English: The Sahara Desert covers most of the North African country of Algeria. Rainfall is scarce, and it comes and goes rapidly, flowing through channels called wadis. When dry, as they are in this true-color Landsat Thematic Mapper image from January 1, 1987, the wadis are revealed as pale tracings across the darker brown and gray shades of the desert landscape. In many cases, these wadis end in small pools which have no outlet. The water flows in from the wadis and gradually evaporates, leaving behind salts and other minerals which appear white in this image. At the upper left of the image, a few pools of bright blue-green water are hanging on despite the aridity, their greenish color probably due to high salt and mineral content, or perhaps the growth of algae.