LATE QUATERNARY DEBRIS-FLOW AGGRADATION OF THE COLORADO RIVER IN CATARACT CANYON, UTAH, USA

Cataract Canyon is incised into Mesozoic and Paleozoic rocks of the Colorado Plateau in south-central Utah. Before regulation of the Colorado River, Cataract Canyon was 63 km long and had 52 rapids created by debris fans issuing from tributary canyons. Debris-flow deposits emanate from nearly all tributaries and steep chutes where the Halgaito Shale is exposed in the cliffs above the river; where this unit is not recognizable in the Paleozoic stratigraphy, debris flows are uncommon. Shales with similar clay minerals are related to high Holocene debris-flow frequency elsewhere on the Colorado Plateau. Between river kilometer 348 and 343 (km decrease downstream), the gradient of the Colorado River is 0.28 m/km, increasing to 2.94 m/km in Cataract Canyon between km 343 and 336. We hypothesize that frequent Holocene debris flows in Cataract Canyon created a natural dam that affects its gradient as far as 75 km upstream. Drill holes near the confluence of the Green and Colorado Rivers (km 348.5) measured more than 38 m of sand beneath the river, and tributary mouths upstream are backfilled with fine-grained sediments, creating bottomlands. The longitudinal profile of the Colorado River is hypothesized to be structurally controlled and strongly influenced by climate; during glacial and interglacial climates in the Rocky Mountains, bedrock downcutting occurs primarily owing to large floods, and during interglacials with monsoonal precipitation, aggradation from local debris flows prevailed. During the Pleistocene-Holocene transition in the Southwest, summer precipitation increased, runoff decreased, and Cataract Canyon had frequent debris flows. Response of the river and its tributary streams to climate changes are recorded in fluvial gravels, fan aprons, and debris-flow deposits. Seven levels of Quaternary terrace gravels (Qtg) are preserved in discontinuous deposits along the Colorado and Green Rivers about 81 km upstream of the confluence; these deposits are not present in Cataract Canyon. The lowest, youngest Qtg deposits on the Colorado River disappear under the present-day water surface between river kilometers 390 and 400. Early Holocene debris-flow activity in Cataract Canyon appears to have aggraded the bed of the Colorado River by as much as 40 m, decreasing its gradient upstream.