Mystery Photo 46 shows Medano Creek in Colorado's Great Sand Dunes National Park.

Medano Creek is not a run-of-the-mill stream.  It exhibits a phenomenon called "surge flow." Since there are not many places in the world where you can see surge flow, this is really special.

Surge flow occurs when the water in a stream flowing across sand advances in rhythmic surges or pulses marked by small waves or wavelets. To get surge flow, three elements must operate together: (1) a relatively steep gradient, which makes the water flow quickly; (2) a smooth, mobile creekbed that offers little resistance; and (3) a high enough volume of water to produce surges. At Great Sand Dunes, both Medano Creek and Sand Creek can produce surge flow in the spring and summer when water is abundantly available.

The process is fun to watch, especially if viewed up close up while wading in the shallow water (which lots of people do in order to reach the great dunes on the far side). As water flows across the sand in the stream bed, a bunch of little "sand dams" (proper name antidunes) form on the creek bed, impeding the water flow in the sections of the stream where the sand dams form. When the water pressure becomes too much for the little dams, they break. This sends water surging down the stream.  At Medano Creek, the surges tend to happen about every 20 seconds or so.  Most are only a few inches high, but in years with unusually heavy water flows the surges can be a foot tall.

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