Ever notice a playa or a cattle tank holding water while lying far above dry land downslope? Strange, because our desert water table often lies hundreds of feet below the surface. So how come these perched pools? The answer, of course, is that they lie in a depression floored by an impervious layer; a layer of material unpenetrable by water. In most of our natural playas, fine particles of silt and clay have been carried into the low spots, packing together close enough to slow down the movement of water to lower depths. Then, cycle after cycle of flood and evaporation deposits minerals carried by in-flowing water into remaining vacuities, plugging any escape. Later, deposits of mineral salts may grow to be many feet deep.
And those cattle tanks? Unless fortuitously located on beds of clay,
stirring the pot, so to speak, may be necessary to first suspend fine particles that
then are allowed to settle into a closely packed, waterproof blanket. Just hope that
during some dry spell, an ambitious rodent doesn't burrow through, pulling the
plug.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.