Here’s the latest installment of the Valley Courier’s Colorado Water 2012 series. Andrew Valdez explains some of the geology — along with the role of surface water — in the formation of the Great Sand Dunes. Here’s and excerpt:
Streams play an important role in the delivery of sand to the sand deposits. The sand of the Great Sand Dunes originates in the surrounding San Juan and Sangre de Cristo Mountains. The weathering process, aided by water, breaks rock down into boulders and smaller fragments that can be transported by water into the San Luis Valley. Saguache and San Luis Creeks and their tributaries collect water in their mountain basins and flow into the valley bringing sediment with them. Coarser material, such as boulders and cobbles, are deposited in alluvial fans at the mountain front as stream flow slows down.
Beyond that, and into the San Luis Valley, the low energy streams continue to transport sand and finer material. Once the streams reach a playa lake/pond, the sand is deposited in a delta and silt and finer material settle in the open water of the lake. Wave action can redistribute the sand forming a beach deposit.
Playa lakes/ponds are temporary features that begin to dry, and then disappear as water input drops below water loss. Once dry, the former beach deposits are exposed to the wind, and can be transported by the wind. Small playas often have crescent-shaped sand ridges on their downwind side that are known as lunettes.
The dominant wind direction on the valley floor is from the southwest, therefore dunes formed from the beach deposits will migrate to the northeast toward the Sangre de Cristo Mountains. Currently some of these dunes migrate an average of 35 feet per year. As they approach the passes of the Sangre de Cristo Mountains, the wind patterns change from unimodal or dominant in one direction, to bimodal, or dominant in two directions. In this case, the bimodal directions are from the southwest and northeast. In this bimodal zone, dune behavior changes and the dunes stop migrating and begin to grow vertically. Along the mountain front, away from the passes, the winds remain unimodal and sand ramps form as dunes migrate up the slope.
More Colorado Water 2012 coverage here.