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This is a neat place to visit. In the spring, there's a "pulsating stream," which is fed by snow melt from the surrounding mountains. When the sun is out, the water is very warm. When the sun goes behind a cloud, the water and surrounding sand becomes very cold in less than a minute -- an excellent real-world example of solar heating.

The pulsating stream is described here (the first place I found it....):

The persistence of Medano and Sand Creeks has prevented the dune field from extending up into the Sangre de Cristo Mountains. Medano Creek flows off Mosca Pass and then along the east, southeast, and south sides of the Great Sand Dunes. Sand Creek flows from the mountains just north of Medano Creek and continues along the east, northeast, and north sides of the Great Sand Dunes. These two creeks wash accumulating sand from the east side of the dune field, carrying it west back to the base of the dune field from where it is again blown east by the prevailing wind. This is recycling on a grand scale and it is accomplished, in part, by the "pulsating flow phenomenon" in Medano and Sand Creeks. The water flowing in these two shallow and wide creeks builds up miniature sand dams. The dams briefly impound the stream flow and then fail, sending a small wave of water down the stream, until it builds a new dam which, in turn, fails. This unique pulsating stream flow was one of the reasons that, in 1932, President Hoover proclaimed the Great Sand Dunes National Monument.