Dunes National Monument (hereinafter called Great Sand Dunes)
is located in south-central Colorado along the eastern edge
of the San Luis Valley (fig. 1). The San Luis Valley is bounded
by the Sangre de Cristo Mountains to the east and the San Juan Mountains
to the west. Great Sand Dunes has an altitude ranging from about
7,500 to 10,300 feet and receives annual average precipitation of
about 11 inches near the Great Sand Dunes Headquarters (fig. 2).
Figure 1. Location of the Great Sand Dunes
National Monument, south-central Colorado.
The predominant feature in Great Sand Dunes is the 30 square-mile
main dune field (fig. 2) which rises up to 750 feet above the valley
floor. The age of the dunes is estimated to be between 2,000 and
12,000 years old (Chatman and others, 1997). Composition of the
sand indicates the predominant source is the San Juan Mountains
to the west. The sand was transported to the San Luis Valley by
the Rio Grande and other local streams and then carried by prevailing
southwesterly winds to the Sangre de Cristo Mountain front. The
shape of the Sangre de Cristo Mountains acts as a sand trap
(Chatman and others, 1997).
The Sand Dunes Need Water
As incongruous as it might seem, existence of the sand dunes is
dependent on water (Chatman and others, 1997). Two streams flowing
from the Sangre de Cristo Mountains, Medano Creek on the east and
southeastern edge, and Sand Creek on the northwestern edge (fig.
2), transport the sand southwestward around the perimeter of the
dunes and deposit the sand on the upwind side, where prevailing
winds transport the sand northeastward toward the Sangre de
Cristo Mountains again. The rate of sand transport northeastward
by wind generally is the same as the rate of sand transport southwestward
by water; together, the two mechanisms maintain the dune field in
a state of quasi-equilibrium (Chatman and others, 1997). Streamflows
typically are largest during spring snowmelt, which corresponds
with the period of greatest sand transport by the creeks.
An important ecological feature of Great Sand Dunes are wetlands
associated with interdunal ponds and depressions that nearly intersect
the water table near the western edge of the dune field. The interdunal
ponds and depressions provide important habitat for a variety of
species that depend on a wetland habitat in an otherwise desert
environment. The interdunal ponds are of special concern because
their numbers have decreased markedly in recent years. The relatively
sudden disappearance of ponds appears to indicate a considerable
change in Great Sand Dunes hydrology. The existence and natural
maintenance of the dune field and the interdunal ponds are dependent
on maintaining ground-water levels at historic elevations.
Two Aquifers Exist at Great Sand Dunes National Monument
A shallow aquifer and a deep aquifer are the two principal aquifers
at Great Sand Dunes. All the water in Medano and Sand Creeks, except
what is lost to evapotranspiration, infiltrates into the ground
to recharge the shallow aquifer. The shallow and deep aquifers are
separated by an impermeable layer of clay about 8 feet thick. Wells
V and Y, located near the western edge of Great Sand Dunes (fig.
2), intercept the clay layer about 300 feet below the land surface.
It is not known if the deep aquifer extends eastward all the
way to the Sangre de Cristo Mountain front.
How Old is Water in the Shallow Aquifer?
Ground water in the shallow aquifer is much younger than ground
water in the deep aquifer (Rupert and Plummer, 2004). Water first
seeps down to the shallow aquifer from Medano and Sand Creek (fig. 2).
Figure 2. Shaded relief map of the Great
Dunes National Monument, south-central Colorado.
Ground water near Medano and Sand Creeks probably is only a few years
old. The ground water then travels in a southwesterly direction,
underneath the main dune field. Additional water from precipitation
on the dune field moves down to and mixes with the ground water
underneath the main dune field. Ground water in the shallow aquifer
also travels to springs that discharge to Big and Little Spring
Creeks on the west side of Great Sand Dunes. Ground-water age dating
using chlorofluorocarbons (FreonTM) and tritium (an isotope of
hydrogen) indicate that it takes more than 60 years for ground water
to travel underneath the dune field from Medano and Sand Creeks
to Big and Little Spring Creeks (Rupert and Plummer, 2004).
How Old is Water in the Deep Aquifer?
Using carbon-14 geochemical dating methods, the age of ground water
sampled from well Y, which is completed in the deep aquifer and
is near the western edge of the dune field (fig. 2), is about 30,000
years before present (plus or minus 3,000 years) (Rupert and Plummer,
2004). This age was determined using methods similar to those used
to date charcoal found in firepits of archeological sites. The last
major ice advance (Wisconsin) during the ice age peaked about 20,000
years before present (U.S. Geological Survey, 1992); ground water
from the deep aquifer is older than that.
Chatman, M., Sharrow, D., and Valdez, A., 1997, Water resources
management plan, Great Sand Dunes National Monument, Colorado: National
Park Service, Water Resources Division, 197 p.
Rupert, M.G., and Plummer, L.N., 2004, Ground-water flow direction,
water quality, recharge sources, and age, Great Sand Dunes National
Monument and Preserve, south-central Colorado, 2000-2001: U.S. Geological
Survey Scientific Investigations Report 2004-5027, 32 p.
U.S. Geological Survey, 1992, The great ice age: U.S. Geological
Survey General Interest Publication, ISBN 0-16-036025-0, 8 p. Accessed
May 29, 2002, at URL http://pubs.usgs.gov/gip/ice_age/