An arroyo is a dry creek bed or gulch that fills with water either seasonally, or after a heavy rain. Arroyos can be natural or man-made.

A good example of man-made arroyos can be found in Albuquerque, New Mexico. There are several miles of open-air concrete drainage channels that eventually drain into the main tributary along I-40 and into a cistern, where the runoff becomes part of the city's water supply. Because of the danger of flash flooding signs are posted, warning people to keep out, but deaths and injuries are a yearly occurance. Graffiti artists regularly deface the arroyos and there has been a big push to curtail their activities.

Although not technically an arroyo, the Los Angeles River is a more famous example of this open drainage system.