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, . 1100 .... Canada .......................... 4~. 40. 3~. 3~. 30. 1200 igure 2, Physiographic provinces of the North American Cordillera: (I) western ranges, (2) ocky Mountains, (3) interior ranges, (4) basin and range (Great Basin), (5) Columbia Plateau, (Ill olorado Plateau and (7) Mexican Plateau, Major river systems draining the Cordillera are (A) tht 'olumbia, (B) the Sacramento-San Joaquin and (C) the Colorado, With strong headwater flows and a 2500 m change in elevation, the river cut eep canyons through the rising plateau, exposing sequentially the geologic istory of the Southwest. In Black Canyon on the Gunnison River (Stanford & lard 1984) and Grand Canyon on the mainstem (Plate I), the river reached the recambrian basement, so that nearly all formations underlying the basin are isible on the canyon walls (Fig. 3). Historically, the rate of basin-wide erosion bove the Grand Canyon has been c. 17 cm per 1000 years (Bishop & Porcella 980), Near the end of mountain building in the Tertiary, the middle of the Grcal asin arched upward from c. 300 m to more than 1500 m above sea level v'eathering yielded the Basin and Range Province, a series of ranges, plateau\ nd buttes (Fig, 2). Drainage from these ranges and from the Sier~a Nevada and ,ocky Mountains accumulated in pluvial lakes on the west (Lake Lahontan) nd east (Lake Bonneville) of the Great Basin. Lake Bonneville (Fig. 4) reach('d laximum development about 28000 years ago, when it was more than 300 m ~ep and 50000 km2 in area. Lake levels oscillated with the climate, and began ) recede 10-15000 years ago. Desert or semi-desert lands claimed the lowland- Jon after the Pleistocene, and the lakes were reduced to saline remnants: thu' ;6 . .....~., ,~. (0) UNCOMPAHGRE PLATEAU GUNNISON R. AKO Atk "oo:'~'::"'. .:~;:;:~:'. F::r:":"~i:f:="C-,"'~:;:'..... M~t~}~~~~fjf~~f18~~~ltFyJf~~;;~i~~~~~1~~3~~~ ;2~;;~~~:' WASATCH (bl PLATEAU 2500 m KAIBA8 PLATEAU ! ~ Shale em S',-"idsrone FCl." Sandstone and E2"] !-ihale miJl.ed ~ Aeolian sandstone ~ Llmes10ne ~ Quarfzite Permian _... ~~~~!~.8.i.~p.i?~... NORTH SOUTH Figure], Geologic formations of the Colorado River Basin: (a) structures in a N-S section of the lower Gunnison Valley, Colorado (based on Chronic 1980; Stanford & Ward unpublished), (b) major exposures in the N-S section from Wasatch Plateau, Utah, to the bottom of the Grand Canyon (after Strahler 1963), Pyramid Lake in Nevada and Great Salt Lake in Utah (Hubbs & Miller 1948; Clark & Stearn 1960). Lake Bonneville was never connected to the Colorado system, but a series of smaller pluvial lakes drained via the White River system_ (Moapa-White River and Meadow Valley Wash: Fig. 1). These flowed south into a large lake, Hualapai, on the Colorado at the mouth of the Grand Canyon. Other pluvial lakes were present on the Gila River and in the Salton Depression near the river mouth (Fig. 4). A unique fauna evolved in the Pleistocene Colorado. Although the fossil rccord is not completely understood, the Pliocene fish fauna diversified in the mesic Pleistocene environment. For example, a unique assemblage of minnows (Cyprinidae) and suckers (Catostomidae) evolved in the mains tern while other minnows and trout (Salmonidae) diversified in the headwaters. Mainly sub- specific differentiation occurred after the Pleistocene, as desertification exerted strong selective pressures on populations isolated in the disrupted White River and parts of the Gila River (Stanford & Ward 1986a), About 8000 years ago the natural character of the river was determined, and remained so until Lake Mead was formed in 1935. 357