An arroyo, from Spanish arroyo, is a dry watercourse that temporarily or seasonally fills and flows after sufficient rain). They are usually found in deserts or other arid climates and completely dry up between periods of activity due to water-less conditions. Arroyos can be natural fluvial landforms or constructed flood control channels). They can cut as deeply as 20 meters into the valley floor, are often wider than 50 meters, and can be hundreds of kilometers long. Arroyos are indicative of a river that formerly coursed through the area that is presently dry.

Flash floods are common in arroyos following thunderstorms). In periods of rain, an arroyo acts like a channel, funneling all of the water into one place. As a result, the arroyo rapidly fills with water, which appears in such a high volume that it cannot percolate through the soil to dissipate. As a result, the water rises, and ultimately turns into a river which can create a flash flood, rapidly tearing through the arroyo and wrenching up trees and rocks. Arroyos can potentially be very dangerous, as they are prone to flash-flooding and erosion.

Arroyos are an important source of water in desert regions. In many rural communities, arroyos serve as the primary transportation routes, while in urban settings they could serve as recreational parks that could be used for biking, used by pedestrians or could serve as equestrian courses). Arroyo formation can be very destructive to agriculture. As soon as arroyo cutting begins, the surrounding water table is lowered making irrigation difficult. Arroyos can quickly remove as much as 25% of their valley floor, covering downstream agricultural land with unwanted flood-borne sediment.

Arroyos are very common in desert areas. Occasional heavy rainfall can cause water to flow over the land, forming a stream bed when the water dries up. Arroyos can be shallow stream beds covered with mud or sand, or they can be cut deeply, forming a narrow channel in the ground.