San Luis Valley, Colorado

James S. Aber
Emporia State University

Table of Contents
Overview Great Sand Dunes
Water resources Zapata Falls

Overview

The San Luis Valley is part of the Rio Grande Rift system that extends from central Colorado southward through New Mexico and West Texas into northern Mexico. The San Luis Valley of southern Colorado has been called the highest, largest, mountain desert in North America (Trimble 2001). The rift system began to form in the Oligocene, as a large graben sank along deep bounding faults. At the same time, tremendous volcanic eruptions and associated intrusions built up the San Juan Mountains to the west and intrusions took place to the east.

Space-shuttle photograph of San Luis Valley and surroundings, southern Colorado. 1 - Rio Grande, 2 - San Luis Lakes, 3 - Great Sand Dunes, 4 - Sierra Blanca Range of the Sangre de Cristo Mountains. JSC Digital Image Collection; photo ID: STS040-151-126

The magnitude of structural movements is demonstrated by more than 30,000 feet (9 km) of Oligocene-Holocene sedimentary and volcanic fill beneath the floor of the valley juxtaposed with surrounding ranges rising more than 6000 feet (1800 m) above the valley floor. Since the creation of the valley, large alluvial fans have accumulated against the mountain front. This is most obvious around the flanks of the Sangre de Cristos, where a series of alluvial fans slopes down toward the valley floor. The upper portions of the fans are composed of coarse, cobble and boulder gravel derived from the crystalline mountains. Lower portions of the fans have progressively finer pebble gravel and sand toward the valley floor.

Great Sand Dunes

Great Sand Dunes National Monument is located in the San Luis Valley adjacent to the Sangre de Cristo Mountain range. Rising 750 feet (230 m) above the valley floor, these are the tallest dunes in North America. The sand dunes cover an area of about 40 square miles (100 km²), and are situated at approximately 2500 m (8200 feet) elevation.

The sand is blown by prevailing southwesterly wind from alluvial sediments of the San Luis Valley. Sand mineralogy indicates that much of the sediment was derived from volcanic rocks of the San Juan Mountains, transported by the Rio Grande, and deposited in the river's huge alluvial fan on the western side of the valley. From there sand, was blown downwind eventually piling up in the Great Sand Dunes. At the mountain front, sand accumulates where the wind is funneled across a low portion of the mountain ridge at three passes--Medano, Mosca, and Music (less than 3000 m). The mountain ridge rises to more than 4000 m elevation to the south (Blanca Peak) and north (Crestone Peak). Dunes have built up in this location since the Ice Age.

Northward view from Zapata Falls toward Great Sand Dunes National Monument. The sand dunes are located on the floor of San Luis Valley in scene center. Crestone Peak of the Sangre de Cristo Mountain range is visible behind the dune field. Image date 6/99; © J.S. Aber.
View toward the southeast from the Great Sand Dunes. A picnic area is visible near the center of this scene, and snowy Blanca Peak can be seen on the right horizon. Image date 6/99; © J.S. Aber.

The general scenario outlined above has been well known for some time (Upson 1939; Johnson 1971). Continuing research has revealed a more complicated situation (Trimble 2001). The center of the valley, in vicinity of San Luis Lakes, is a sabkha environment, in which sand is cemented just below the surface by salt precipitation from shallow ground water. At present it would be difficult, if not impossible, for wind to transport sand from the Rio Grande alluvial fan across the sabkha. Between the sabkha and the Great Sand Dunes lies a sand sheet consisting of low parabolic and longitudinal dunes that are largely stabilized by scrubby vegetation. Again, it would be difficult for wind to carry much sand across this environment nowadays.

Thus, current conditions do not favor further supply of sand across the valley to the Great Sand Dunes. Accumulation of the Great Sand Dunes must have occurred under different climatic conditions that allowed sand movement across the San Luis Valley. Such conditions were likely during the mid-Holocene when climate was generally hotter and drier, a period known as the altithermal or hypsithermal in the western United States. For example, large dunes of the Nebraska Sand Hills formed 8000 to 5000 years ago in the mid-Holocene. Following an interval of stability, dunes were again active in the late Holocene, 3500 to 1500 years ago (Swinehart 1990). In this regard, the Great Sand Dunes should be considered a relict landform of previous climatic conditions.

