To the west of the Rocky Mountain belt in the United States there exists a region embracing about 210,000 square miles, which sends no stream to the ocean. This vast and in large part desert tract is known as the Great Basin. The climate is characterized by its aridity. The annual precipitation is small and evaporation active. All the water reaching the land is returned to the air by evaporation, either directly or from the streams and lakes. Many of the lakes do not overflow and are more or less alkaline and saline, while some of them, as Great Salt Lake, Utah, and Mono and Owens Lakes, California, are dense with mineral matter in solution. [Illustration: FIG. 20.--The Great Basin.] The Great Basin is not a single level-floored depression, as one might infer from its name, but is traversed by rugged mountain ranges, which divide it into a large number of minor valleys. Some of these secondary basins have lakes and streams which escape from them into lower depressions, but in many instances under present climatic conditions they have no surface water, all the moisture that reaches them being absorbed by the thirsty soil or evaporated without forming lakes. The Great Basin proper, as it may be termed, embraces nearly the whole of Utah and Nevada, together with small portions of the southern parts of Idaho and Oregon and a large area in southeastern California. While the drainage conditions limit the application of the name to this group of associated basins which send no tribute to the sea, the climatic and to a less extent the topographic and geological conditions that characterize it have much wider, although indefinite boundaries. This wider region which resembles the Great Basin proper, extends from British Columbia southward to beyond the city of Mexico, and includes the eastern half of Washington and Oregon, a large part of Idaho, and much of Arizona, New Mexico, and western Texas. In this outer region both to the north and south of the Great Basin proper there are drainless valleys, as those of central Mexico, in which the conditions characteristic of the desert valleys of Utah and Nevada are repeated. The greater region of arid valleys and desolate mountains surrounding the Great Basin proper is crossed at the north by the Columbia and in the central part by the Colorado. Each of these large rivers has its source in the Rocky Mountains and flows to the Pacific.