Walking on Water
When you walk on shallow Medano Creek at the base of the dunes, you are walking on water that extends deep below the surface. The dunes sit on top of an aquifer that extends up to a mile below the valley floor. Streams flow on top of the high water table, and most wetlands here are actually the visible top of the aquifer, where it fills in the lowest depressions in the dunes and valley floor. Recharged each year by stream runoff from the mountains, the aquifer is two–layered, with an unconfined upper layer and a deeper layer largely confined by seams of blue clay. Instead of flowing into rivers that eventually reach the ocean, streams flow on the valley surface, then sink down through the sandy soil, primarily into the unconfined aquifer. Because of these "disappearing" streams, and long underground faults around the aquifer, water in the northern half of the San Luis Valley is trapped into a closed basin. The dunes rise at the eastern edge of the basin, at the base of the Sangre de Cristo Mountains. This dramatic setting is the foundation of the massive dunes and their hydrological system, from mountain peaks to verdant wetlands.
The Heartbeat of the Dunes
The cradling, flowing arms of Medano Creek on the east and south sides of the dunes, and Sand Creek on the north are the coronary arteries of the Great Sand Dunes. Sand grains blow into the dunes from the valley floor, bouncing up and down over a sea of sand, until they drop into Medano or Sand Creeks. The water in the creeks captures the sand, carrying it back downstream to the valley floor, only to be picked up again by the wind. The wind/water recycling continues, piling the sand to gigantic heights. To a large extent the creeks’ flow depends directly on the high water table beneath them. If the water table were permanently lowered for any reason - even a few feet lower - the creeks’ surface flow would diminish or stop, since the water would sink below the surface to meet the lowered water table. Rather than flowing around the dunes, streams would sink below the surface near the mountains. Sand recycling would slow down or cease. The dunes, unbounded by the water, would eventually spread out over a larger area. By protecting lands west of the dunes, the National Park Service protects the massive but sensitive aquifer beneath the dunes. The mountain watershed, streams, and many of the wetlands of the dunes system are now also protected as part of Great Sand Dunes National Park and Preserve.
Streams With a Pulse
Medano Creek and Sand Creek are unusual in another way. This is one of the few places in the world where one can experience surge flow, a stream flowing in rhythmic waves on sand. Three elements are needed to produce the phenomenon: a relatively steep gradient to give the stream a high velocity; a smooth, mobile creekbed with little resistance; and sufficient water to create surges. In spring and early summer, these elements combine to make waves at Great Sand Dunes. As water flows across sand, sand dams or antidunes form on the creekbed, gathering water. When the water pressure is too great, the dams break, sending down a wave about every 20 seconds. In wet years, waves can surge up to a foot high!
Sunny White Beaches?
The high water table supports other geologic features important to the dunes. A sabkha is a whitish plain where sand is cemented together by evaporated salts from seasonal ponds. The sabkha here has developed in the large wetland region west of the dunes. These sensitive wetlands are held at the surface by the high water table, and are subject to sub–surface changes in the aquifer. Without regular saturation and evaporation, the sabkha would slowly disintegrate. Rare shore birds, mammals, amphibians, and the globally threatened slender spiderflower (Cleome multicaulis) all depend on these desert wetlands for their existence.
Dig down anywhere in the dunes a few inches and you will find wet sand anytime of year. The source of this moisture content - 7% throughout the dunes - mostly comes from precipitation captured over time. The dry sand on top actually serves as a moderate barrier to escaping moisture. About 11 inches of precipitation falls on the dunes in an average year, while about 50 inches falls on the high peaks above the dunes, most of which flows down into the valley’s aquifer. At the base of the dunes, the water table can moisten the sand up to about three feet above the actual water level through capillary action, allowing wetlands to form higher than the water table. The sand’s unexpected moisture enables specialized plants, the Ord's kangaroo rat, dunes insects, and even some amphibians to survive the dunes environment.
Pure and Clean
Water quality is monitored regularly throughout the park and preserve. The mountain water in Medano Creek is of such purity that it qualifies under rigid standards for "Outstanding Waters" designation by the state of Colorado. Though the water gains natural salinity as it reaches the sabkha, it remains essentially unpolluted. Streams and ponds throughout the park and preserve provide an ideal habitat for all wildlife, especially for species that are threatened in other areas due to water pollution or disappearing habitat.
Recent research has focused on dating the water at various depths and localities in the park. Carbon-14, Tritium-dating, and other methods have been used to estimate ages of the different waters. By dating the water, scientists can learn how water moves through the dunes system. Water emerging into streams just west of the dunes predates the 1940s, and may be far older. The oldest water in the park, beneath the dunes in the deep aquifer, fell as precipitation before the last ice age.
The Past and Future
Protecting this entire natural hydrological system was the driving force in the 2004 expansion of the national monument to a large national park and preserve. In the 1980s and 1990s, commercial water development north of the dunes threatened the entire dunes system. As a result of citizen support statewide, Congress acted, creating a national park and preserve to better protect the system’s waters. Scientific research helped us to initially understand the importance of this hydrological system to the Great Sand Dunes. Ongoing hydrological research and monitoring will help us better understand the nature of this complex system. By collecting a variety of data across many decades, we can begin to understand trends and changes in trends, and the complex relationships between precipitation, streamflow, and the dunes’ aquifer. Protecting this system for future generations requires a continuing commitment to understand, value, and enjoy this national treasure.
K-12 Hydrology Learning Activities
Ground-Water Flow Direction, Water Quality, Recharge Sources, and Age, Great Sand Dunes National Monument, South-Central Colorado, 2000-2001 US Geological Survey, Michael G. Rupert and L. Niel Plummer
Hydrogeology of the San Luis Valley, Colorado, An Overview—and a Look at the Future (.pdf file, 44 kb) Philip A. Emery
San Luis Valley Project, Closed Basin Division, Colorado (.pdf file, 1.21 mb) Alamosa Field Divsion Staff, Bureau of Reclamation
The Role of Streams in the Development of the Great Sand Dunes and their Connection with the Hydrologic Cycle (.pdf, 5.7 mb) Andrew D. Valdez, Great Sand Dunes Geologist