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, . 1150 , 40. 40. 3.. 3.. , o 100 ~ km igure 4, The maximum extent of late Pleistocene lakes and known fluvial connections to th~ olorado River: (I) Lake Pattie, (2) Lake LeConte, (3) Lake Morrison, (4) Lake Hualapai. (5! [uvial White River system and (6) Lake Bonneville (after Smith 1978; Miller 1981), lorphology and discharge he Colorado originates in a subalpine meadow (3105 m) in Colorado's Rock:- 10untain National Park, and from there flows 2320 km to the Gulf of 'alifornia. With the Green River, which originates at Peak. Lake in the hnd River Mountains of Wyoming, the Colorado is more'than 2700 k m I length (Fig. I). Major sub-basins include the Gila (I45000km2), Green . 15000km2), Upper Mainstem (above the Green confluence; 67000km'l. ittle Colorado (67000 km2), San Juan (66800 km2) and Virgin (28500 km:) he total basin area within the United States is 629 I II km2 (U.S. Geological urvey data), with another 8600 km2 forming the delta in Mexico (SyJ...c' >37). :;8 ~"" ' . . , 40 I O;scharge I ,/1'/'1"'\,1 I " , / '''', 1944-1962 1:----- 1963-'977 Q 30 M -'" C 20 o E ": 10 E 20 r' ...1-- - - i-___ -'1944 -1962 ..r"'" 1963-1977 f..' .' J....... E ~;r N;'" D~C J~n F~b I , I , I . , Mar Apr May June July Aug 5{1p Mon1h Figure 5, Monthly means and ranges (bars) of discharge and temperature in the Colorado River at Lee Ferry before (1944--62) and after (1963-77) impoundment of Lake Powell (after Paulson & Baker 1981), All the major tributaries originate in mountains above 3000 m, The head- waters of the Upper Basin, with more than 130cm precipitation annually, are more mesic environments than the headwaters of the Lower Basin, More than 40% of the annual flow is derived from the Upper Mainstem sub-basin (Irons et at. 1965). Precipitation decreases dramatically below elevations of 15?0 m (c, 70% of the basin), and much of the desert below the Grand Canyon receives less than 15 cm annually. The virgin flow of the Colorado River cannot be accurately estimated because the various dams and diversions pre-date discharge measurements near the river mouth. However, the average virgin flow at Lee Ferry is estimated at 18.35km3 a-I for 1914-65 (Bishop & Porcella 1980). The annual hydro graph peaked in spring, in response to snowmelt (Fig. 5), generating flows of 2400-5600m3s-1 in the Grand Canyon (Carothers & Minckley 1981). Most virgin flow came from the Upper Basin, but winter storms often fl~ode? the lower river with silt-laden waters from the Little Colorado and Gila fivers, Maximum runoff in the Gila Basin usually occurred in winter, with flows up to J500m3s-1 (Sykes 1937). Based on U,S, Geological Survey data, the pre- regulation average annual flow at the Colorado Delta was c. 19.0 km3 a-I (602m3s-I). The highest recorded flow is 7100m3s-1 on 22 January 1916, but flows greater than 10 000 m3 S-I probably have occurred in the last 200 years (Dill 1944). Lowest flows occurred in mid-late summer; the minimum for 1902-34 was less than lOm3s-1 on 25-27 August 1934. The annual unit-area discharge for the entire basin (637711 km2) is 29800 m3 km-2, indicating one of the driest river basins in the world, The sediment~ of the Colorado Plateau are highly erodible, so that slight flow increases accelerate sediment loading in all sub-basins. Sediment movement 359