All of the water flowing through the Siphandone area might generally be described as part of the mainstream of the Mekong River. There are more than a dozen braids of the river that separate upstream of the area, spread over more than 10 km, and then reunite downstream at the border with Cambodia.

However the Don Sahong project does not include a dam spanning the mainstream. The project will be entirely within the Hou Sahong, and convey on average 15% of the total Mekong River flow. The project is planned on one of the many braided channels of the mainstream, which does not carry the full mainstream flow in either the wet or dry season. The Sahong channel naturally carries about 5% of the total annual flow of the Mekong River through Siphandone. This cannot be considered a significant flow of the main stream. By contrast, the Somphamit and Phapheng channels account for 60% and 30% respectively more than 90 percent of the mean annual flow.