I'll see if I can ID the river to the west after dinner.
I don't think it's considered a tributary of the Mekong as it has its own estuary. However, it could be an arm. But an arm of that size and length would ordinarily be considered a river in its own right as I think I detect that it has its own drainage pattern.
Get back to you later.
From what I can see on these two links the Bassac River was strictly PBR country, but I didn't follow all the links.
http://www.pacifier.com/~sivie/pbr/p0469.htm
http://www.pcf45.com/sealords/sealords.htm
Hope this helps.