River:
Also called the Great River, clearly this is one of the largest, if not the largest, river in the north-western part of Middle-Earth in the Third Age. It started in the Grey Mountains and roughly paralleled the Misty Mountains on the eastern side. At one point in history, hobbits lived on the banks of the river, though they later migrated North and West.
Only three permanent crossings are described. The first is the Carrock, near the High Pass. It is a ford with a rock in the middle. The second is called the Old Ford, fairly close to the Carrock. The third was destroyed by the time of the War of the Ring. It was the bridge in Osgiliath. The only other method of crossing was by swiming (as attempted by Isildur) or by boat.
The river passed by or through the realms of Gondor, Rohan, Lothlórien and the region of Rhovanion. Landmarks along it's course included the Gates of Argonath, Tol Brandir, and Rauros.
For many years, from the Battle of the Gladden Fields, when Isildur was slain, the River hid the One Ring. It was several centuries later that the Ring was found by two hobbits, Déagol and Sméagol.
During the War of the Ring, when the Company was passing down the River, the
Eastern side of the lower Anduin was dangerous because it was patrolled by
Orcs. The western bank however, was still believed to be safe, although this
was disproved when the Company was attacked at Parth Galen.
The Lord of the Rings