Corridor of Cataclysm -- mapping the perilous geologic journey of the Owyhee River

P. Kyle House, PhD

University of Arizona

Arizona Geological Survey

Over the last 2 My, the Owyhee River—aided by diverse geologic processes and events—has carved a spectacular landscape into the high-desert volcanic terrain of southeastern Oregon. River-corridor-focused geologic mapping reveals a repeating ‘triad of catastrophe’: immense lava dams that fossilize valleys, regrade canyons, and eventually breach; channel-blocking landslides that reorganize drainage and generate outburst floods; and pluvial lake outburst floods from remote sources that sweep through the corridor. Yet through each episode, the river reasserts itself, resuming incision until the next cataclysm. This catastrophic cycle is regenerative. A late Miocene, pre-river framework of volcanic and lacustrine deposits is susceptible to slope instability. As the river incises, it exposes additional unstable strata, while lava damming redirects channels into susceptible slopes and reloads the canyon with basalt flows and lacustrine deposits. In this way, lava damming reconstitutes the vulnerable stratigraphy promoting future instability.

When lava enters the corridor, momentum drives lobes upstream, where explosive contact with the river builds a prograding delta of interbedded subaerial lava, pillow lava, and hyaloclastite, a structurally weak assemblage. As the lava dam and lake grow, the canyon downstream becomes a conduit for lava that repaves and regrades the dry channel with tens of meters of basalt resulting in a steep tapered wedge of rock. The battle upstream produces the dam’s internal vulnerability; the downstream flows produce its mass. Together they construct a blockage that self-destructs. Because water fills reservoirs faster than sediment, the first phase of dam degradation is hydraulic. The dam begins consuming itself via plucking and transporting coarse monolithologic gravel we termcannibal gravels. Lowering of the crest accelerates delta advance, and once the delta reaches the declining outlet, its rich sediment load accelerates incision, while the poorly consolidated lava-delta margins serve as landslide food, rebuilding slope instability along the canyon walls. The river then resumes incision into an even narrower canyon where the triad looms. Drawing on nearly two decades of field mapping, this presentation traces and maps the Owyhee’s course through repeated impoundment, catastrophic release, and drainage reconstruction. Moreover, this remarkable record reveals how rivers persist, and landscapes evolve in the face of multiple origins of channel blockage and outburst flooding.

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