Title: Separating Fact from Fiction at Great Sand Dunes
Author: Andrew Valdez, Geologist Great Sand Dunes National Park and Preserve
Publication: The Outcrop, November 2005, p. 1, 6-8
The Great Sand Dunes of south-central Colorado are a paradox to many visitors. Large dunes rise to over 700 feet above the valley floor. Combined with the rugged but lush Sangre de Cristo mountains in the background, this area has the sense of contrasting environments that should not be found together. Geologists have been studying the area since the early 1870s and the National Park Service (NPS) has been managing Great Sand Dunes since 1932. This fascinating system has been a mystery to them as well.
Interpretations of the Great Sand Dunes that have not withstood the test of time were the result of: 1) a lack of resource expertise by local NPS staff, 2) geologic studies conducted before aeolian geology was well-organized, 3) assumptions that the Great Sand Dunes originated from late Pleistocene glacial outwash, and 4) attempts to show that groundwater withdrawal threatens Great Sand Dunes. The suspect information included the identification of dune types, the source of the sand, the age of the dunes, and what factors stabilize the dunes.
Great Sand Dunes’ foundation for scientifically sound information began in the late 1970s when a US Geological Survey group led by Ed McKee began studying dune systems, including Great Sand Dunes. Prior to that, work by earlier geologists produced information that described the dunes in the context of the limited dune science of the day. As the understanding of dune systems has evolved, those previous interpretations turned out to be inadequate. Interestingly enough, the park staff of the time chose to focus on the factoids produced by the flawed studies and disregarded the groundbreaking work done by the McKee group.
The big push for knowledge about Great Sand Dunes came during the late 1980s. American Water Development Inc. (AWDI) purchased a large ranch bordering the dunes with the intention of developing groundwater production. Things remained calm until AWDI announced that their groundwater model predicted a lowering of the water table by 50 to 150 feet at the NPS boundary. Park management was alarmed, but was unsure of how to evaluate the consequences that the lowering of the water table would have on the dune system. The need to evaluate this potential threat led to the establishment of a resource management division in 1991 , which was tasked with learning the details of how the Great Sand Dunes system functions. One of the biggest obstacles was obtaining reliable, credible, scientific information on the hydrologic and aeolian system in the Great Sand Dunes area that would form the basis for new or modified interpretations.
Below are some of the interpretations that have been modified with new information:
The major dune type at Great Sand Dunes dune field was originally classified as transverse dunes by Ross Johnson (1967). Later, the McKee group identified them as reversing and star dunes. The correct answer comes from the McKee group. Dune classification is based on dune morphology and is controlled by sand supply and wind regime, and in some cases vegetation growth and topography. Transverse dunes are ridge-shaped dunes that form in unimodal wind regimes and tend to migrate over time. As migratory dunes they are very asymmetrical with long gentle upwind slopes, broad tops, and short, steep downwind slopes containing a slipface that extends to the base of the dune form. Reversing dunes are ridge shaped dunes that form in bimodal or complex wind regime and tend to grow vertically over time. Since reversing dunes respond to winds coming from several directions, the slopes on both sides of the dunes are relatively steep, the dune top is usually pointed, and the slipface is a small feature located near the ridge top that can be found on either side of the ridge. Star dunes form where two or more reversing dunes intersect. Great Sand Dunes has classic examples of reversing dunes, both in form and behavior.
The traditional sand supply for Great Sand Dunes was believed to be fluvial, from the Rio Grande. USGS geologist Rich Madole has proposed that Great Sand Dunes is a playa-sourced system. He is currently working on the details of his theory, but mapping of the aeolian deposits show that they originate in a playa area. Upwind of the playa system and toward the Rio Grande there are no aeolian deposits other than small dunes generated by exposed cropland. Fluvial fed dune systems tend to be found with braided stream channels. The Rio Grande has a meandering channel upwind of Great Sand Dunes. Keep in mind that this is still a work in progress.
