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in 1956, exports water from the Colorado River
<br />headwaters to the South Platte drainage in eastern
<br />Colorado (Pennak 1963). Water is stored in Willow
<br />Creek Reservoir, Lake Granby, Shadow Mountain
<br />Lake, and Grand Lake and is delivered to the
<br />eastern slope through the 21-km Adams Tunnel for
<br />flood control, irrigation, municipal supplies,
<br />hydroelectric power, and recreational facilities.
<br />The Roaring Fork River joins the Colorado River
<br />at Glenwood Springs, Colorado. The largest
<br />tributary of the Upper Main-stem Colorado, the
<br />Gunnison River, enters the Colorado at Grand Junc-
<br />tion, Colorado. In Utah, the Dolores River enters the
<br />Colorado; it is the last major tributary upstream of
<br />the Colorado-Green River confluence.
<br />The San Juan Sub-basin
<br />The San Juan Sub-basin is the drainage between
<br />the junction of the Green and Colorado rivers and
<br />the Lower Colorado River Basin. Its 99,200 km' area
<br />is in Colorado, Utah, New Mexico, and Arizona.
<br />The Colorado River below the mouth of the Green
<br />River passes through Cataract Canyon and remains
<br />entrenched in a deep canyon as it flows about 200
<br />km southwestward to Lee Ferry (Frontis.). Much of
<br />the stretch of river below Cataract Canyon is now
<br />part of Lake Powell. Glen Canyon Dam and Reser-
<br />voir (Lake Powell) were authorized by the Colorado
<br />River Storage Project Act in 1956; the dam, a few
<br />miles below the Utah-Arizona line, was completed in
<br />1964 (Upper Colorado River Commission 1980).
<br />Principal Colorado River tributaries which now
<br />enter Lake Powell are the Dirty Devil, Escalante,
<br />and San Juan rivers.
<br />The San Juan River arises on the southern slopes
<br />of the San Juan Mountains in southwestern Col-
<br />orado, flows southwestward into New Mexico, and
<br />then turns back into the southwestern corner of
<br />Colorado before entering Utah. It flows through a
<br />deep canyon before entering Lake Powell. The
<br />Navajo Storage Unit of the Colorado River Storage
<br />Project was completed in 1963 to regulate San Juan
<br />River flows for irrigation and municipal purposes
<br />(Upper Colorado River Commission 1980).
<br />The Paria River joins the Colorado River about 25
<br />km below Glen Canyon Dam and 1.6 km north of Lee
<br />Ferry.
<br />Dams, Reservoirs, and Water Diversions
<br />We have not attempted to mention all dams,
<br />reservoirs, and water diversions in the Upper
<br />Colorado River Basin. Spofford (1980) referred to
<br />nine principal federal reservoirs in the basin. A com-
<br />plete list of the 21 participating projects of the
<br />Colorado River Storage Project authorized by Con-
<br />gress appeared in the Thirtieth Annual Report of
<br />the Upper Colorado River Commission (1978). Some
<br />of these are still in planning phases or under con-
<br />struction. Other, non-federal, projects such as the
<br />Moffat and Roberts tunnels of the Denver Water
<br />Board also exist. A complete list of existing and
<br />planned projects is very difficult to develop and
<br />keep up-to-date.
<br />Many diversions, like the Colorado-Big Thompson
<br />Project, result in export of water from the Colorado
<br />River Basin. Water diverted from the Upper Col-
<br />orado River Basin is used in the Arkansas River,
<br />Platte River, and Rio Grande River basins and in the
<br />Great Basin. Martin (1981) reported, for example,
<br />that 27% of Colorado's legal share of Colorado River
<br />water is diverted to eastern-slope cities from Fort
<br />Collins south to Pueblo and that water needs of
<br />these cities are expected to increase by at least
<br />200% in the next 30 years. Schad (1980) noted the
<br />irony in the Colorado River Basin, which drains
<br />some of the nation's more and lands and has the
<br />lowest run-off per square mile of any major river
<br />basin, being the source of such a large number of in-
<br />terbasin transfers.
<br />Conditions in the Basin
<br />Iorns et al (1965), Joseph et aL (1977), and Bishop
<br />and Porcella (1980) summarized conditions in the Up-
<br />per Colorado River Basin. A broad range of climatic
<br />and streamflow conditions exist in the basin; annual
<br />precipitation varies from over 127 cm in mountains
<br />to under 15 cm in desert areas.
<br />Seasonal streamflow is derived primarily from
<br />snowmelt in mountainous areas, and historic unit
<br />discharge rates decrease rapidly as tributary
<br />streams flow from their headwaters into less humid
<br />areas (Bishop and Porcella 1980). Significant varia-
<br />tions in annual discharge have occurred from year to
<br />year and over periods of years (due to long-term
<br />climatic trends). Progressive 10-year running
<br />averages of estimated "virgin flow" (if the stream
<br />were in its natural state and unaffected by the ac-
<br />tivities of man) at Lee Ferry have ranged from 16.0
<br />to 17.8 billion m' since 1970, and the 1896-1980 long-
<br />term annual average virgin flow at Lee Ferry is
<br />about 18.3 billion m' (Upper Colorado River Commis-
<br />sion 1980). The Upper Main-stem Colorado Sub-basin
<br />contributes the greatest volume of water and the
<br />San Juan Sub-basin the least. Joseph et aL (1977)
<br />and Spofford (1980) summarized flow data from
<br />selected U.S. Geological Survey gaging stations in
<br />the Upper Colorado River Basin.
<br />Joseph et aL (1977) recognized three distinct
<br />stream zones in the Upper Colorado River System.
<br />Their upper (headwater) zone was characterized by
<br />cold, clear water, high gradient, and rocky or gravel-
<br />ly substrate and was regarded as ideal habitat for
<br />cold-water fishes. In this zone primary production
<br />(mainly by "periphytic" algae) was considered
<br />"significant" and benthic invertebrate production
<br />"substantial". An intermediate zone occurs as
<br />streams flow from the upper zone; there, water
<br />warms, discharge increases, waters are turbid dur-
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