Rock that is easily eroded is one of the essential ingredients of arroyo formation. In semi-arid climates, where there is little vegetation on the landscape, the abundant erosion products are washed downhill, where they accumulate in alluvial fans. This fan, composed of fine-grained sediment, has a small arroyo incised into it.

Numerous fans merge into continuous valley fills of soft, easily eroded material. If slopes are steep enough, vegetation cover low enough, and precipitation high enough, then the stage is set for incision of arroyos.

Volume of material

The arroyos of the main channel of the Rio Puerco have a volume of about 0.30 cubic kilometers. Imagine a pile of dirt 1 kilometer (0.62 miles) wide on each side and 300 meters (the length of three football fields) high.

Material is added to the channel by two major processes: collapse of arroyo walls and piping. Collapse occurs when wall materials are weakened and saturated either by channel flow or by local precipitation. Piping occurs when local precipitation flows into the soft arroyo wall materials and encounters favorable flow paths that are enlarged until they become voids. In some places in the Rio Puerco, piping has created voids that can swallow automobiles. Once introduced into the channel, the disrupted wall materials are readily transported by the stream during periods of high flow. The relative volumes of material that are produced by collapse and piping are unknown.

In-channel sediment movement

Arroyos of the Rio Puerco are ephemeral streams, which means that they carry water only part of the year or, in especially dry years, not at all. One effect of infrequent flow is the absorption of water into the channel. As a storm flood passes downstream, it diminishes in volume; its capacity for sediment transport diminishes; and it gradually drops its sediment load.

An upstream section of an arroyo may pick up material from its bed, thus lowering the channel bottom, while a section downstream will accumulate sediment that can no longer be carried because the channel has absorbed water. Under certain conditions of climate and upstream sediment contribution, a channel can completely refill with sediment.

Storage and movement of sediment

After initial incision, an arroyo typically displays an inner channel and adjoining terrace(s). When high-volume flow exceeds the capacity of the inner channel, then the terrace(s) flood, but their vegetative cover - which develops because flow occurs there less frequently than in the inner channel - slows the flow and causes sediment to fall out. What results is a contest in sediment balance between accumulation on the terrace surface and removal from the terrace wall. Scientists do not yet know the laws that govern rates of retreat of terrace walls; laws that govern sediment deposition on terrace surfaces are relatively better known, but are site specific and complex.

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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