Permanent streams are normal in humid regions, but almost all desert streams are dry most of the time (Figure 6.2%). Deserts have ephemeral streams, which means that they carry water only in response to specific episodes of rainfall. A typicel ephemeral stream might flow only a few days or perhaps just a few hours during the year. Ln some years the channel may carry no water at all.
This fact is obvious even to the casual observer who, while traveling in a dry region, notices the number of bridges with no streams beneath them or the number of dips in the road where dry channels cross. However, when the rare heavy showers do occur, so much rain falls in such a short time that all of it cannot soak in. Because the vegetative cover is sparse, runoff is largely unhindered and consequently rapid, often creating flash floods along valley floors (Figure 6.2%). Such floods, however, are quite unlike floods in humid regions. A flood on a river such as the Mississippi may take many days to reach its crest and then subside, but desert floods arrive suddenly and subside quickly. Because much of the surface material is not anchored by vegetation, the amount of erosional work that occurs during a single short-lived rain event is impressive. In the dry western United States a number of different names are used for ephemeral streams. Two of the most common are wash and arroyo. In other parts of the world, a dry desert stream may be called a wadi (Arabia and North Africa), a donga (South America), or a nullah (India).
Humid regions are notable for their integrated drainage systems. But in arid regions streams usually lack an extensive system of tributaries. In fact, a basic characteristic of desert streams is that they are small and die out before reaching the sea. Because the water table is usually far below the surface, few desert streams can draw upon it as streams do in humid regions. Without a steady supply of water, the combination of evaporation and infiltration soon depletes the stream.