This has been brought up before, John. I think caution is advised unless one of us can produce a detail map showing, say, a 10-mile square patch at the point where the Mekong passes over the border. I'm not familiar with that particular geography, but until I am, I'm willing to concede that a zoom-in may show that the Mekong takes a short wind right along the border itself before it crosses into Cambodia. We need that map.
Try this:
http://rapidfire.sci.gsfc.nasa.gov/gallery/?2003239-0827/Mekong.A2003239.0330.250m.jpg
http://academic.uofs.edu/faculty/gramborw/atav/maps.htm
