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. The arroyos of the main channel of the Rio Puerco have a volume of about 0.30 cubic kilometers. 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.