Horizontal Grading

January 10, 2008


Provided by: Thomas McGuire
Summary author: Thomas McGuire

Horizontal sorting occurs when a transporting medium, in this case an ephemeral stream in Arizona, slows down. In streams like this, the size of the sediments (boulders, cobbles, pebbles, sand, silt etc) in an indication of the speed of the stream. Clearly, this section of Cottonwood Wash in Cave Creek, Arizona shows a decrease in both slope and stream velocity. Also note the steeper bank on the outside of the curve where the fastest water is swept to the outside by inertia. Similar horizontal sorting occurs when a river enters the calm water of a lake or ocean. The largest particles being transported by the river are deposited first near the mouth of the river, whereas progressively smaller sediments are carried farther from the shoreline.

Cottonwood Wash has had several dramatic flood events since the Cave Creek Complex fire burned 150 square miles (389 square km) of high desert two summers ago. Measurements by U.S.G.S. hydrologists have shown that the water flowed as much as 6.5 ft (about 2 m) deep in this location. Erosion in these desert washes has been compared a to war: months of boredom interspersed with moments of sheer terror.

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