<P> The San Luis Valley is a region in south - central Colorado with a small portion overlapping into New Mexico . It is the headwaters of the Rio Grande . It contains 6 counties and portions of 3 others . The 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 . Prior to the Mexican war the Spanish and Mexican governments had reserved the valley to the Utes, their allies . During the 19th century Anglo settlers settled in the valley and engaged in mining and irrigated agriculture . Today the valley has a diverse Anglo and Hispanic population . </P> <P> It is an extensive high - altitude depositional basin of approximately 8,000 square miles (21,000 km) with an average elevation of 7,664 feet (2,336 m) 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 valley is approximately 122 miles (196 km) long and 74 miles (119 km) wide, extending from the Continental Divide on the northwest rim into New Mexico on the south . The San Luis Valley has a cold desert climate but has substantial water resources from the Rio Grande and groundwater . </P>

Where is the san luis valley in colorado