Fluvial Response to Intra-Canyon Lava Flows, Owyhee River, Southeastern Oregon

Cooper Cooke Brossy
January 2006

Abstract

At least six lava flows have entered the Owyhee River Canyon north of Rome, Oregon, since the Pliocene and directly impacted the Owyhee River. The effects on the river of the two youngest lava flows, the West Crater (60–80 ka) and Saddle Butte (> 60–90 ka), are readily apparent. These two lava flows entered a paleo-Owyhee Canyon several kilometers wide via three different tributary drainages. The flows dammed the Owyhee River, created lakes, and effectively confined the river to the opposite side of the valley from the flows’ entrance. Lava from these flows filled a paleo-Owyhee Canyon to depths of up to 25 m for distances of at least 12 km. Several minor river channels were incised into these lava flows, but abandoned once the river began to incise into the less-resistant adjacent and underlying geologic units. The Upper AM-PM lava is older than the Saddle Butte lava. It likely represents distal portions of the Clarks Butte lava flow and records a time when the Owyhee River channel was at a higher elevation than today but at a very similar gradient to the modern river for over 26 km.

The Bogus Point, Bogus Rim, and Greeley Bar lava flows are the oldest (up to 2 Ma) and their effects on the river are mostly obscured by younger geologic units. From cosmogenic dating of fluvial features and 40Ar/39Ar dating of lava flows, estimates of mean incision rates through and the around the West Crater flow range from 0.27 mm/yr at Airplane Point to 3.3 mm/yr at Dog Leg Bar. No deposits from catastrophic outburst floods resulting from lava dam failure have been found, supporting gradual incision rather than catastrophic failure of the dams. The lava dams were long and low, and the river commonly eroded around the dams, reducing the likelihood of catastrophic lava dam failure. Today, the river flows through a narrow inner gorge often less than 500 m wide within the wider Owyhee Canyon.

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