Arroyo (watercourse)

An arroyo—from the Spanish word for “brook”—is a dry channel that only carries water after enough rain falls, often filling and flowing seasonally or temporarily. Thunderstorms can trigger flash floods in these washes. The term typically describes a watercourse in sloped or mountainous areas of dry, desert climates. Similar landforms go by other names around the world: wadi in North Africa and Western Asia, chapp in the Gobi, laagate in the Kalahari, donga in South Africa, nullah in India, fiumare in Italy, and dry valley in England.

Arroyos can be natural features or man-made flood-control channels. In many rural areas, they serve as main transportation routes; in cities, they often become parks with trails for biking, walking, and horseback riding. Flash floods can carve deep arroyos or deposit sediment on flooded land, lowering the local groundwater level and making farming difficult. However, a lowered water table in desert arroyo valleys can also reduce salty seepage and alkali buildup in topsoil, which may improve conditions for irrigated agriculture.

Natural arroyos form through a process called arroyo-cutting, which happens in arid regions like New Mexico. Heavy rains can enlarge rivers, cutting into surrounding rock and creating ravines that stay dry in normal weather. But whether stormy periods alone cause this cutting is debated; long-term climate changes also play a role. Overgrazing by livestock has removed surface vegetation, reducing how much rain soaks into the ground and increasing runoff, which speeds up and strengthens high-flow events. Groundwater pumping adds to the downcutting. Arroyo-cutting in the southwestern United States during the 1900s caused serious farming problems, including a lowered water table and loss of agricultural land.

In agricultural areas needing irrigation, farmers have long used small constructed arroyos, acequias, zanjas, or ditches to distribute water. A larger example is in Albuquerque, New Mexico, where miles of open-air concrete drainage channels feed into the North Diversion Channel, a tributary of the Rio Grande. Signs warn people to stay out because flash floods can turn a dry channel into a roaring river in under two minutes, even without local rain. Between 2017 and 2021, twelve people died in Albuquerque drainage channels from sudden flooding, despite most residents knowing the risk. The Arroyo Seco and Los Angeles River in Southern California are famous examples of natural seasonal arroyos that were turned into constructed open drainage systems.