The San Luis Valley is a region in south-central Colorado with a small portion overlapping into New Mexico. The valley is approximately long and wide, making it the largest alpine valley in the world. It extends from the Continental Divide on the northwest rim into New Mexico on the south. It contains 6 counties and portions of 3 others. It is an extensive high-elevation depositional basin of approximately with an average elevation of above sea level. The valley is a section of the Rio Grande Rift and is drained to the south by the Rio Grande, which rises in the San Juan Mountains to the west of the valley and flows south into New Mexico. The San Luis Valley has a cold desert climate but has substantial water resources from the Rio Grande and groundwater.
The San Luis Valley was ceded to the United States by Mexico following the Mexican–American War. Hispanic settlers began moving north and settling in the valley after the United States made a treaty with the Utes and established a fort in the early 1850s. Prior to the Mexican war the Spanish and Mexican governments had reserved the valley to the Utes. Later in the 19th century Anglo settlers began to settle in the valley and engaged in mining, ranching, and irrigated agriculture. Today the valley has a diverse Anglo and Hispanic population.
History Utes Prior to 1868 the Capote (Kapota) band of Ute Indians lived in the valley. The Utes made a treaty of peace with the United States in 1849 after the Mexican War. Shortly thereafter settlers from New Mexico established several small settlements in what is now Colorado and in 1868 the Utes were removed to a reservation in western Colorado. They continued to play a role in Saguache in the northwestern corner of the valley from the Los Pinos Agency to the west of Saguache until they lost their extensive reservation as the result of the Meeker Massacre in 1879.
Spanish and Mexican administration The area was administered as part of the Spanish, later Mexican, province of Nuevo Mexico. In 1843, the government of New Mexico created the Sangre de Cristo Land Grant consisting of of the valley and granted the family of Carlos Beaubien. The area was ceded to the United States in 1848 after the Mexican–American War in the Treaty of Guadalupe Hidalgo.
United States administration Extensive settlement began in the San Luis Valley, primarily by Hispanic farmers and ranchers from New Mexico, in the 1850s. Early settlers built a church in the village that is now called San Luis and dedicated it on the Feast of Saint Louis, 21 June 1851. At present, the San Luis Valley has the largest native Hispanic population in Colorado; many families are directly descended from the original New Mexican settlers. The surge of immigration followed the construction by the U.S. Army of Fort Massachusetts for protection against the Utes, who had previously barred settlers. The history of the U.S. military presence in the valley is preserved at Fort Garland and other historic preserves in the valley.
The San Luis Valley became part of the Territory of Colorado in 1861. The original Ute population was confined to the Southern Ute and Ute Mountain Indian reservations in the late 19th century.
Unlike the rest of Colorado, the United States surveyed the lands in the San Luis Valley using the New Mexico Meridian and Baseline. The San Luis Valley was one of eight candidate sites to detonate the first atomic bomb with White Sands Proving Ground ultimately selected for the Trinity nuclear test. The outlaw Felipe Espinosa operated in the San Luis Valley.
Geography The San Luis Valley is the broad, generally flat, valley at the headwaters of the Rio Grande in south central Colorado and far north central New Mexico. The northern portion of the San Luis Valley is an endorheic basin; surface water does not exit this area. Irrigated agriculture is possible in the area due to groundwater and streams fed by the average 100 inches of snow the surrounding mountain ranges receive. The southern portion is drained by the Rio Grande.
There is no clear southern boundary but the term is generally used to include the San Luis Hills of southern Colorado and the Taos Plateau of northern New Mexico. About 50 miles from east to west and about 150 miles from north to south, the valley is bounded on the east by the Sangre de Cristo Mountains and on the west by the San Juan Mountains.
Within Colorado the San Luis Valley is generally considered to comprise five Colorado counties: Saguache, Alamosa, Rio Grande, Conejos, and Costilla. The principal towns are: Alamosa, Monte Vista, Del Norte, South Fork, Saguache, Center, Moffat, Fort Garland, San Luis, Blanca, Antonito, La Jara, Capulin, Manassa, Sanford, Romeo, Crestone, Villa Grove, Hooper, Mosca, San Acacio and a number of smaller locations. A few other counties of Colorado have some land in the Rio Grande Basin including Archuleta, Mineral, Hinsdale, and San Juan.
Blanca Peak is prominent in the Sierra Blanca at the southern end of the northernmost section of the mountains, which is known as the Sangre de Cristo Range. There are several passes, with elevations between , giving access to the valley. North La Veta Pass, through the Sangre de Cristo Mountains, is used by U.S. Highway 160 and by the Colorado Pacific Rio Grande Railroad (formerly a branch of the Denver & Rio Grande Western) tracks. Other passes used historically were Medano, Mosca and Sangre de Cristo Passes.
