COLORADO RIVER SYSTEM OF THE SOUTHWESTERN U.S.: LONGITUDINAL PROFILES, DIFFERENTIAL INCISION, AND A HYPOTHESIS FOR QUATERNARY TECTONISM AT BOTH ENDS

The Colorado River is the principal drainage system for the western slope of the southern Rocky Mountains; understanding its evolution can provide insight into long-standing questions regarding the geomorphic history of the southwestern U.S. Here we couple analysis of the longitudinal profiles of the river and its main tributaries with a new compilation of incision rates (inferred from fluvial terraces) in an effort to explore hypotheses for the evolution of the drainage system. The longitudinal profile of the Colorado River from the Rockies to the Gulf of California exhibits a double concave-up shape juxtaposed across a major knickpoint near Lees Ferry, AZ. Although bedrock strength (of both the channel substrate and the transported material) may be a factor in localizing this knickpoint, with resistant Paleozoic carbonates below and Mesozoic sandstones above, we explore an alternative hypothesis that the knickpoint is a transient feature in the river system. We advocate in support of this hypothesis on the basis of profiles of tributary streams joining the main stem; tributaries downstream of Lees Ferry typically exhibit knickpoints that appear to be systematic (in elevation and position along the tributary) with respect to the river below Lees Ferry. In contrast, tributaries upstream of Lees Ferry have fairly simple, lower gradient, concave-up profiles. Published incision rates exhibit a large degree of scatter, but the best available Quaternary incision rates indicate different average incision histories along different reaches of the Colorado River. Below the knickpoint, incision rates are ~140 m/my, while above, rates are ~ 100 m/my (Wolkowinsky and Granger, 2004). To explain the Lees Ferry knickpoint, variations in incision rate, and the double concave profile of the river system, we propose that Quaternary tectonism modulates incision at opposite “ends” of the river system. In the lower Colorado River basin, incision rates over the past 600 ky are modulated by Quaternary normal faulting (Pederson et al., 2002), while in the upper basin variable incision may be explained by drainage reorganization and stream capture reflecting ongoing tectonism in the southern Rocky Mountains. Variable incision through the Colorado Plateau, however, may reflect a transient incisional wave generated during cutting of Grand Canyon.