An arroyo is a dry riverbed which tends to fill with water in the rainy season. The term “arroyo” is Spanish in origin; such geographical features are known as wadis in Africa and the Middle East. Arroyos can potentially be very dangerous, as they are prone to flash-flooding and erosion. As a result, it is a good idea to learn to identify arroyos so that you are not inadvertently caught in one during inclement weather.
A characteristic arroyo takes the form of a deep cut in the desert. An arroyo may also feature scattered rocks and logs, testament to a previous period of flooding. In some cases, green shrubs and small trees line an arroyo, indicating that there is a source of underground water which these plants exploit. In populated areas, signs may designate an arroyo, to alert people to the potential danger.
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. People and animals who are caught in the arroyo during the flood will be swept away.
In some regions, an arroyo will support a small, shallow stream or river during the wet season. This can be irritating in municipal areas, as the water can provide a breeding ground for mosquitoes and algae before it evaporates entirely in the late spring or summer. In regions where mosquitoes transmit diseases like malaria and West Nile virus, some cities treat these temporary accumulations of water to ensure that they remain inhospitable to insects.
One of the major concerns with arroyos for people like farmers is that their banks can become quickly eroded. A small channel in a field can turn into a gaping chasm during a period of heavy rain, as the flash flood eats away at the banks of the temporary river, undermining them and causing them to collapse. The flood also typically carries away lightweight usable topsoil, leaving useless sand and rocks behind. The decision to cut channels into fields for the purpose of transporting water should be undertaken with care to ensure that these channels do not cause problems later.