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. Radiocarbon dates from detrital charcoal fragments and shell have been used to constrain large flood packages and determine flood frequency. Periods of historic and prehistoric occupation from ~160-2300 cal years BP have been identified between flood events that correlate laterally within the flood stratigraphy. 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. These more extensive, coarse-grain and crossbedded flood deposits are observed in the inset stratigraphic sequences beginning 740-900 cal years BP, and may represent Snake River channel migration concurrent with regional climate change. Depositional changes and continued terrace development at Redbird Beach during this time also appear to coincide with a decrease in prehistoric human occupational use of the site. 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.