Flowing water adjacent to the Great Sand Dunes

I don’t know about you, but when I think of water and sand dunes together I think about dunes on the seacoast. When I visited Te Paki in New Zealand in February, I was surprised to find a stream separating the parking lot from the dunes. The same thing is true in Great Sand Dunes National Park, where people who want to walk to the dunes from the parking lots have to cross Medano Creek (médano is a Spanish word for ‘dune’). What’s strange about Medano Creek is that it pulses. The phenomenon is known as surge flow, and here’s what the website of Great Sand Dunes National Park says about it:

This 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!

I noticed the phenomenon when I went to take pictures of sand patterns in Medano Creek. No sooner would I compose and take a few photographs, than a “wave” of water would flow downstream and obscure my subject. The picture above shows the shallow regular flow of Medano Creek; the picture below shows a moment of surge flow.

© 2017 Steven Schwartzman