[Illustration: In the San Luis Valley, runoff from the San Juan and
Sangre de Cristo Mountains sinks into layers of sand in the Alamosa
Formation. Flowing along the sand layers toward the center of the
valley, it provides artesian water for irrigation of valley farmlands.]
SAN JUAN MOUNTAINS
LIMIT OF FLOWING WELLS
HUBBARD’S WELL
OTTOWAY’S WELL
ALAMOSA WELL
GEORGE NEWSOM’S WELL
CALKIN’S WELL
LIMIT OF FLOWING WELLS
Moraine
Alluvial Slope
SANGRE DE CRISTO MOUNTAINS
Sands, lava beds, gravels, conglomerates, etc.
Alamosa formation
Granites
WEST
SANTE FE FORMATION
SANTE FE FORMATION
EAST
Groundwater is extremely important to Colorado, especially in the
Prairie Province and the San Luis Valley. Below these two areas lie a
number of distinct and productive groundwater aquifers, several of them
artesian. In Otero County, for example, there are five major aquifers:
three separate Quaternary gravel deposits, the Cretaceous Dakota
Sandstone, and the Cheyenne Sandstone Member of the Purgatoire
Formation, also Cretaceous. All these aquifers are characterized by
their high porosity and permeability, which allow water to flow rapidly
through them. Wells in the younger, shallower aquifers produce as much
as 2,000 gallons per minute; those in the older, deeper aquifers produce
about eighty gallons per minute, some of it with an artesian “head.”
The San Luis Valley supports intensive agriculture, made possible by a
great artesian water supply. A thick series of soft interlayered clays
and sands, the Alamosa Formation, slopes down toward the center of the
basin from the surrounding mountains. Water entering the sandstone beds
at the mountain edges flows through the sand layers held there by the
impermeable clay beds. By the time it reaches the center of the valley,
it has developed considerable hydrostatic head, and the water rises in
wells without pumping. Unfortunately, both the irrigation water and the
soils in the San Luis Valley are highly alkaline. Constant evaporation
from the irrigated fields has concentrated the alkali near and on the
surface, rendering some of the land less usable than it was originally.