FLUVIAL RESPONSE TO INTRA-CANYON LAVA FLOWS, SOUTHEASTERN OREGON

At least four basaltic lava flows in the last 4 million years have entered the Owyhee River Canyon north of Rome, Oregon. The lava flows temporarily dammed the river, and locally reduced rates of river incision while they were being downcut. Each lava flow created a lake many tens of meters deep; however, no deposits from catastrophic outburst floods have been directly linked to individual lava dams. Although their exact age is still unknown, the Bogus Point and Bogus Rim lava flows are the oldest, and their effects on the river are obscured by younger geologic units. The two youngest lava flow complexes are from West Crater (37-86 ka) and Saddle Butte, which is approximately an order of magnitude older. The younger lava flows entered a paleo-Owyhee Canyon several kilometers wide via three different tributary drainages, dammed the Owyhee River multiple times, created lakes and shifted the position of the river. Individual lava flows filled a paleo-Owyhee Canyon to a depth of at least 25 m for distances of up to 27 km. Channels formed on the lava flows until the river began to incise into the less-resistant adjacent and underlying geologic units. In the bedrock canyon downstream of West Crater, the river incised directly through the lava flows, leaving several remnants of basalt lava perched at two consistent heights above the channel on strath terraces in the rhyolite bedrock. Preliminary maximum estimates of incision rates through the lava flows range from 0.18 to 2.3 mm/yr. These different flows highlight a range of ages, environments and geological settings for river/lava flow interactions, and point to the importance of discriminating multiple flows from the same vent in interpreting the response of rivers to lava flow incursions.