Developer Joel Burgess, who was River's owner, has announced the demise on Twitter and went over River's impact on Fallout 4's development team to understand a dog companion. River was also filmed, recorded, and had her behaviour watched as a reference for Dogmeat.
Burgess explained in detail about how River influenced his desire to have a dog companion in Fallout 4 be more than a weapon, but a true friend to the player. Burgess' design goals for the character was of bringing the personality of a loving, protective, outgoing dog to Fallout 4 and therefore River's looks, movements, and specially the personality were reflected in the in-game character. Dogmeat's propensity to bring you large objects, even ones you didn't want, was based on River.
"River really loved making people happy...sometimes by bringing us large objects. Her intentions were pure, but her judgement wasn't always perfect," said Burgess, accompanied by a picture of River carrying a large log.
Likewise, a walk with animator Jean Simonet inspired Dogmeat's behaviour: River would often walk ahead of Burgess, but consistently stop to look back and check on him.
River's behaviour in the game is also based on the behaviour of German Shepherds, protective dogs who will put themselves between the thing they want to protect and danger. Dogmeat does just that. "This didn't just give Dogmeat a distinctive combat style from other companion NPCs; it was a character choice," Burgess said.
It is really not easy to lose a friend, especially when that friend is a constant companion like a dog. It made all of us sad and our hearts go out to Joel Burgess, and everyone who knew River. So let's raise a glass to River's memory today. As Dogmeat, at least, she can keep bringing people joy (and large sticks) for years to come.