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 Perched
Aquifer at Medano Creek
Medano Creek flows on the surface of the sand in spring and early summer
because of the aquifer which supports it is "perched" atop a
mostly impermeable layer of "hardpan" of caliche (calcium carbonate).
During the winter the perched aquifer shrinks and surface flow disappears
until spring snowmelt refills the perched aquifer and Medano
Creek begins to flow on the surface again, usually in April or May.
Note that the regional unconfined aquifer (water table) without Medano
Creek's perching layer is roughly 120 feet below the surface.
Move your cursor over the image above to see how the perched aquifer
changes during seasonal fluctuations.
(Diagram adapted from McCalpin, Geomorphology, hydrology, and Hydrogeology
of Medano Creek, Great Sand Dunes National Park, Colorado, in
Schenk, C.J., 1999.)
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