"Medano Creek is one of the few places in the world where one can experience surge flow, a stream flowing in rhythmic waves on sand. Three elements are needed to produce the phenomenon: a relatively steep gradient to give the stream a high velocity; a smooth, mobile creekbed with little resistance; and sufficient water to create surges. In spring and early summer, these elements combine to make waves at Great Sand Dunes. As water flows across sand, sand dams or antidunes form on the creekbed, gathering water. When the water pressure is too great, the dams break, sending down a wave about every 20 seconds. In wet years, waves can surge up to a foot high."