4 The Colorado River: lifeline of the American Southwest Clarence A. Carlson Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, CO, USA 80523 Robert T. Muth Larval Fish Laboratory, Colorado State University, Fort Collins, CO, USA 80523 1 Carlson, C. A., and R. T. Muth. 1986. The Colorado River: lifeline of the American Southwest. Can. J. Fish. Aguat. Sci. In less than a century, the wild Colorado River has been drastically and irreversibly transformed into a tamed, man-made system of regulated segments dominated by non-native organisms. The pristine Colorado was characterized by widely fluctuating flows and physico-chemical extremes and harbored unique assemblages of indigenous flora and fauna. Closure of Hoover Dam in 1935 marked the end of the free-flowing river. The Colorado River System has since become one of the most altered and intensively controlled in the United States. Many main-stem and tributary dams, water diversions, and channelized river sections now exist in the basin. Despite having one of - the most arid drainages in the world, the present-day Colorado River supplies more water for consumptive use than any river in the United States. Physical modification of streams and introduction of non-native species have adversely impacted the Colorado's native biota. This paper treats the Colorado River holistically as an ecosystem and summarizes current knowledge on its ecology and management. "In a little over two generations, the wild Colorado has been harnessed by a series of dams strung like beads on a thread from the Gulf of California to the mountains of Wyoming. The living river that Powell knew has been sec- tioned into placid desert lakes throughout much of its length, and the river's primordial task of carrying the massif of the Colorado Plateau to the sea, bit by grainy bit, has been interrupted, and will remain interrupted for the lifetimes of our children's children and beyond." - Watkins (1969, p 165) 2 Despite years of study, the Colorado River has rarely been viewed holistically as an ecosystem (sensu Minshall et al. 1985), and much informa- tion on its biota is available only in "gray" literature. We welcome this opportunity to summarize current knowledge on what has been called the most used, dramatic, and highly litigated and politicized river in the United States, if not the world (Fradkin 1981). The Colorado heads on the Never Summer side of Rocky Mountain National Park, Colorado, and flows 2320 km to the Gulf of California in Mexico (Fig. 1). The Green River, which joins the Colorado in Canyonlands National Park, Utah, originates in the Wind River Range of southwestern Wyoming, 2735 km above the Gulf of California. As Madson (1985) suggested that the Upper Mississippi is a tributary of the Missouri River, a case can be made for considering the Mainstem Upper Colorado (called the Grand River until 1921) a tributary of the Green River. The Colorado's drainage basin encompasses 632,000 km2 in the United States and Mexico (Plummer 1983) and ranges in elevation from sea level to 1450 m (Miller et al. 1983). It occupies one-twelfth of the land area of the United States. The upper basin (above Lee Ferry, Compact Point in Fig. 1) consists of the Green (115,800 km2), Upper Mainstem (68,600 km2), and San Juan (99,200 km2) sub-basins (Carlson and Carlson 1982). Sub-basins in the lower basin are the Little Colorado (67,000 km2), Virgin (28,500 km2), Bill Williams (14,000 1cm2), and Gila (145,000 km2) (Sykes 1937). Conditions in the basin were reviewed by LaRue (1916), Sykes (1937), Iorns et al. (1965), Bishop and Porcella (1980), Carlson and Carlson (1982), Graf (1985), and Stanford and Ward (1986). 3 The Colorado River drainage spans three geologic provinces: Rocky Mountains, Colorado Plateau, and Basin and Range (Hunt 1974; Graf 1985). Igneous and metamorphic rock underly the headwaters region, but the river contacts marine deposits containing salts and fine-grained sediments as it flows downstream (Miller et al. 1983). As the Rocky Mountains and Colorado Plateau emerged from the Cretaceous sea during the Laramide Orogeny of the early Tertiary, lakes of the northern Plateau received small streams draining the western slope of the new Rockies (Hunt 1969). With time, the lakes filled with sediment and overflowed to the south as the Colorado Plateau tilted. New pondings developed farther south, ancestors of today's upper- basin streams converged from the east, and the present northern Colorado River drainage was established by the end of the Miocene. Hunt (1969) believed that the lower Colorado River in the Basin and Range Province was an estuary of the Gulf of California in Miocene and Pliocene time and that a drainage from the Colorado Plateau must have joined it by then. McKee et al. (1967) suggested that the ancestral Colorado River flowed southward and southeastward along the present course of the Little Colorado River in Oligocene and Miocene time to an inland sea (Lake Bidahochi) in northeastern Arizona. Streams of the Hualapai Drainage System on the western flank of the Kaibab Upwarp extended their channels in an upstream (south- easterly) direction during the Pliocene and encountered the ancestral Colorado, which was diverted westward across the upwarp to form the modern drainage pattern through the Grand Canyon. About 150 m of canyon deepening occurred during the Quaternary throughout most of the Rocky Mountains and Colorado Plateau (Hunt 1969). This region was lowered about 3.2 km by the Colorado River System since the middle Oligocene. Major geologic strata underlying the basin were summarized by Stanford and Ward (1986). 