EVOLUTION OF THE COURSE OF THE COLORADO RIVER IN DETRITAL VALLEY AND BOULDER CANYON, LOWER LAKE MEAD REGION, NEVADA AND ARIZONA

The Colorado River in the Lake Mead region of the southern Basin and Range province flows westward through The Narrows of Boulder Canyon into Boulder Basin where it turns southward into Black Canyon toward the Gulf of California. Exposed river gravels in Detrital Valley (a north-south trending valley east of Black Canyon) indicate the Colorado River occupied this valley in the late Miocene. Why did the river exit westward into Boulder Basin instead of down the axis of Detrital Valley? To identify additional river deposits and potential ancient outlets we analyzed existing geologic maps, digital elevation models, and MASTER imagery to guide field work. A predictive map of the quartz-rich river deposits based on their thermal spectral characteristics in MASTER imagery proved especially useful.

The highest ancestral Colorado River deposits in Detrital Valley were deposited at the top of the upper Miocene Hualapai Limestone at an average elevation of 700 m. Householder Pass is at 690 m elevation with a bedrock sill at 680 m. This pass would have provided the Colorado River a short outlet southwestward from Detrital Valley into the lower reaches of Black Canyon. The highest and oldest Colorado River gravels and sand preserved in Detrital Valley lie 24 km south of the present river channel, and 13 km northeast of Householder Pass, at an elevation of 620 m. An 80 m thick exposure of these deposits has southward-directed current indicators, although no evidence has been found that the river ever flowed through the pass. Instead, the present course is incised more than 550 m deep across the north-trending northern Black Mountains, which there have a range crest higher than 800 m elevation. This suggests that the Black Mountains near Boulder Canyon may have been lower than the Hualapai Limestone at the time of river inception (~5 Ma) and subsequently were uplifted and incised after the river established its course. Longwell’s observations, before Lake Mead was impounded, of tilted river gravels against a fault west of Boulder Canyon support the idea that the river abandoned Detrital Valley and re-established its course into Boulder Basin prior to the uplift of the Black Mountains.