The Arroyo Problem in the Southwestern U.S. (page 2 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.

Causes of Arroyo Formation

Three factors may cause arroyo formation, but the relative contribution of each is difficult to discern. The main factor is thought to be a change in climate that produced unusually heavy rainfall. Land-use practices, such as grazing, may have enhanced arroyo formation in the southwest during the most recent period of erosion (A.D. 1865-1915). A natural cycle of erosion and deposition caused by internal adjustments to the channel system is a third possibility (Graf, 1988; Schumm and Hadley, 1957).

Climate

Flooding caused by heavy rain may produce arroyos. Although climate records in the southwest were not systematically kept before about 1900, recent studies have found evidence for unusually heavy rainfall in Tucson, Arizona during the late 1800's (Betancourt and Turner, 1993). This rainfall was caused by strong and frequent ENSO (El Nino Southern Oscillation) events, suggesting that heavy rain was a regional phenomenon. Thus, the climate of the Southwest during the most recent period of arroyo entrenchment was conducive to large floods (Hereford, 1993). Drainages may have been especially vulnerable to arroyo cutting, if unusually wet ENSO conditions occurred immediately following a period of below normal precipitation. During a dry period, the enervated vegetation would not have its normal capacity to protect the soil from rain-drop impact or to absorb and slow runoff.

Land use

With the settlement of the West came the rapid introduction of cattle, sheep, and horses. From 1870 to 1890 the number of livestock in New Mexico increased from 300,000 to 2,300,000 (Peterson, 1950). Similar increases were reported in other Western states during this time. Valley floors, which were the most dependable forage areas for the animals, were quickly overgrazed. The fragile vegetation was consumed, and the soil was compacted and left extremely susceptible to erosion. To further exacerbate the soil conditions, both humans and livestock created trails along stream channels and nearby hillsides forming small ditches, leaving the land surface susceptible to arroyo formation.

Nevertheless, earlier periods of arroyo formation predated the introduction of livestock, and thus overgrazing cannot be judged solely responsible. Spanish and Mexican ranchers, moreover, introduced large numbers of livestock in the 1700s without associated erosion. For these reasons, many researchers conclude that other factors such as climate change may have played a more important role in arroyo formation.

Natural Internal Adjustments

A third explanation for arroyo development involves external forcing and climate change as triggering mechanisms for incision. This theory postulates that the system has to be in a state ready for incision and involves random, heavy rain and flood events along with internal adjustments in the channel system. If a cloudburst occurs over a drainage, it may deeply erode a single channel, leaving hanging valleys where tributaries enter. Future runoff through the tributaries would cause incision at their mouths and arroyo elongation through upstream migration of headcuts (Dellenbaugh, 1912). As sediment accumulates in stream valleys, the gradient of the water course changes. When the slope builds to a critical point, entrenchment may be triggered causing arroyo formation. This type of arroyo formation causes the channel to shift laterally across the stream bed with each cycle of incision. An example of this phenomenon is illustrated from the 800-year entrenchment history of Red Creek in Utah based on tree-ring data from trees growing in the flood plain. The author concluded that lateral movement of the stream channel in its flood plain was responsible for controlling arroyo development (LaMarche, 1966).

Summary

While it is arguable which component has contributed the most to arroyo formation in the Southwest, it is widely accepted that climatic events, human settlement and land use, and naturally occurring internal adjustments in drainages are probable causes. The temporal coincidence of the causes may have magnified the effect of each factor.

Page 3: Effects and Corrective Treatments
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