Water and the Dunes
The Great Sand Dunes consists of four sand deposits that cover much of the northeast San Luis Valley. The sand deposits begin in the center of the valley and change with elevation and wind energy in an eastward direction. These deposits have developed in response to a number of geologic, hydrologic, and biologic processes. These processes are: rifting, sand movement by wind, sand movement by streams, vegetation growth, and evaporation of groundwater.
The Role of Surface Water
The Great Sand Dunes are located in the closed basin portion of the San Luis Valley where rift-related subsidence is the greatest, and a large depression has formed. The largest streams that flow into the closed basin are Saguache Creek out of the San Juan Mountains and San Luis Creek from the Sangre de Cristo Mountains. Historical flow from these streams can reach the bottom of the closed basin, filling ponds and lakes. During very wet periods, the lakes can fill the closed basin depression and become tributary to the Rio Grande River. This was last documented to have occurred during the 1920s and possibly the 1940s.
Current land use now requires that most stream flow be diverted for agricultural use, so the opportunity for the closed basin to fill is diminished. Periodic input from the Rio Grande may occur since sand deposits located along the southern margin of the closed basin have a stream channel morphology and trace up gradient to the Rio Grande.
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.
Streams also modify the perimeter of the dunefield and help give the Great Sand Dunes their majestic character. Medano Creek flows along the east and southeast dunefield margin and Sand Creek flows along the northern margin. These streams erode sand from the upper margins and deposit it along the more distal margins of the dunefield, giving the dunefield its crescent shape. Each lobe of the crescent is built by stream-deposited sand.
Medano Creek flows extensively along the margin and its effect is dramatic. The upper dunefield margin is truncated by the creek, resulting in a large slipface. The sand deposition on the southeast floods the area with sand resulting in closely spaced north-south trending dunes. The excess sand fills the troughs of the north-south trending dunes resulting in the northeast-trending ridge seen on the horizon. The large dunes that are seen as one drives into Great Sand Dunes National Park is a sand ridge built by sand supplied by Medano Creek. Edges of the dunefield that lack stream-supplied sand have much smaller dune forms.
The presence of water is often associated with high levels of biological diversity. At the Great Sand Dunes it also leads to greater diversity in the physical system. Using good science to understand the role of water at Great Sand Dunes has helped the National Park Service manage and protect Great Sand Dunes National Park and Preserve. This includes: evaluating potential treats, such as groundwater withdrawal, expansion of the National Park Service boundary to protect the entire system (the previous boundary protected only the dunefield), and the ability to interpret the dunes to the public with greater depth and accuracy.
For more information on the Great Sand Dunes National Park and Preserver, please call 719-378-6399.
For more information on Water 2012 in the Rio Grande Basin, please see their website www.rgwcei.org or www.water2012.org.