Erosion in the Desert Southwest

July 15, 2011

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Photographer: Thomas McGuire
Summary Author: Thomas McGuire

The photo above features the Paria River in southern Utah. It was taken during a period of normal discharge – non-flood or drought conditions. Note the brown color of the water indicating that the river is transporting a relatively large sediment load. The white deposits on the riverbank are the mineral gypsum.

While most parts of the southwestern deserts of the U.S. receive far less rainfall than the eastern U. S., erosion occurs much faster in the Southwest than it is in any other region of the country. For example, it’s estimated that before dams were built on the Colorado River, the Colorado carried, on average, about 400,000 tons of sediment per day. Comparatively, the Hudson River in New York, in a much more humid climate, historically transports about 1,300 tons of sediment per day. That’s approximately one-thirteenth the erosion rate per square mile of the Colorado River. Yet, the average rainfall in the Hudson River basin is about three times that of the Colorado River watershed. Thus, in spite of the dry climate, erosion in this example is more than ten times as rapid in the desert as it is in the humid eastern U.S.

Why is this the case? Several factors contribute to the high erosion rates in the desert Southwest. There’s relatively little plant cover to protect the rock and soil, and a large portion of precipitation comes in intense monsoon thunderstorms (especially in Arizona and New Mexico). In addition, active regional tectonic uplift in the Southwest has resulted in steeper river gradients, so water moves down-slope at a faster rate.

Photo Details: Camera: FUJIFILM FinePix S9000; Focal Length: 12.8mm; Aperture: f/6.4; Exposure Time: 0.0018 s (1/550); ISO equiv: 80; minor adjustments made in Adobe Photoshop.