Anarroyo (/ə.ˈrɔɪ.oʊ/ (from theSpanisharroyo (Spanish:[aˈroʝo], "brook"))) orwash is a drywatercourse that temporarily or seasonally fills and flows after sufficientrain.[1]Flash floods are common in arroyos followingthunderstorms. It is akin to theCatalanrambla, which stems from theArabicrámla, "dry river".
Similar landforms are referred to aswadi (in North Africa and Western Asia),chapp in the Gobi,laagate in the Kalahari,donga in South Africa,nullah in India,fiumare in Italy, anddry valley in England.[2][3]
Thedesert dry washbiome is restricted to the arroyos of the southwestern United States. Arroyos provide a water source to desert animals.
Arroyos can be naturalfluviallandforms or constructedflood control channels. The term usually applies to asloped ormountainous terrain inxeric anddesert climates. In addition: in many rural communities arroyos are also the principal transportation routes; and in many urban communities arroyos are also parks and recreational locations, often with linear multi-use bicycle, pedestrian, and equestrian trails.Flash flooding can cause the deep arroyos or deposition of sediment on flooded lands. This can lower thegroundwater level of the surrounding area, making it unsuitable for agriculture. However a shallow water table lowered in desert arroyovalleys can reducesaline seeping andalkali deposits in thetopsoil, making it suitable for irrigatedfarming.
TheDoña Ana County Flood Commission in theU.S. state ofNew Mexico defines an arroyo as "awatercourse that conducts an intermittent or ephemeral flow, providing primary drainage for an area of land of 40 acres (160,000 m2) or larger; or a watercourse which would be expected to flow in excess of one hundred cubic feet per second as the result of a 100 yearstorm event." Research has been conducted in thehydrological modeling relative to arroyos.[4]Natural arroyos are made through the process known as arroyo-cutting. This occurs in arid regions, such as New Mexico, where heavy rains can lead to enlargement of rivers cutting into surrounding rock creating ravines which are dry under normal weather conditions.[5]It is argued, however, whether these excessively stormy periods are the sole cause of arroyo-cutting as other factors such as long-term climate changes must also be taken into account.[6] Further,overgrazing by livestock throughout the 20th century and today has removed vast amounts of surface vegetation which decreases ground infiltration of precipitation and increased runoff, increasing speed and energy of high flow rain events. Coupled with groundwater pumping this increases downcutting in arroyos as well. Arroyo cutting which occurred in the 1900s in the southwestern United States caused serious farming issues such as a lowered water table and the destruction of agriculture lands.
Inagricultural areas in climates needing irrigation,farmers traditionally relied on small constructed arroyos,acequias,zanjas oraqueduct channels and ditches for the distribution of water.
An example of larger constructed arroyos is inAlbuquerque, New Mexico. There are several miles of open-airconcrete lined drainage channels that drain an area into the main North Diversion Channel, atributary of theRio Grande joining upstream of Albuquerque. After the San Juan Project Water Treatment Plant here, the Rio Grande's flow exceeding that needed for the river'ssilvery minnowhabitat is available for municipal water supply diversion. Signs are posted at the constructed arroyos warning to keep out due to danger offlash flooding, which can turn a dry channel into a roaring river in under two minutes, even when there is no rain in the local area. From 2017 to 2021, twelve people were killed by sudden flooding of drainage channels in Albuquerque, despite most residents being aware of the risk.
TheArroyo Seco andLos Angeles River are more famous examples inSouthern California of former natural arroyo seasonal watercourses that became constructed open drainage system arroyos.
The term "wadi" (Arabic) is also used to designate a torrent valley ... whereas others are torrents during the rainy season but dry up completely during the rainless season.
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