Medano Creek: The River That Acts Like an Ocean

Every spring, something happens at the base of the tallest sand dunes in North America that makes no physical sense until someone explains the geology. A creek runs along the eastern edge of Great Sand Dunes National Park, between the dune field and the Sangre de Cristo range. In spring snowmelt, that creek swells wide and shallow, and the flow does something no other river in Colorado does: it generates waves. Not white-water turbulence, but slow, rhythmic pulses of water that roll across the sandy shallows like small surf.

The phenomenon is called surge flow, and it happens because the sandy creek bed is so permeable that water saturates it, momentarily sealing the surface, and then the built-up water releases in a wave. The cycle repeats every few seconds, and the result is a mountain stream that behaves like a shallow beach. Families wade in it every May and June. People bring sleds to the dunes and towels to the creek, which is a combination that exists nowhere else in the American West.

The dunes themselves formed over roughly four hundred and forty thousand years, carried by wind from the San Luis Valley floor. The Sangre de Cristo range stopped them. The mountains catch the sand; the mountains catch the precipitation; the mountain streams bring the sand back down each time the wind pushes it too far east, recycling it back into the field. The system is in a kind of dynamic equilibrium, the dunes neither growing nor shrinking over long time scales, just moving.

What you are standing in front of is a four-hundred-thousand-year process that still has not finished deciding what to do with itself.