Comparison of Natural Dams from Lava Flows and Landslides on the Owyhee River, Oregon

Numerous large lava flows and mass movements have temporarily dammed the Owyhee River in southeastern Oregon at various temporal and spatial scales. These channel-encroaching events potentially play a significant role in creating and maintaining the geomorphic features of river canyons in uplifted volcanic terranes that compose a significant part of the western U.S. Abundant landslides and lava flows have the capacity to inhibit incision by altering channel slope, width, and bed character, and burying valley- bottom bedrock under exogenous material; or promote incision by generating cataclysmic floods through natural dam failures. The natural dams vary in their source, morphology, longevity and process of removal, which in turn affects the extent and duration of their impact on the river. The 3 most recent lava flows filled the channel 10-75 m deep and flowed up to 26 kilometers downvalley, creating long, low dams that were subject to gradual, rather than catastrophic, removal. In the last 125 ka, the Saddle Butte and West Crater lava dams created reservoirs into which 10-30 meters of silt and sand were deposited. The river overtopped the dams and in most reaches eventually cut a new channel through the adjacent, less resistant bedrock buttresses. Terraces at several elevations downstream and upstream of the West Crater dam indicate periods of episodic incision ranging from 0.28 to 1.7 mm/yr., based on 3He exposure ages on strath surfaces and boulder-rich fluvial deposits. In contrast to the lava dams, outburst flood deposits associated with landslide dams are common along the river. The mechanisms of failure are related to the geologic setting, and include rotational slump complexes, cantilevered blocks and block slides, and massive earthflows. Most large-scale mass movements occur in reaches where the Owyhee canyon incises through stacks of interbedded fluviolacustrine sediments capped with lava flows. The frequently observed association of landslides and flood deposits, coupled with an almost universal absence of associated lacustrine deposits or fill terraces, supports a model of short-lived landslide blockages and catastrophic breach-flooding. The exceptions are massive earthflow dams in the Hole-in-the- Ground reach, which appear to be stable enough to accumulate aggradational deposits upstream, but are still linked with outburst flood bars. The impact of the different types of mass movements is a balance between the cumulative effect of the more numerous, smaller block falls and slumps, which introduce colluvial blocks to the channel and can generate dam-breach floods, and the massive, less frequent landslides and earthflows that persist longer in the geomorphic record.