4 Human occupation of the Colorado's canyon country dates back 4000 yr. Agricultural Anasazi, Fremont, Moholon and Hohokan cultures flourished at various times into the 1200s, then declined, perhaps in part through over-exploitation of natural resources. They were succeeded by their descen- dants (Hopi, Taos, Zuni, and Pima-Papago) and immigrant Navaho, Apache, Southern Paiute, Havasupai, and Hualapai people (Watkins 1969; Barnes and Pendleton 1979; Fradkin 1981; Graf 1985). Spanish explorers in search of riches encountered the lower Colorado River in 1540 (LaRue 1916; Dellenbaugh 1902). New Spain was subsequently established in the American Southwest, and new explorers set forth in search of religious converts and a route to California. The river was given several names before Father Kino applied the name 'Colorado' on a 1705 map of his passage westward (Bolton 1919; Hughes 1967). Members of Father Escalante's 1776 expedition were the first Euro- peans to encounter the Green River (Crampton 1952). Trappers continued exploration of the canyon country in the early 1800s; William Ashley's party first navigated the upper Green River in 1825 (LaRue 1916; Watkins 1969). Mormon colonizers established towns along the river, and John Lee was sent by church leaders to establish a crossing below Glen Canyon (Fradkin 1981; Rusho and Crampton 1981). Lee's ferry site has played a prominent role in the law of the river. The lower Colorado was navigated by commercial steamships and the U.S. Army in the 1850s (Lingenfelter 1978). John Wesley Powell's scientific investigations of the Colorado began in 1869, when he floated from Green River, Wyoming, through the Grand Canyon. In documenting this and a second expedition begun in 1871, Powell shattered many myths regarding commercial development of the Colorado basin and established a basis for federal science in the United States (Stegner 1982 ; Rabbitt 1969). 5 Watkins (1969) and Fradkin (1981) have contributed excellent comprehen- sive reviews of subsequent Colorado River history. Many human activities have drastically altered the river ecosystem, but none has been more signifi- cant than construction of high dams on the mainstream river. The first of these (Hoover Dam) was closed in 1935, marking the demise of the free- flowing Colorado. Stanford and Ward (1986) noted that the natural character of the river was determined about 8000 yr ago and was intact until 1935. In the remainder of this report, we summarize the status of the Colorado River ecosystem prior to 1935, human manipulation of the river, and the current condition and future of the river system and its biota. The Colorado River before 1935 Prior to 1935, the Colorado flowed essentially unchecked from its sources to the sea, ending in a 'live delta' of meandering streams, silt, and shifting land patterns (Hundley 1966). Then, as now, much of its basin consisted of relatively barren deserts. High relief, sparse vegetation, and desert storms combined with montane snowmelt to produce spectacular vari- ations in discharge (Minckley 1979). Flows at Yuma, Arizona, ranged from near zero to over 7,000 m3/s (U.S. Geological Survey 1975). Mean annual virgin flow from the upper basin during the past four centuries was 16.65 1cm3 (Weatherford and Jacoby 1975; Welsh 1985). The upper basin produced most of the river's discharge, and peak flows occurred after snowmelt in spring and early suaner (Bishop and Porcella 1980). Maximum runoff in the lower basin often followed winter rainstorms (Sykes 1937). Lowest discharge typically occurred in mid to late summer, but long periods of low flows accompanied droughts. 6 At times of moderate flows and during droughts, the river ran clear, but sediment transport was remarkably high during floods. Dolan et al. (1974) noted that maximum sediment transport in the Grand Canyon before closure of Glen Canyon Dam was 25 million t on 13 September 1927 at a discharge of 3540 m3/s. Except where the river breached hard-rock barriers, bottoms must have continuously shifted as bedload was transported (Minckley 1979). Unstable flows and other conditions resulted in arroyo cutting in small and medium- sized streams before the arrival of technologically advanced human cultures, and floods and droughts resulted in alternating braided and meandering channels over time (Graf 1985).