Erosion in the Rio Puerco
How arroyos work
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.
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.
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.