Age of the Dunes
Great Sand Dunes was often cited as being 12,000 years old. The source of that information is unknown, but it may have been based on the assumption that the end of the Ice Age would have been a time of increased sediment supply to the area which stimulated dune development as occurred on the Great Plains. The age of the dunes is still unknown; however it is known that Great Sand Dunes is not a surficial feature; aeolian sand extends well into the subsurface. The total depth of the aeolian deposit beneath the dunes is unknown. Well drilling at the base of the dunes has penetrated to a depth of 125 feet and encountered only aeolian sand. The sand collected at the 110-115 depth yielded an OSL age of 20,000 years BP. Further upwind in the valley, on a low-lying deposit known as a sand sheet, the aeolian sand extends to depths of 300 and 350 feet. The local valley is a rift valley and it is very likely that it took over 12,000 years to create and fill 300 feet of accommodation space.
Photos taken of Great Sand Dunes during the 1920s to the present show that the large dune forms appear to be relatively stable. Largely in response to the AWDI proposal, a new theory was presented that stated that dune stability was the result of sand moisture being found six to twelve inches below the dune surface. The theory also stated that the moisture was sourced by groundwater and if the groundwater were lowered, the dunes would dry up and blow away. The theory was never tested, but its sensational nature made it popular with the press and park interpretation adopted it as an explanation of the dune behavior.
The sand moisture/dune stability theory falls apart when applied to other dune fields. Many large dunes in arid to hyper-arid regions are completely dry, yet they are stable forms due to the multiple wind directions that they are exposed to. Algodones Dunes in California are one example. Many coastal dunes have a higher moisture content than Great Sand Dunes, yet they migrate in response to prevailing winds coming onshore. Oregon Dunes are a good example. Sand moisture does create cohesiveness and does dampen sand mobility, but only if the surface is wet. It is an important process along beaches. A desert dune surface is almost always dry with sand available to be transported by wind and whether the dune core is moist or dry makes very little difference on dune behavior. It is a false assumption to expect all dunes to be migratory. The large dunes at Great Sand Dunes are exposed to winds from multiple directions, so one would expect them to be stable. They would only move away from their current position if the wind regime were to become unimodal.
It is also very unlikely that the dune moisture source is groundwater. In many cases, that would require moisture traveling up sand column hundreds to perhaps a thousand feet above the water table. Small migratory dunes at Great Sand Dunes show that precipitation is the source of sand moisture. Near the slipface of these dunes, at the youngest portion of the dune, the sand throughout is dry or just wet near the surface. With distance upwind on these dunes, the dune has been in place longer and moisture penetrates to deeper levels and the dry sand at the base of the dune form thins.
The sand moisture is the portion of groundwater that gravity cannot drain, known as specific retention. As long as the sand is a foot or two below the surface, energy from the sun cannot drive off the moisture and will remain moist, regardless of the position of the water table far below. There is little chance that the dunes would dry up due to a drop in groundwater levels.
Thanks to a perceived threat, Great Sand Dunes now has an active research and monitoring program. Geologic processes such as rifting, sand movement in response to wind regime, sand movement by stream flow, sand stabilization by vegetation growth, and mineral precipitation from ground water contribute to the development of the Great Sand Dunes aeolian system. The NPS and cooperating researchers have focused on measuring these processes to understand them.
The benefits of using scientifically valid information have been great at Great Sand Dunes. First and foremost, whenever the information is presented in scientific forums it is accepted. Secondly, understanding what geologic processes affect the development of Great Sand Dunes has allowed the NPS to fairly evaluate how potential changes to the natural system can impact natural processes. Data and modeling show a strong relationship between extent of stream flow and the position of the water table. Lowering the water table has the potential to decrease stream flow. Stream erosion, transport, and deposition of sand have modified the perimeter of the dune field and contribute to the development of the large dune forms found adjacent to the creeks. Reducing stream flow would negatively impact the stream’s ability to erode and transport sand. Other features, such as interdunal ponds and gaining streams could disappear with just a five-foot water table drop. These wetlands contribute greatly to the diversity of the area. Thirdly, studying the area has led the NPS to realize the dune field is just the tip of the iceberg in a larger aeolian system that includes not only dunes but sand sheets, playas, sabkhas, and wetlands. With that information, a boundary expansion was proposed and granted in 2000 to protect a larger part of the aeolian system. Great Sand Dunes has been re-designated from a national monument to a national park and it encompasses an area now five times larger than the previous national monument.