The Arroyo Problem in the Southwestern U.S. (page 1 of 3)

Author: Brandon J. Vogt, adapted from http://geochange.er.usgs.gov/sw/impacts/geology/arroyos/, a product of the USGS Earth Surface Dynamics Program.

"The main street of Silver City, New Mexico, is a drainage channel that was originally two or three feet lower than the adjacent ground. In 1887 this channel began to deepen, bridges were built for the crossing and a wooden drop structure was constructed to prevent further headward cutting. This drop structure was carried away in a flood about 1892 and erosion proceeded until, in 1917, the channel was over one hundred feet wide and thirty-seven feet deep. Numerous buildings have been carried away in the progress of this erosion (Olmstead, 1919)."

Description of Arroyos

An arroyo is a nearly vertically walled, flat floored stream channel that forms in fine, cohesive, easily eroded material. Arroyos can cut as deeply as 20 meters (65 feet) into the valley floor, are often wider than 50 meters (165 feet), and can be hundreds of kilometers long. Arroyos exist throughout the western United States, but are most common in arid and semi-arid climates in the Southwest. The rapid widening and deepening of arroyos have both changed the physical environment and been a costly nuisance in the west since settlement began in the mid-1800s.

Earlier Cycles of Cutting and Filling

The most recent period of arroyo formation in the southwest, which occurred from about 1865 to 1915, was not unique. Previous cycles of arroyo cutting (erosion) have occurred at about 2000 years Before Present (B.P.) and 700 years B.P. (Emmett, 1974). These cycles of arroyo cutting were both followed by slower periods of filling (alluviation). Since about 1915, arroyo development, with a few notable exceptions, has slowed with many arroyos showing aggradation (Cooke and Reeves, 1976). The primary modifications since this time have been widening of trenches, grading of walls, aggradation, and slow changes in bed levels.

Page 2: Causes of Arroyo Formation
Page 3: Effects and Corrective Treatments
References and Resources