A winterbourne is a stream or river that is dry through the summer months. A winterbourne is sometimes simply called a bourne, from the Anglo-Saxon word for a stream flowing from a spring, although this term can also be used for all-year water courses. Winterbournes generally form in areas where there is chalk (or other porous rock) downland bordering clay valleys or vales. When it rains, the porous chalk holds water in its aquifer, releasing the water at a steady rate. During dry seasons the water table may fall below the level of the stream's bed, causing it to dry out.