When a valley is filled with fine sediments, as is the valley of the Rio Puerco, erosion causes steep-walled gullies called arroyos. These channels create significant barriers to transportation, access of livestock to water, and diversion of water for crop irrigation. The process of cutting arroyos into valley bottoms is called incision.

The Rio Puerco has not always had continuous arroyos. Prior to the 1880’s, the area around the nearly abandoned town of San Luis was a broad flood plain. Annual renewal of soils by flood-borne silt encouraged vigorous grass cover. Historical accounts state that this was prime grazing land, and that residents of San Luis irrigated crops by diverting river waters into their fields.

Incision of the arroyo caused drastic changes. The river now flows entirely within the arroyo; without floods, the valley bottom supports desert shrubs rather than grass. Diversion of river water to fields near San Luis would have to originate far upstream, and it is impractical to maintain the needed diversion and transport structures.

As streams in the basin entrenched into the valley floor the water-dependent riparian habitat shrank laterally and entrenched. Cottonwood trees, once prevalent in the valley bottoms, are now scarce. Reduction in wooded habitat has provoked an inevitable reduction in population of birds, grazing animals, rodents, and other species that depend on shade, food, and concealment provided by the riparian forest environment. Despite the reduction in riparian area, the arroyos and their terraces still support populations of mule deer, raccoons, coyotes, and other animals. In the many parts of the basin where arroyos are now refilling with sediment, terraces that store the accumulating sediment widen over time as a result of meandering of the channel within the arroyo. Much of the vegetation on these new terraces, however, is exotic: Russian olives and tamarisk (salt cedar) trees displace native cottonwoods. Thus, the current cycle of incision has caused significant changes in habitat and ecological resources of the basin.

Arroyos have little respect for human structures. The soft materials of the valley bottom are readily removed by flowing water. Meandering streams inevitably impinge on the bases of structures like this bridge, eventually removing the bridge approach. Engineering solutions require extensive armoring of channel walls using materials brought in from quarries at some distance, which makes these solutions expensive. It is often better (as in the case of this illustration) to abandon threatened structures and to choose stream crossings at locations that have some form of natural protection, rather than provide artificial protection.

For more information on historical changes of arroyos, please see The Rio Puerco Arroyo Cycle & the History of Landscape Changes.

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U.S. Department of the Interior
U.S. Geological Survey
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Last modified: 15:04:23 on 15-Mar-2006