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113- . 1 J ... LAKE MEAD / I Figure 1. The Grand Canyon region. ARIZONA , o "" 112- U.KE POWEll \ GLEN CANYON DAM , 50 CIIdIn. CrMIl: t 25 km reach between Glen Canyon Dam and Lees Ferry is a blue-ribbon trophy fishery for non-native rain- bow trout (Oncorhynchus mykiss). The river provides essential water for humans as well. Water from the Colorado River has been diverted to southern California for 90 years and is being increasingly diverted to cit- ies in the Wasatch Front in Utah, the Front Range in Colorado, southern Nevada, and central and southern Arizona. The discharge of the Colo- rado River is relatively small for the basin's size: The mean annual dis- charge at Lees Ferry was only 505 m'/s (15.9 x 10' m'/yr) between 1912 and 1963, before the dam was built (USGS 1996). Therefore, large res- ervoirs have been constructed to as- sure water availability, and the Colo- rado River has the largest reservoir storage capacity in relation to an- nual discharge of any major water- shed in the United States (Hirsch et al. 1990). The potential for flood control and sediment retention by these reservoirs is nearly complete, and restoration or rehabilitation can be achieved only by changing the dams or their operations. Many aspects of water-release Table 1. Controlling factors and ecological processes of the Colorado River in the Grand Canyon that can, and cannot, be manipulated by Glen Canyon Dam. Relation to tbe existence or operation! of Glen Canyon Dam Controlling factors Regional climate; regional geology and geomorphology; human activities (prehistoric settlement, spatial patterns of water use, growth in water demand, tegionalland-use changes, recreational demand); non-native species invasions Unrelated Discharge; water temperature; sediment; nutrients; woody debris; non-narive species introductions Related Ecological processes Regional land use; tributary floods and debris flow!!; stage- discharge relations of the Colorado River; solar insolation and downstream rate of water warming; regional expansion of some native and non-native species Lake Powell limnology and mainstem sediment transport; stratification; sediment accumu- lation on river bed; transfer of sediment to eddies and sediment accumulation in eddiesj sandbar stability; ice formation and trans- port; dissolved load transport; woody debris transport and decomposition; aquatic and terrestrial productivity 736 policy from Glen Canyon Dam are controlled by statutory or adminis- trarive rules that are related, directly or indirectly, to the seven-state Colo- rado River Compact of 1922 thar allocated water use among the states. Water released from Glen Canyon Dam constitutes a delivery of water from the upper basin to the lower basin because the division point be- tween the basins is near Lees Ferry. Glen Canyon Dam is the largest dam ofthe Colorado River Storage Project (CRSP); its power plant produces approximately 75% of the total CRSP power for a six-state region. Lake Powell holds 80% of the upper Colorado River basin's stored water supplies. The management of that portion of the Colorado River that flows through the Grand Canyon reflects the interests and values of many man- agement and regulatory agencies. The federal agencies that manage or moni- tor Glen Canyon Dam and the eco- logical resources of the Colorado River include the US Bureau of Rec- lamation, the National Park Service, the US Fish and Wildlife Service, and the Grand Canyon Monitoring and Research Center. The Western Area Power Administration markets the power produced by the CRSP and partly derermines the daily releases from each CRSP dam, The Arizona Game and Fish Department man- ages the sport and native fish popu- lations. Approximately 200 km of the left side of the river (facing down- stream) forms the reservation bound- aries for the Navajo and Hualapai tribes, and five additional Native American tribes have vested inter- ests in Grand Canyon river manage- ment. Other interested parties in- clude numerous municipalities and agricultural organizations that use water and electrical energy, national and regional environmenral groups, commercial river-running compa- nies, and professional trout-fishing guides. ~ Ii-: ,;,( ~:. " !o" .":;. " ..":J.. ~';: 'i:" ,., l, ':-: ~ :. t: ,.. " ,:. f.... , . f', t<j,. u C' r:~ f. I. f I' [ i: r. , , . " ~.~ , :..~ ::.it c:- The Colorado River ecosystem in the Grand Canyon The Colorado River ecosystem in the Grand Canyon is sustained by the flow of water and nurrienrs released by Glen Canyon Dam, but other con- trolling factors are unrelated to rhe BioScience Vol. 48 No.9