English: Medano Creek and Sand Creek exhibit a globally rare phenomenon call surge flow, a stream flowing in rhythmic waves on sand. Three elements are needed to produce waves: 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 late spring, these elements combine to make waves at Great Sand Dunes.

As water flows across sand, sand dams or 'antidunes' form on the creek bed, gathering water. When the water pressure is too great, the dams break, sending down a wave about every 20 seconds.

This photo was taken near peak flow on May 26, 2023. Due to lower snowpack, it was a lower-flow season.

NPS/Patrick Myers