The longest river of Third Age Middle-earth rising in the Grey Mountains and flowing south for some 1500 miles to issue through a delta into the Bay of Belfalas.

The northernmost Vales of Anduin were historically the cradle of both the Hobbits and the horsemen of the Eotheod. Later in the Third Age they were occupied by Woodmen and Beornings, but plagued by goblins and wargs of the Misty Mountains and other evil influences issuing from Mirkwood. In The Hobbit Thorin and Company cross the Anduin at The Carrock, some way north of the the Old Ford that continued the east/west road on into the forest.

Further south the Anduin passed by the Gladden Fields, Lothlorien and the Field of Celebrant.  Beyond this point it formed a barrier between the Wold and the Emyn Muil before issuing through the Argonath into the lake of Nen Hithoel and tumbling from there over the Falls of Rauros.

Below the falls the Anduin formed an even more important barrier between Mordor and Gondor, passing the isle of Cair Andros, Ithilien, the ruined city of Osgiliath and on to the port of Pelargir (where Aragorn seized the pirate ships in the Return of the King).

Little is said of the delta at the Mouths of Anduin through which the river flows into the Bay of Belfalas, but there is no doubt thatin the Second and Third Ages, the Anduin was the most important river in Middle Earth serving as a natural barrier between lands under the sway of Mordor and its vassals, and those controlled by Gondor and its allies.

NOTE: Additional information from Cirdaneth.