I don't know much about that confluence, but a similar kind of river/mixing phenomenon can be seen in estuaries where rivers with large sediment load reach the ocean. the brown color of the river water is caused by loads of fine sediment in suspension. when salt levels in the water increase through mixing with sea water, the fine sediment naturally coagulates and settles. this paper has a fairly obtuse and technical explanation of what's going on. in short, higher salt levels in water generally encourages fine sediment to glom together, making larger particles that are denser and more readily sink.