7.1 SAN LUIS VALLEY & RIO GRANDE BASIN
The two major hydrogeologic units in the San Luis Valley are the upper unconfined aquifer and the lower confined aquifer, predominantly within the Alamosa Formation (Table 7.1-1). A series of clay layers in the upper Alamosa Formation forms the confining layer between the two aquifers. Depth to the confining clay layers varies from about 100 feet in the northern part of the basin to about 40 feet in the southern part of the basin.
The unconfined aquifer consists of Quaternary alluvial fan deposits with some well-sorted aeolian sands and the uppermost sandy layers of the Alamosa Formation. The alluvial deposits are underlain, in most areas, by the Alamosa Formation which is composed of fine dark sands interbedded with discontinuous blue, gray, and green clays and silts. The geologic materials comprising the upper portions of the confined aquifer vary with location within the valley. In the northern and central part of the San Luis Valley, this unit consists of unconsolidated sand and gravels of the lower Alamosa Formation. At the western edge of the valley, in the Monte Vista Graben and on the Alamosa Horst, the confined aquifer consists of unconsolidated sands and gravels of the Los Pinos Formation, and in the Baca Graben, it is composed of unconsolidated to semi-consolidated sands and sandstone of the Santa Fe Formation.
In addition, there are alluvial deposits along the Rio Grande and its tributaries and along other streams that are hydraulically connected with the basin-fill deposits within the basin. In the mountainous area of the Rio Grande watershed the alluvium is considered to be separate from the surrounding crystalline-rock aquifers. Alluvial stream deposits consist of a mixture of gravel, sand, silt, and clay and are generally only tens of feet thick.
The Rio Grande becomes a shallow, meandering river as it enters the San Luis Valley. Alluvial deposits associated with the river are in hydraulic connection with
the basin-fill deposits of the valley. Photo by V. Matthews, CGS.