DescriptionAt the confluence of the Palouse and Snake Rivers, the Lyons Ferry area, an immense gravel bar formed from the flow of the monumental Missoula flood waters, as shown in Figure 1 (Bretz et al, 1956; Baker, 1973; Busacca, 1989). The Missoula floodwaters were forced to divide at this location because of the Snake River’s steep valley walls (Bjornstad, 2006). The division of water up and down the Snake is thought to have slowed the water enough to allow portions of the material in the outburst floods to deposit the gravel bar.
Water smashing into the valley wall would have effectively slowed the current and created the large-scale features shown. Current ripples within the bar demonstrate that, after splitting, the flow was directed both up and down the Snake River (Busacca, 1989). The bar itself consists of primarily basaltic gravel, but also material that is notably different from the sediment roaring down in the Snake River. The size, location, and types of rocks in the gravel bar signify the origins from the Channeled Scabland (Bjornstad, 2006).
The greatest cataclysmic flood that occurred was probably the older Wisconsin glacial episode because of the high energy deposits found within this time, such as the gravel bars. Some of the gravel deposits contain Mount St. Helens ash (Busacca, 1989). The slackwater (pooled up water) deposits, some of which contain the gravel bars, are overlain by non-flood deposits, which in turn are overlain by rhythmites (Busacca, 1989). Rhythmites are what geologists call periodically layered sediment. As the floodwaters overtopped the soils, some think that the pressure caused more water-rich layers to explode into overlying units, referred to as clastic dikes (Bjornstad, 2006). This sequence of deposition is concrete evidence that there were indeed dozens of mighty floods roaring through Washington State. Figure 1 shows the junction of the Missoula Flood split at the Snake River valley that created the Lyons Ferry gravel bar.