The Great Sand Dunes are a famous feature of the valley. They lie directly to the west of the Sangre de Cristo Mountains. The dunes can reach high. The Great Sand Dunes National Park and Preserve is now in place to protect both the dunes and the numerous archeological sites found in the area. The natural valley aquifer is close to the surface in this part of the valley, and helps with maintenance of water levels in the San Luis Lakes, just to the west of the sand dunes.
Elevation rises as you go north in the valley to Poncha Pass, used now by U.S. Highway 285 and historically by the narrow gauge tracks of the Denver and Rio Grande Railroad. Otto Mears, then of Saguache, built and operated a historic toll road over Poncha Pass at the north end of the valley into the San Luis Valley during Colorado's 19th century mining era when the valley was the gateway to the San Juan and Gunnison country and the Ute agency was in the mountains west of Saguache. Mount Otto on the east side of the valley is named after him.
Cumbres Pass is a 10,015 ft. pass between Antonito, Colorado and Chama, New Mexico. The pass is traversed by State Highway 17 and the Cumbres and Toltec Scenic Railroad (originally built as the San Juan Extension of the Denver and Rio Grande Railroad that ran to Durango, Colorado). From the headwaters of the Rio Grande Wolf Creek Pass is the route of U.S. Highway 160 between Del Norte, Colorado and Pagosa Springs, Colorado, while Spring Creek Pass is the route of State Highway 149 between U.S. Highway 160 and Lake City, Colorado. Stony Pass, sometimes spelled Stoney Pass, a historic wagon road to the mining camps of the San Juans, is now a jeep trail. At the north end of the valley, North Pass is the route of State Highway 114 between Saguache, Colorado and Gunnison, Colorado, bypassing the original route over Cochetopa Pass, now a county road.
The Rio Grande follows a course through the southern valley from Del Norte southeastward via Alamosa to New Mexico. South of Alamosa it is joined by several streams from the west including the Alamosa River and the Conejos River and Culebra Creek from the east. Most of the northern valley is an endorheic basin called the San Luis Closed Basin. Generally, within the Closed Basin the major streams such as Saguache Creek, San Luis Creek, and the streams from the west face of the Sangre de Cristos flow only a short distance onto the valley floor as surface streams. Only in very wet years, perhaps every 20 years, does the stream system in the Closed Basin flow as a contiguous unit into San Luis Lake at the low point of the Closed Basin west of the Great Sand Dunes.
Land ownership and management
About 50% of the in the San Luis Valley is privately owned. Much of the land in the south part of the Valley, in Conejos and Costilla counties, was originally part of large Mexican land grants and is private land.
on the borders of the valley (generally adjacent to National Forest Lands) are managed by the Bureau of Land Management, BLM, a division of the United States Department of the Interior. This land is usually leased to neighboring ranches for grazing for a nominal fee. Part of the value of a ranch is its continuing lease of BLM or National Forest lands.
Public lands in the mountains surrounding the San Luis Valley are generally part of the Rio Grande National Forest and are managed by the United States Forest Service.
Large areas of private lands have either been subdivided into small "ranch" lots or have been sold or donated to the Federal government and make up portions of the Great Sand Dunes National Park and Preserve, other wildlife preserves, and various state wildlife sites.
Geology
The San Luis Valley contains an alluvial basin, the Alamosa Basin, which lies at the north end of the Rio Grande rift. Deposits include the deeper alluvial strata, overlying Precambrian basement rock, Eocene alluvial deposits from a shallow Laramide depression, the Blanco Basin Formation; and Oligocene ash and lava flows associated with the Conejos Formation, is the Santa Fe formation, mixed alluvium and lava outflows from the San Juan volcanic field to the west; later strata comprise the Alamosa formation, alternating layers of sand, gravel, and clay. The relatively impermeable clay layers trap ground water in the "confined aquifer" which, if tapped, supplies artesian wells. Surface waters, including the Rio Grande and other streams, are hydrologically connected to the "unconfined aquifer" a shallow ground water formation which underlies much of the valley. The deeper confined aquifers have sufficient hydrological connection with the surface waters of the valley that they are not considered for the purposes of Colorado water law "nontributary" waters which could legally be mined. Streams which flow into the valley, particularly the Rio Grande, have deposited alluvial fans where they emerge from higher terrain onto the valley floor.
Ancestral Rockies, the Central Colorado Trough, and the San Luis-Uncompahgre Highlands The gap between the Precambrian basement rock and the Eocene alluvial deposits in the San Luis Valley is considered evidence that much of southern Colorado, including the western portion of the San Luis Valley, was highlands in the period prior to the Eocene Epoch, 56 to 34 million years ago. There were lowlands to the north in central Colorado, the Colorado sag, an east–west basin. It is from those highlands to the west, in Paleozoic, Pennsylvanian, and Permian times that the coarse alluvial deposits of the Sangre de Cristo Formation such as the Crestone Conglomerate originated. The Minturn Formation, a limestone formation exemplified at Marble Mountain on the crest of the Sangre de Cristos, is evidence of a sea in the Central Colorado Trough during the era of the Ancestral Rockies.
Western Interior Seaway