The Snake River watershed spans a large geographic region from the Rocky Mountains to the inland Pacific Northwest, and a comprehensive paleoflood chronology on the mainstem of the river is key to identifying the frequency and magnitude of large prehistoric floods within the region. We examined and compared four sites of slackwater deposits along a 20-km reach of the Lower Hells Canyon on the Snake River, Idaho. The sites contain evidence of up to 34 paleofloods within the last 1700 years. Stratigraphic breaks, soils, and in-situ plant or archaeological materials demarcate distinct layers that represent discrete paleoflood events. Radiocarbon dates from in-situ and transported charcoal constrain the ages of stratigraphic sequences with similar sedimentological characteristics.
The spatially coherent pattern in the paleoflood deposition and chronology over the last 1700 years in the lower Hells Canyon indicates a relatively consistent geomorphic environment in which the accumulation and preservation of paleoflood sediments is not significantly influenced by variations in the morphology of individual sites. This coherence is likely due to a slight widening of the canyon where the bedrock transitions from the hard, accreted metamorphic terrane of Hells Canyon to the basalt of the Columbia Basin, which accommodates the abundant slackwater deposition of fine sand and silt downstream of alluvial fans and in long benches along the channel margins. These geomorphic settings provide longer-term stability or protection of deposits from erosion by channel migration and undercutting from subsequent floods.
Hydraulic modeling of the study reach using HEC-RAS and Lidar data indicates that the flow necessary to overtop the existing deposits is approximately 6,500 m3 s-1 (230,000 ft3 s-1). The Snake River Flood Terrace has a record of 2-4 large flood events within the last 300 years. Combining this information leads to the conclusion that these four recent prehistoric floods were larger than 6,500 m3 s-1 (230,000 ft3 s-1) in magnitude. A comparison with the historic record indicates that the largest flood in the 62-year gage record at Anatone, WA of 5,520 m3 s-1 (195,000 ft3 s-1) in 1974 is insufficient to overtop the Snake River Flood Terrace. However, one of the flood deposits from the last 300 years may be the result of a large flood in 1910 recorded within the 110-year gage record from Weiser, Idaho
