What advantage did the colonies from the Red River to the Sabine river have over others?

They had union objectives. They also had a plan they wanted to control their own river. And most of them were confederate that's also how they had a advantage.

This area has easy fluvial communication with the Mississippi River. This is why this area was first settle by french colonists, then by spanish colonists, and finally by american colonists. Let's not forget that first recorded ferry use in the area was as early as 1794. In Jack Kerouac's novel, On the Road, the sabrine river is mentioned.