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

Effects of Arroyo Cutting

Swamps in the Southwest during the Last Century

Observations before 1865 describe verdant river bed marshes, known as cienegas, containing beaver ponds, fish, and tall grasses which were nourished by high water tables (Bryan, 1925). These marshes have since been drained by arroyos, altering the flora and fauna of the area by widening and deepening the original stream channel.

Decreased Agricultural Productivity

Arroyo formation can be very destructive to agriculture. As soon as arroyo cutting begins, the surrounding water table is lowered making irrigation difficult. Arroyos can quickly remove as much as 25% of their valley floor (Cooke and Reeves, 1976, p. 3), covering downstream agricultural land with unwanted flood-borne sediment. This sediment does not improve the fertility of the underlying alluvial soil because it contains large quantities of sand and gravel that originate from subsoils and deposits of soil forming materials (Cooperrider and Hendricks, 1937).

Flooding

Sediment deposits in the Little Colorado River

The often excessive deposits of sediment from upstream arroyo formation can decrease flood protection by reducing the natural regulatory functions of stream channels. Sediment from upstream arroyo erosion fills channels that otherwise would store flood water. Arroyos also increase flood severity by changing the geometry of the stream channel. Development of an arroyo in a previously braided or meandering drainage straightens and shortens the channel which limits flood water dispersal and increases velocity.

Displacement of People

Because of the loss of land to arroyos and the increased difficulties of farming, humans have occasionally been forced to either change their agricultural practices or to relocate. Where cropping had depended on irrigation, problems of increasingly fluctuating and decreasingly reliable water sources and difficulties of transferring water to fields drove out farmers or forced a change to grazing (Gregory and Moore, 1931). Other damages include destruction to roads, railroads, bridges, culverts, fences, and irrigation works. In the late 1880s, the entrenchment of the Rio Puerco in New Mexico forced the desertion of the towns of San Ignacio, San Fernando y Blas, and San Francisco (Bryan, 1925). Prehistoric arroyo cutting may have been one of the main factors leading to abandonment of southern Utah and northern Arizona by the Anasazi (Hereford et al., 1995).

Corrective Treatments

Attempts to mitigate damage from arroyo cutting date back to the Civilian Conservation Core in the 1930s when attempts were made to slow the erosion of headcuts and banks by reducing grazing and installing control structures. Livestock growers, although fully conscious of the erosion menace, are generally not convinced that their herds are responsible for erosion or that their removal will effect a cure (Peterson, 1950, p. 425). In one study, isolated tracts of land near arroyo banks were fenced to keep livestock out to promote revegetation. The results varied widely. In some locations, the increase in vegetation stabilized the arroyo walls, whereas in others recovery was insignificant (Peterson, 1950, p. 426). Other methods of erosion control involve tree planting along banks, the introduction of debris into the channel to slow the flow, and the construction of spreader dikes to catch silt. Unfortunately, such erosion controls are costly. There is no known solution to the arroyo problem.

Continued Research and Education

Further research is needed to better understand the rapid and often destructive erosion caused by arroyos in the Southwest. To better conserve soil, we must continue to study the effects of land use practices on arroyo formation and develop efficient and cost-effective erosion control methods. Finally, we must work to model and forecast the effects that future climate change may have on arroyo development.

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Page 1: Description of Arroyos

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