The San Luis river may be based off the Rio Grande river in the east near Hennigan's Stead, and the Colorado River in the west near Gaptooth Ridge. Both Rivers separate Mexico from the U.S at points, with the Colorado River separating California from Baja California and the Rio Grande separating Texas from Coahuila and Chihuahua.
This is one of the few areas where ducks can be found.
In Redemption 2, spending time on the northern shore adds brief bits of Nuevo Paraíso's ambient soundtrack from the first game into the background music.
It is possible to follow the river downstream in Red Dead Redemption 2, by first glitching into Mexico and then using a horse to swim past the former map border south of Nosalida. While there is no vegetation and some textures are of low quality, the coastline remains solid for a significant distance. The river eventually ends in an open area that has slightly different texturing and geography from it's surrounding out-of-borders areas -- on several edges it's surrounded by high cliffs, and compared to it's direct south, east and west the area has lighter textures and has more variation in it's elevation. It is unknown if this perceived uniqueness is intentional however.
In Redemption 2, if the player is traveling on a boat, heading west from the far western part of Flat Iron Lake where the river flows out from, the HUD will show the San Luis River as part of West Elizabeth, which is not labelled on the on-paper version of the map.
If the player rides along the shores of the river in Red Dead Redemption 2, the game will identify it as part of Río Bravo, regardless of which region the player is actually in.