A word, Arabic in origin, that generally refers to a river. However, owing to the desert climate in most parts of the word where Arabic is spoken, it more commonly denotes a dry river-bed. Wadis are often, but not exclusively, found near mountainous areas.

Over many centuries, water falling on the hills during the spring rains carves channels into the rock, in a similar manner to most conventional rivers. These channels become deeper and deeper over time, and provide convenient routes through the mountains for caravans. Highways needing to traverse mountain ranges are often built along the valleys carved out by the wadis.

Sometimes, when the channels are carved deep enough, they reach the water table, and strings of oases form along the wadi route. These are often favourite camping spots for people in the area, and also can make good day trips for picnics, swimming and so on. The activity of taking a four-wheel drive up into the mountains along a wadi, either for the purpose of getting to the other side, or with the aforementioned oases as a destination, is called wadi bashing.

Wadis can be very beautiful places. However, they can also be very dangerous. It is not uncommon for flash floods to occur, caused by rains at the wadi head, which may be fairly distant. These floods can bring a literal wall of water coursing down the wadi at extreme velocity. They can happen with little to no warning and will sweep away anything in their path, including humans and vehicles. Where wadis pass through towns, or when highways are built along their valleys, they often have bridges over the wadi despite the fact that they contain no water. Buildings are almost never built directly in the wadis. It is not unheard of for floods to carry ten tonne rocks down their path. Unfortunately, these floods do not deter the wild animals, including mangy wadi dogs that often live in the wadis and cause disturbances in surrounding neigbourhoods.

Many wadis do not run out to the ocean, but rather end in large gravel plains called sabkhas, where the water is finally able to seep into the ground.

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