Quaternary geology and geomorphology of the lower Deschutes River Canyon, Oregon.

Abstract

The morphology of the Deschutes River canyon downstream of the Pelton-Round Butte dam complex is the product of the regional geologic history, the composition of the geologic units that compose the valley walls, and Quaternary processes and events. Geologic units within the valley walls and regional deformation patterns control overall valley morphology. Valley bottom morphology is mostly the result of Quaternary events. These include several large landslides, which have caused retreat of the canyon rims but have also constricted the valley bottom with immense volumes of debris. In at least two instances (as at Whitehorse Rapids), landslides blocked the channel, resulting in ponding, breaching of landslide dams, and downstream floods. Large floods from other mechanisms have also formed many valley-bottom features along the lower Deschutes River. A large Pleistocene lahar resulting from a circa 0.07 Ma eruption of Mount Jefferson left bouldery deposits along the valley margins for most of the canyon length. The 15,000-12,700 ¹⁴C yr BP Missoula floods backflooded up the Deschutes River from the Columbia River and mantled the downstreammost 60 km of Deschutes River valley with bedded silt and clay. A large, possibly meteorologic flood between 6,500 and 3,000 ¹⁴C yr BP left abundant boulder bars and high sand and silt deposits that flank the channel in wider valley-bottom locations. In contrast, large historic main-stem floods, such as December 1964 and February 1996, had few effects on channel geomorphology due to the volume and coarseness of valley bottom deposits left by the earlier and larger floods and landslides.

Citation

O''Connor, Jim E.; Curran, Janet H.; Beebee, Robin A.; Grant, Gordon E.; Sarna-Wojcicki, Andrei. 2003. Quaternary geology and geomorphology of the lower Deschutes River Canyon, Oregon. Oregon Water Science and Application. 7: 77-98