The Tsauchab River, which also feeds the Sossusvlei in good rainy seasons with water, has over millions of years eaten its way into the limestone and formed this 30 meters deep and one kilometre long canyon. Sesriem, translated into English means "Six belt". It is often claimed that this name originates thereof, that the first settlers had to knot six ox wagon belts together to draw water here for their oxen. But this is not proven. There is a second variant where it got its name from. At this Canyon poachers were caught with oryx carcasses and were busy manufacturing the sixth belt from the skin. What the truth is, I'll tell you on the edge of the canyon.

More information can be found here:
http://en.wikipedia.org/wiki/Sesriem