Introduction: CRevolution 2: Origin and Evolution of the Colorado River System II

A 2010 Colorado River symposium held in Flagstaff, Arizona, in May 2010, had 70 participants who engaged in intense debate about the origin and evolution of the Colorado River system. This symposium, built on two previous decadal scientifi c meetings, focused on forging scientifi c consensus where possible, while also articulating continued controversies regarding the Cenozoic evolution of the Colorado River System and the landscapes of the Colorado Plateau–Rocky Mountain region that it drains. New developments involved hypotheses that Neogene mantle fl ow is driving plateau tilting and differential uplift, with consensus that multidisciplinary studies involving differential incision studies and additional geochronology and thermochronology are needed to test the relative importance of tectonic and geomorphic forcings in shaping the spectacular landscapes of the Colorado Plateau region. In addition to the scientifi c goals, the meeting participants emphasized the iconic status of Grand Canyon for geosciences, and the importance of good communication between the research community, the geoscience education/interpretation community, the public, and the media. Building on a century-long tradition, this region still provides a globally important natural laboratory for studies of the interactions of erosion and tectonism in the shaping landscape of elevated plateaus. BACKGROUND Studies of the origin and evolution of the Colo rado River System are central to understanding the Cenozoic tectonic and geomorphic evolution of the western U.S. orogenic plateau. This region was uplifted from sea level in the late Cretaceous, to present elevations that exceed 4 km in the Rocky Mountains and 1.5 km over large regions of the Colorado Plateau. The Colorado River is the trunk river of the single river system that drains the western slope of the Rockies and the entire Colorado Plateau, and hence is central to understanding the uplift and erosion history of the region. The timing of the initial development of the Colorado River, and its evolution into the drainage network seen today, have been the focus of over a century of research, since the early scientifi c trips of J.W. Powell down the Green and Colorado river systems. This fi eld laboratory, because of its spectacular exposure, has been at the forefront of scientifi c breakthroughs in geomorphology, stratigraphy, paleontology, and tectonics for over a century (Dutton, 1882). In early syntheses (Powell, 1875, 1879; Dutton, 1882), the Colorado River system was presumed to be ancient and antecedent, following the path of today’s west-fl owing river system that carries snowmelt from the Rocky Mountains to the Pacifi c. Longwell (1928, p. 143) noted the problem (“Muddy Creek problem”) that the Colorado River did not exit the western edge of the Colorado Plateau during the Pliocene, the lower boundary of which was placed at ~11 Ma until the early 1970s. Blackwelder (1934) proposed that the regional river and canyon system did not exist until the Pleistocene, before which time there was a general lack of integrated river systems. Hunt (1956) outlined the evolution of the entire region since Cretaceous time, and his Colorado River synthesis (Hunt, 1969) involved discussions of interacting geomorphic and structural controls on Colorado Plateau drainages through time. In summary of the continuing debate, Hunt (1969, p. 63) stated, “The view that the Colorado River is an ancient river considers the river as a whole from the time of fi rst uplift of the present Rocky Mountains; the view that the river is young is based on particular segments.” This was a glimpse of subsequent controversies that attempt to reconstruct the regional picture by study of both regional uplift history and individual segments of the river system. Additional advances in our understanding of the complexities of the river system have been punctuated by three collaborative meetings in northern Arizona, in 1964, 2000, and 2010. This paper provides brief refl ections on the fi rst two meetings and a summary of the 2010 meeting. Our goal is to foster continued research on western U.S. landscape evolution at all scales. 1964 MEETING: MUSEUM OF NORTHERN ARIZONA COLORADO RIVER SYMPOSIUM The fi rst meeting had 21 participants. It was an outgrowth of discussions between Eddie McKee and Dick Young during visits to McKee’s U.S. Geological Survey offi ce in Denver, Colorado, related to Young’s PhD (Young, 1966), funded in part by the Museum of Northern Arizona (MNA awards were $1000 each for the summers of 1962–1965). Young’s fi eldwork on the Hualapai Reservation and Ivo Lucchitta ’s work in the Lake Mead region (Lucchitta, 1966, 1972) evolved with close interaction. Contrary to the account in Ranney (2005), the new data For permission to copy, contact [email protected] © 2012 Geological Society of America 1 Geosphere; December 2012; v. 8; no. 6; p. 1–7; doi:10.1130/GES00716.1; 1 table. Received 28 April 2011 ♦ Accepted 15 August 2012 ♦ Published online 16 November 2012 CRevolution 2: Origin and Evolution of the Colorado River System II themed issue *Emails: Karlstrom: [email protected]; Beard: [email protected]; House: [email protected]; Young: [email protected]; Aslan: aaslan@ coloradomesa.edu; Billingsley: gbillingsley@usgs .gov; Pederson: [email protected]. as doi:10.1130/GES00716.1 Geosphere, published online on 16 November 2012