NEOGENE PROFILE OF THE COLORADO RIVER: DIFFERENT REACHES AND DIFFERENTIAL INCISION WITHIN A YOUNG RIVER SYSTEM

The Colorado River system is the major drainage for the southwestern Rocky Mountains and is thus a sensitive gauge for evaluating models of drainage development and integration and the relative importance of Neogene tectonics versus denudation of an elevated Laramide plateau. The longitudinal profile of the Colorado River is not a simple concave-up profile, but rather shows numerous complexities and differences in river gradient that define distinct reaches: 1) the headwaters to the confluence with the Green River (1.9 m/km); 2) Green River to Lee’s Ferry (0.33 m/km); 3) eastern Grand Canyon from Lee’s Ferry to Shinumo Creek (2.3 m/km); 4) western Grand Canyon from Shinumo Creek to Hoover Dam (1.2 m/km); and 5) lower Colorado River from Hoover Dam to the Gulf of California (0.32 m/km).

Incision rates over the last 10 million years have been compiled based on precise ⁴⁰Ar/³⁹Ar dates on basalt flows, tephrochronology in terrace fills, and U-Series dates on travertine. Incision rates on the mainstem and major tributaries vary by an order of magnitude along the river system and are both temporally and spatially variable. It seems that higher rates correspond to higher gradients in the above numbered reaches: 1) 160 m/my (over 10 my) and 300 m/my in some segments (over 0.64 ka); 2) unknown; 3) 140 m/my (over 0.35 my); 4) 95 m/my (over 0.35 my); 5) 23 m/my (over 4.4 my).

The Colorado River system is a very young river system that continues to become integrated and adjusted to an ultimate base level that was established 6 Ma. Differential incision in the lower Colorado River system (reaches 3,4,5) is demonstrably related to Neogene normal faulting due to Basin and Range extension whereas differential incision in the upper Colorado River system may be explained by recent (<10 my) major stream captures and knick point migration (Unaweep Canyon and Black Canyon of the Gunnison) possibly influenced by Neogene epeirogenic uplift related to the buoyant Aspen mantle anomaly in central Colorado. The profile thus may subtly record different tectonic influences (active faulting in the Basin and Range and epeirogenic uplift in the Rocky Mountains) superimposed on the long term erosion of the Laramide orogenic plateau.