Checking billy barr’s snow log…
The Gunnison River, a major tributary of the Colorado River houses the Black Canyon, one of the narrowest (350 m) and deepest (700 m) bedrock canyons in North America and is an excellent location to study Cenozoic river evolution. The modern longitudinal river profile exhibits a prominent knickpoint within the Black Canyon. In the last 640ka, average bedrock incision rates surrounding the knickpoint are unbalanced and vary from 130-150 m/Ma (downstream) to 470-600+ m/Ma (within) to 90-95 m/Ma (upstream), necessitating knickpoint migration. The 640ka profile, reconstructed from terraces containing Lava Creek B ash, indicates ~25km upstream migration. The best age constraint and incision rate of Black Canyon comes from projecting the ~640 ka Shinn-Boswick tributary to its intersection with the Gunnison, suggesting 350-400 m of incision 700m total depth in the last 640ka. Assuming steady average rates this indicates that Black Canyon has been carved in the last 1.4 Ma. Ten strath terraces ascend from the North Fork-Gunnison River to 670 m above the modern river. A cosmogenic burial date of ~ 1 Ma on the seventh terrace anchors the 640ka paleoprofile (at Qt 5/6) giving an average incision rate of 220 m/Ma. Approximately graded with the North Fork terrace, the Redlands Mesa pediment is tentatively assigned the ~1Ma age. Strath terraces and pediments are inferred to record glacial-interglacial stages. Long-term incision rates on the Gunnison River over the time span of 10 Ma are ~160 m/Ma below and ~55 m/Ma above the knickpoint. A paleo-Gunnison was in approximately its present course ~30 Ma as indicated by ash flow units, but the bedrock strath was ~500 m lower than at 10 Ma; hence bedrock incision rates were small (or negative due to surface uplift) and gradients were low from 30 Ma to 10 Ma. Although driving forces remain poorly constrained, a hybrid model includes recent rapid incision, knickpoint migration, drainage reorganization, and unequilibrated river profiles are responding to base level fall accompanying increased climate erosivity, local neotectonics, and epeirogenic surface uplift with accompanying basaltic magmatism.
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