The great river that carries Boromir’s funeral boat is the Anduin. After Boromir dies defending Merry and Pippin, Aragorn, Legolas, and Gimli place his body in one of the Fellowship’s Elven boats and set it on the river.
The funeral takes place near Parth Galen, where the Fellowship breaks apart. Boromir is laid out with his sword, cloak, belt, helmet, broken horn, and the weapons of the Orcs he killed. Aragorn and Legolas sing a lament as the boat drifts toward the Falls of Rauros.
The Anduin is the longest and one of the most important rivers in Middle-earth. It flows south between major regions including the Misty Mountains, Lothlórien, Rohan, and Gondor. The Fellowship travels along part of it after leaving Lothlórien.
Boromir’s boat eventually reaches the southern parts of the river. Faramir later sees the boat near Osgiliath and recognizes his brother’s body. The other options are rivers or streams in Middle-earth, but none carries Boromir’s funeral boat.