HOLOCENE PALEOFLOOD HISTORY OF THE LOWER SNAKE RIVER, IDAHO

A chronology of paleofloods over the last 2000 years was constructed at four sites within a 20-km reach of the Snake River, between the lower Hells Canyon and the confluence with the Clearwater River near Lewiston, Idaho. The stratigraphy at each site of slackwater deposition was analyzed and described using stratigraphic breaks and or indicators of subaerial exposure, such as soil horizons and in situ plant material, to identify distinct layers that represent discrete paleoflood events. Radiocarbon dates from in situ charcoal at the upper boundaries of slackwater deposits constrain the ages of stratigraphic sequences with similar sedimentological characteristics and identify variability in paleoflood frequency. Each site contains evidence of up to 30 paleoflood events within the last 2000 years. Similarities in age and stratigraphy across all four sites suggest that they preserve deposits from many of the same paleoflood events. This spatially coherent pattern in paleoflood deposition indicates a relatively consistent geomorphic environment in which the accumulation and preservation of the paleoflood sediments is not significantly influenced by variations in the morphology of individual sites. The canyon widens through this section of the lower Snake River as the bedrock transitions from the hard, accreted metamorphic terrane of Hells Canyon to the relatively soft basalt of the Columbia Basin, which accommodates the abundant slackwater deposition of fine sand and silt along the margins of the channel.

The results of this study will be combined with those from previous investigations on other parts of the river to increase our understanding of the paloehydrologic and geomorphic history of the lower Snake River. The Snake River watershed integrates multiple climatic zones from the Rocky Mountains to the inland Pacific Northwest, and the addition of a comprehensive paleoflood chronology on the mainstem of the river is key to addressing long-term links between climate changes and large floods within the region.