Geomorphic evolution of the Grande Ronde and John Day Rivers in northeastern Oregon.

Matt Clausen and Molly Gerber

Department of Geology, Whitman College, Walla Walla, WA 99362

Molly Gerber and I studied the geomorphic evolution of the Grande Ronde and John Day Rivers in Northeastern Oregon with Bob Carson. Molly concentrated on the Grande Ronde River and I on the John Day River. We also worked on a field guide to the geology between Milton-Freewater and Wallowa Lake via La Grande that was used for the field trip for the Keck Geology Constorium meeting at Whitman in April 1999, and may be published by the Oregon Department of Geology and Mineral Industries.

The work on the John Day River focussed on the main branch from its headwaters in the Blue Mountains to Clarno, Oregon. Items used to interpret the history of the John Day River system included terraces, pediments, Mazama ash deposits, truncated alluvial fans, and intracanyon lava flows. Two of the more remarkable things observed included an intracanyon lava flow in the Clarno Formation (Eocene) near Clarno, Oregon and the large extensive surface of the Rattlesnake Formation (Miocene) in the John Day-Mt. Vernon, Oregon area. The former, idicates the presence of a river system in north-central Oregon 40 m.a. The latter indicates the presence of the John Day River system at 5 m.a.

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