Not much of River's background is ever revealed except that he was friends with Blue Eyes and Ash. He didn't like that Blue Eyes blamed his father for nearly dying and initially looked up to Koba after he told them the worst atrocities humans had committed. When they rediscovered humans after one of them shot Ash, River wanted Caesar to give them the order to kill them but he just yelled at them to go with the other apes joining.
After Caesar was supposedly killed by the humans and burned their home, River was so enraged he joined Koba in assaulting their home but was very uncomfortable using guns and much preferred his spear but was overruled by Koba. After the humans started firing back, River's rage was quickly replaced with fear and watched as Koba killed a human mounting a tank and used it to blow open the gates. After Caesar was revealed to be alive and that Koba shot him, River went back to serving Caesar after he killed Koba. For years he fought the Alpha-Omega army and started courting a female chimpanzee named Kami.
One day when he was guarding a trench, an explosion rang out and the apes were attacked and River was shot in the shoulder and fell off his horse and the body of another ape named Spark fell on top of him and struggled to move before reinforcements arrived and killed the humans save four. River was later present when Caesar talked to the prisoners and let them go to send a message the apes just wanted to left alone. River eventually managed to reach the oasis that would become their new home and told Maurice stories about his life so he could write them in a book Maurice planned to give to Cornelius when he was old enough.
- In an early draft of Dawn, River was originally the name of Caesar's eldest son before it was changed to Blue Eyes.