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I. ABIOTIC COMPONENTS Introduction The upper Colorado River basin is a dynamic system with an ever- changing physical and chemical character. Snowmelt in the spring swells the many tributaries dramatically and adds large amounts of suspended and dissolved material. Summer rainstorms have a similar effect, but changes occur over relatively short periods of time. As a result, typical basin streams and rivers have widely variable flows and chemi- cal characters as well as constantly shifting channel substrates. Man-induced changes have altered the physical and chemical charac- teristics of the upper basin. Some activities have enhanced the rate and magnitude of change while others have had a moderating effect. Ir- rigation practices, water exports, dam construction, unsound grazing and forestry practices, and municipal and industrial development have most affected the abiotic components of the system. Man's controlled use of water from the upper Colorado basin dates back to the horticultural practices of the Anasazi culture between the fifth and fourteenth centuries A.D. (La Rue 1916). The basin remained largely unexplored by modern man until the discovery of gold in 1859. Major John Wesley Powell's explorations in the 1860's introduced much of the basin to a new culture of man; and mining, farming, and ranching activities flourished in the basin around the turn of the century. Demands on the Colorado River water supply increased through- out the early 1900's as steadily increasing amounts of water were with- drawn for irrigation and domestic uses. Since 1963, the volume of water removed from the river has increased dramatically with expanding populations and for Colorado River water both from within and outside of the basin. The literature on water-related resources of the Colorado River is extensive. The most reliable and readily available water data are in the reports of the U.S. Geological Survey. Much of the early hist- ory of the river is documented in the earliest water-supply papers (La Rue 1916), some professional papers (Iorns et al. 1965) and more 7