The dunes are remarkably stable because of ground water derived from the mountains. The ground water infiltrates from Medano Creek along the eastern side of the dunes. Ground water seeps under the dunes and emerges again in springs west of the dunes. The ground water beneath the dunes limits wind erosion and removal of the sand. Erosion by Medano Creek removes sand from the eastern side of the dunes, and the stream deposits sand on the western side. In this manner, sand is constantly recycled within the dune complex against the mountain front.

Interior of the Great Sand Dunes field. The highest dune (right of center) is a so-called "star dune." The San Luis Valley can be seen in the left background. Image date 6/99; © J.S. Aber.
Massive dune ridge along the southern side of Great Sand Dunes. The steep slip face of a dune can be seen to lower left. Image date 3/02; © J.S. Aber.

Water resources of the San Luis Valley

The San Luis Valley is a true desert, receiving less than 20 cm (8 inches) of precipitation per year. The Rio Grande drains the southern part of the valley through a gorge in volcanic rocks along the Colorado-New Mexico border. In vicinity of Great Sand Dunes, however, the San Luis Valley is a closed depression with no surface outlet for drainage. Surface runoff from the Sangre de Cristos soaks into alluvial fans, and ground water migrates toward the low point at San Luis Lake (< 2300 m altitude). Abundant ground water gives rise to many ephemeral lakes, wetlands, springs and flowing wells, and supports considerable irrigation in the valley.

View over San Luis Lake with the Sierra Blanca Range in the background. Surface runoff from the mountains soaks into alluvial fans and moves as ground water toward the center of the valley, where it emerges in numerous springs and shallow lakes. Image date 3/02; © J.S. Aber.

Throughout most of the San Luis Valley, depth to ground water is less than 12 feet (4 m). Ground water is produced from two major aquifers within the valley (Emery 1971).

Excessive use of surface water has led to water logging of soils in many parts of the valley, water-logged soils have become alkaline, and ground water has become highly mineralized from concentration of salts. Although irrigated crop production is good in some areas, much water use is nonbeneficial. Meanwhile, Colorado is obligated to supply water via the Rio Grande southward under terms of the Rio Grande Compact with New Mexico and Texas (Emery 1971). In order to do so, ground water is "salvaged" via high-capacity wells in the northern portion of the valley and transported in canals southward to the Rio Grande.

Water diversion canal at San Luis Lake. Water in this canal is produced from 160 high-capacity wells and delivered to the Rio Grande to the south. Great Sand Dunes is visible in the left background; Sierra Blanca Range appears on the right horizon. Image date 3/02; © J.S. Aber.

Zapata Falls

Zapata Falls is located at the boundary between an alluvial fan of the San Luis Valley and crystalline bedrock of the Sierra Blanca massif. As South Zapata Creek flows off the crystalline bedrock, it has carved a deep and narrow cleft (or glen) into the rock. The only way to reach the falls is to climb through the cold stream. Well-rounded cobbles and boulders consist of gneiss, schist, phyllite, vein quartz, and other crystalline rocks. The stream drains a glacial valley on the northwestern side of Blanca Peak.

View from the floor of San Luis Valley looking southeast toward the Sierra Blanca massif. Zapata Falls (ZF) is located at the head of a large alluvial fan on the side of the mountain range. 1 - California Peak, 2 - Blanca Peak, 3 - Twin Peaks. Image date 3/02; © J.S. Aber.
The entrance to Zapata Falls on the western side of the Sangre de Cristo Mountains. The falls pass through a narrow cleft, and the only way to reach the falls is to wade through ice-cold water in the stream. Note people standing in shadow to right. Image date 6/99; © J.S. Aber.

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