There is a river in the real US state of Georgia called the Lanahassee Creek. It is a tributary to Kinchafoonee Creek.
Lanahassee is a name taken from the Muskogee language which most likely means "rancid yellow water". Variant spellings are "Lanahasee Creek", "Lannahasee Creek", and "Lannahassee Creek".
There are two ways to pronounce the name of the river. One way is "Lah-na-hass-ee", commonly used by Americans and characters with a strong southern accent, like Arthur Morgan. Another way is "Lah-na-ha-chee", which is commonly used by characters who don’t speak English as a first language, like Javier Escuella. Arthur and Javier speak about this discrepancy in one of the fishing missions.
The river in early development had an outlet to the sea, but for unknown reasons it was withdrawn and replaced with land, turning the Lannahechee River into a river without an outlet as it also flows from Flat Iron Lake. This would be because the island of Guarma was planned to be much larger and to be able to be explored in free roam, being able to get there by buying a boat ticket in Saint Denis. This outlet can be seen on the map leaked before the game release.
If the player gets to a certain point whilst travelling the river, the territory will be known as Guarma.
Saint Denis seen from the southern part of the Lannahechee River
The out of bounds swamps, south of Lannahechee River