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The Study Area. The watershed of the San Juan River takes in major portions of Arizona, Colorado, New Mexico and Utah and is more-or-less centered on the Four Corners, the only spot in the United States where four states meet at a common point. Figure 1 shows the
Fig. 1. Hydrology of the San Juan River watershed showing major drainages.
major drainage systems of the San Juan River and Fig. 2 shows the major physiographic and political entities in the study area. The entire area encompasses 65,382 square kilometers (25,244 square miles), an area the size of West Virginia and about half the size of Alabama, Arkansas, New York, or North Carolina. The highest point is 4292 m (14,083 ft.) at Mt. Eolus in the San Juan Mountains and the lowest point is 1130 m (3708 feet) where the San Juan River flows into Lake Powell (the Colorado River). Because of this elevational gradient, vegetation in the study area varies from alpine tundra, to coniferous forests, to mountain shrublands, to lowland sagebrush, to blackbrush, to the sparse communities seen on naked rocks and the scorching sides of low-elevation canyons.
The study area has been formed by the interaction of major hydrological and geological forces (summarized in Baars,1995). Major hydrological processes have produced the main drainage systems. In the northeast, the San Luis Uplift formed the main body of the San Juan Mountains. The Zuni Uplift in the south and the Monument Upwarp in the northwest are prominent features near the edges of the study area. The Defiance Uplift produced the Chuska Mountains, which are capped by sedimentary strata. The study area is dominated by sedimentary strata that have variously been uplifted, dropped, deformed and eroded. Navajo Mountain, on the western edge of the study area, although a volcanic laccolith, is still capped with sedimentary rock. Major igneous structures are the La Plata Mountains, Ute Mountain, Ship Rock, Agathala Peak, the Carrizo Mountains, and the Abajo Mountains. The Grenadier Mountains consist of metamorphic rocks. The oldest rocks in the study are the granites of the Needle Mountains.
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