SEDIMENTARY RECORD OF TWO LAVA-DAM IMPOUNDED LAKES, OWYHEE RIVER, SOUTHEASTERN OREGON

Lacustrine and fluvial sediments preserved behind late Quaternary lava dams on the Owyhee River, southeastern Oregon, document the longevity of the dams and the tempo of dam removal. The fluvial response to these natural dams facilitates our understanding of how this type of episodic perturbation affects the evolution of a river through time and space. The West Crater (60-90 ka) and Saddle Butte (~140 ka) lava flows created dams with crest elevations of 1030 m and 1042 m, respectively, and impounded lakes that reached at least 29 km upstream. Rounded gravels in overflow channels on top of the West Crater dam suggest that lacustrine sediment filled the reservoir and allowed the Owyhee River to flow over the dam. The West Crater lacustrine sections are up to 23 m thick in close proximity to the dam, and are probably derived from local Miocene sedimentary bedrock units. The main section includes a fining upward sequence containing basal ripple-laminated sands and massive clays towards the top, possibly recording the filling of the reservoir. River gravel deposits overlying this section mark the existence of a through-flowing river during dam incision. This section also includes two tephra layers correlated with Mt. St. Helens Set C series (~36 to 50 ka). Additional tephra correlations will provide sedimentation rates. ³He cosmogenic ages on river strath and fill terraces within and upstream of the dam record the timing, rates and process of incision through the dam. Ages range from ~40 ka on a paleochannel on top of the dam 50 m above the river to ~10 ka on a boulder bar 11.5 m above the river. The Saddle Butte lacustrine sediments are primarily preserved in a 22-m thick section, 2 km upstream from the Saddle Butte dam. This section does not contain any tephra layers, but does record fluvial incision into undated massive lacustrine clays, which likely records the breaching of either the West Crater or Saddle Butte lava dam downstream of the section and the reestablishment of the Owyhee River. Correlation of lacustrine sections will be used to reconstruct paleocanyon topography and sediment accumulation dynamics. Diatoms found in the West Crater and Saddle Butte lacustrine sediments will provide environmental information on the lakes.