People living along our desert rivers know that as one climbs out of the valley, there often is a series of relatively steep inclines separated by gently-slopping, step-like areas, or terraces. A discerning person might notice such terraces occurring at the same elevations on both sides of the valley. What causes this?
The answer lies in the climatic oscillations of the Ice Age. Rivers
deepen or fill their valleys depending on many things, including their load of
sediments and the volume of water carried by the river. During glacial times, with cool
summer temperatures and respectable precipitation, rivers carried large volumes of
water. Well-vegetated surroundings produced relatively small amounts of sediments
carried from the watershed. Lightly-loaded rivers thus picked up bottom sediments,
excavating the valley floor. In warmer, drier times, loss of vegetation increased
erosion of sediments into low-volume river waters unable to handle the load. Each
terrace pair represents such a period of deposition left high and dry by subsequent
down-cutting. As the climate cycled, terrace after terrace was left behind in the
overall excavation of the valley.
Contributor: Arthur H. Harris, 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.