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.
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.
The most obvious features of the Great Basin and of the northward and southward extensions of the belt of country
having much in common with it, depend on climatic conditions. The rainfall is small throughout the entire belt from the Canadian boundary to south-central Mexico. The average mean annual precipitation, judging from such observations as are available, is probably less than 15 inches, but this broad statement does not truly represent the diverse conditions. The rainfall is confined almost entirely to the winter season, and frequently comes in short heavy downpours. During the summer season, the valleys especially, become so parched that only such plants can grow as are adapted to long-continued droughts. The topography is rough and diversified by many mountain ranges, and the precipitation is more abundant on the uplands than in the valleys. Over large areas in Nevada and southeastern California the mean annual rainfall is less than 5 inches. The author, while carrying on geological work in this region, was informed by some of the older settlers that at times for fully eighteen consecutive months no rain whatever fell in certain of the valleys. From the accounts of travellers in central Mexico, it seems as if some of the interior basins in that region must be fully as arid as those just referred to.
One conspicuous result of the lack of moisture is the absence of forests. Except on the mountains mainly at the north the Great Basin and its extensions, as defined above, is nearly destitute of trees. The valleys are in many instances thickly covered with desert shrubs, notably the sage-brush, but the floors of the driest basins are in many instances almost absolutely without vegetation, and are frequently white with saline incrustations.
Many of the depressions in the Great Basin, as well as some of the outlying valleys referred to, have rivers and lakes which exhibit certain interesting features that are unfamiliar to people dwelling in humid lands. The streams are fed in part by the small precipitation on the desert valleys, and by springs, frequently of heated water, but mainly by the rain and melting snow on the mountains. Many rills and rivulets are born on the valley sides of a single storm, but are absorbed by the thirsty soil or evaporated during the succeeding hours of sunshine. Other streams
have a greater lease of life and flow down to valleys and basins, suffering evaporation and absorption as they progress, which cause them to diminish in volume, and finally to vanish. The stronger streams, such as Sevier and Bear Rivers in Utah, the Humboldt River in Nevada, and the Truckee River in California, maintain their existence throughout the year, and expand into lakes in which the inflow is balanced by evaporation.