At Phnom Penh, the Mekong River divides into two principal arms, flowing nearly parallel to each other through Vietnam to the South China Sea. The E arm, and principal waterway used by ocean-going vessels, is known as Song Tien Giang, and the W arm is known as Song Hau Giang or
Bassac. (
from this link)
Also, note name of town from your map in Post 29 and from this tourist writeup of a hotel: "Situated on the banks of Bassac River in Chau Doc.. "