BIRTH AND EVOLUTION OF THE SAN JUAN RIVER IN THE PAST 5 MA

The San Juan River (SJR) is a 600-km-long tributary of the Colorado River (CR) with headwaters in the San Juan Mountains in Colorado, a central reach in the Colorado Plateau near Bluff, UT, and a confluence upstream of Lee’s Ferry in Glen Canyon. Its profile shows a concave-up shape with no major knickpoints and is graded to above the Lees Ferry knickpoint. New and published incision rate data help constrain the birth and evolution of the SJR. The Animas River, in the upper SJR drainage basin, yields incision rates of 190 -130 m/Ma over the past 0.64 Ma. In middle reaches, apatite thermochronology shows rapid cooling of the Monument uplift area after 5 Ma indicating that about 2 km of Mesozoic cover has been stripped since then and that initial integration (the birth) of the SJR was after 5 Ma. Bluff (140 m) terraces yielded an average incision rate of 110 m/Ma based on cosmogenic burial dating but new detrital sanidine (DS) dating from the same terrace reveal a younger age and a rate of 134 m/Ma. Lower reaches have numerous high terraces (150 – 244m) but attempts to date them so far with DS yield only old, San Juan volcanic age, grains. Little Colorado River (LCR) incision rates above its major knickpoint are 100 to 150 m/Ma over past 2 Ma. The relief of SJR canyons in the lower, middle, and upper reaches are: Glen Canyon 430 m, Goosenecks 365 m, Animas Gorge 450 m. Given data for acceleration of incision in the past 2 Ma in the LCR, our hypothesis, testable by continued DS dating using strath-to-strath comparisons, is that the young (< 5 Ma) SJR has incised semi-steadily in lower, middle, and upper reaches at 100-150 m/Ma over the past ~2 Ma, to carve about half of the ~ 365-450 m canyons, with slower rates before that. The implication of semi-steady uplift over the 2 million year time frame is that incision is being driven by regional epeirogenic uplift amplified by isostatic rebound due to erosion.