GEOMORPHIC CONSEQUENCES OF PLIOCENE TO PLEISTOCENE INTRACANYON LAVA FLOWS, OWYHEE RIVER, SOUTHEASTERN OREGON

Intracanyon lava flows have influenced the geomorphic evolution of the Owyhee River canyon throughout the late Cenozoic. Our work has shown that up to six lava flows entered and dammed the river canyon during two periods, ca. 1.7 Ma to >780 ka and 250-70 ka. New analyses show the six lava dams added an additional 300-360 m of cumulative vertical channel elevation through which the river had to incise. The lava dams represent different scales of temporal perturbations in the vertical profile of the canyon that are superimposed on a regional long-term drop in base level. For example, calculations of cumulative long-term incision of the Owyhee River canyon range from approximately 0.08-0.18 mm/yr, but incision rates through individual lava flows were up to an order of magnitude greater. The relatively greater incision rates through the younger, lower-volume lava dams may have been enough to reestablish the previous river channel elevations on a scale of 10⁴ years. In contrast, the Bogus Rim lava dam ca. 1.7 Ma resulted from the largest of the intracanyon lava flows in this study, and its effects on the river profile appear to have persisted for >10⁵ years. The dam remnants and extant fluviolacustrine deposits present across large extents of the Owyhee Canyon and upland indicate that the lakes created by the Bogus Rim and other lava dams ranged in size from 10² to 10³ km². Recent paleomagnetic investigations and mapping of additional lava flows outside the modern canyon further demonstrate the broad extent of these early lakes. Many of these lava flows show evidence of lava-water interaction at their termini where they form large, conspicuous rims at elevations consistent with paleolake sediments.

The progressive erosion of intracanyon and rim-forming lava flows is integrally connected to the location, timing and morphology of the large landslides in this reach of the Owyhee River. Following a lava flow incursion the re-incision of the river induces landsliding at various scales. This promotes progressive canyon widening following the abrupt narrowing related to lava incursion. These coupled interactions between lava flows and landslides produce additional lasting effects on channel evolution in the Owyhee River that may occur in similar rivers in uplifted volcanic plateaus.