RELATIONSHIPS BETWEEN SNAKE RIVER PALEOFLOODS AND OCCUPATIONAL PATTERNS AT REDBIRD BEACH ARCHAEOLOGICAL SITE IN LOWER HELLS CANYON, IDAHO

The Snake River basin drains 282,000 km² of the northwestern U.S. and is the largest tributary to the Columbia River. Redbird Beach, an archaeological site located in the lower Hells Canyon reach of the Snake River, contains extensive vertical exposures of archaeological materials interbedded with Snake River flood sediments. Redbird Beach formed in the lee of the Redbird Creek debris fan, and is composed of interfingering deposits from large floods on the Snake River and locally-derived alluvial sediments from Redbird Creek. Through stratigraphic analyses of slackwater deposits, this study compares the temporal and spatial patterns of human occupation at Redbird Beach with variations in the magnitude and frequency of floods from the Snake River. As many as 30 Snake River flood events formed the oldest portion of the Redbird Beach terrace at the downstream end of the site. Additional flood deposits are preserved in a series of inset stratigraphic sections that are progressively younger toward the upstream end of the terrace. Periods of historic and prehistoric occupation extending back at least 2500 years have been identified between flood events and correlate laterally within the flood stratigraphy. The oldest evidence for human occupation overlies pre-terrace forming paleotopography, and may represent a depositional environment much like the modern analogue at the upstream end of the terrace. Radiocarbon dates from detrital fragments and shells collected from organic material will constrain the ages of specific flood events and to determine the flood frequency. The late Holocene flood chronology of this reach of the Snake River, flowing between Washington and Idaho, has not been previously studied. Results of this study will form a key component of a regional synthesis of floods and climate change in the inland Northwestern U.S. and will contribute to our understanding of the archaeological record along this major regional waterway.