Between 2003 and 2007, North Gasherbrum Glacier on the northeastern slope of the Karakoram mountains in Asia underwent a dramatic acceleration, during which a velocity wave propagated down the glacier. There was a significant transfer of ice from up-glacier downstream, which resulted in a strong surface elevation increase over the lower tongue, but only a moderate advance of the glacier snout. We interpret this behaviour as that of a glacier surge, and we explain the observations by means of a simple version of the Kamb drainage-switching theory.