A ravine is a very narrow, steep sided crevice in the Earth's surface. Ravines are smaller than valleys, but larger than gullies, although a ravine has the potential to develop into a valley, over the course of thousands of years. Ravines are especially common in urban areas, although they can be found in other parts of the world as well, and caution should be used when walking around ravines, as the sides can crumble, causing people to fall in and be injured.
Typically, a ravine is formed through the process of erosion, and it starts out as the site of a small stream or river. Over time, the water wears a deep groove into the Earth, which attracts water as it drains from other locations, speeding the erosion process up. Eventually, a ravine may lose its stream, or have only intermittent water flow, because it cannot sustain water year-round. Sometimes, however, a ravine will have a year-round watercourse.
Urban areas tend to have more ravines than other locations because urban drainage is usually poor, and this promotes the formation of ravines. It is common for water to collect in a large mass in urban areas, creating a rushing a torrent, because it cannot percolate naturally through the soil to drain away. Because the water has to go somewhere, it can end up creating a cutting in periods of flooding and heavy rain, and this will develop into a ravine.
Ravines have historically been used for garbage disposal, because of their depth and steep sides. Although this practice is largely discouraged today, ravines still tend to collect garbage, which is carried by the water which periodically pours through them as well as being tossed in by careless litterers. In urban areas, service organizations may designate a day each year to clean up local ravines, canyons, and waterways so that the garbage is not allowed to accumulate for too long.
Depending on the location of a ravine, it may also serve as habitat for local wildlife, especially in regions where wildlife is under pressure due to human habitation. Humans tend to avoid ravines, since they are difficult to navigate, and this allows a variety of creatures to move in and live unmolested at the site. As a result, ravines are sometimes great spots for birding and other forms of wildlife observation.