Sallis and Durant, Mississippi: Opposite Sides of the River

Durant (Holmes County) lies in west-central Mississippi, near the Big Black River but not on the Mississippi River itself.

Sallis (Attala County) lies in central Mississippi, east of the Big Black River.

Neither town is on the Mississippi River. The Mississippi forms the state's western boundary ~50–80 km west of both. The Big Black River (tributary) runs between them—Durant west of it, Sallis east.

If the question implies the Mississippi River: both are east of it. If it implies the Big Black River: Durant is west, Sallis is east. Context matters.