INFLUENCES OF QUATERNARY TECTONISM ON THE PROFILE AND INCISION HISTORY OF THE COLORADO RIVER SYSTEM OF THE SOUTHWESTERN U.S

The Colorado River system provides a sensitive gauge of the effects of both neotectonics and climate change on the geomorphic history of SW. Tectonic influences on both the profile and the incision history are evident in various reaches. In western Grand Canyon (RM 207 to 268) incision rates of ~60 m/my for the last 600 ka are documented by dating of elevated basalt remnants that overlie river gravels. This contrasts with rates of ~140 m/my for the last 350 ka in eastern Grand Canyon (RM 57 to 177). The difference is explained by west-down block motions of 80 m/my across the Hurricane-Toroweap fault system. Near Lees Ferry, a major knickpoint in the longitudinal profile is interpreted to be a migrating transient related to initial integration of the lower Colorado river system across the Grand Wash cliffs. Tributaries downstream of Lees Ferry also exhibit knickpoints whereas tributaries upstream have lower gradient, concave-up profiles. Incision rates appear to be ~ 100 m/my above the knickpont. In the upper basin, variable incision may be explained by drainage reorganization and stream capture reflecting ongoing tectonism in the southern Rocky Mountains. Hanson (1987) used the ~600 ka Lava Creek B ash to infer incision rates of ~300 m/my in the Black Canyon and ~100 m/my upstream at Blue Mesa Reservoir. Terraces and remnant fluvial gravels are well preserved at the confluence of the Black Canyon and North Fork of the Gunnison. Major terraces lie about 6, 24, 37, 43, 61, 70, 130, 165, 190 m above river level, with remnant gravels at ~230, 250, and 305 m. Redlands Mesa contains the Lava Creek B ash within its gravel cap. Profiles projected from Redlands Mesa to Gunnison River terraces indicate that the gravels were graded to a level between 190 and 250 m above the modern river suggesting an incision rate of > 300 m/my for the lower Black Canyon. Grizzly Gulch was a paleo-tributary to the Black roughly 2 Ma, but now drains north due either to NE tilting of the Gunnison uplift and/or stream capture of middle Grizzly Creek by northward-flowing tributaries of the Smith Fork, Causes for rapid incision of the Black Canyon may include broad-scale epeirogenic uplift related to buoyant mantle, and/or knickpoint migration following late Pliocene capture and rerouting of the Colorado River from Unaweep Canyon around the Uncompahgre Plateau.