The first major sediment deposit following the eruptions is the Ringold Formation, which was placed by the Columbia River between 8 and 3 million years ago. Further deposition came as a result of the Missoula Floods. At the end of the last glacial maximum, an ice dam blocked the Clark Fork River in Montana. The pressure from the resulting lake would periodically build to the point that the dam would fail, sending massive amounts of water cascading to the Pacific Ocean. The flood's movement was impeded by the Horse Heaven Hills, creating a temporary lake known as Lake Lewis. This abrupt halt in flow allowed the floodwater to drop a significant amount of sediment before passing through Wallula Gap toward Hermiston. During the largest floods, the water's surface reached above sea level. This completely covered all of the land within Kennewick's city limits.
The first major sediment deposit following the eruptions is the Ringold Formation, which was placed by the Columbia River between 8 and 3 million years ago. Further deposition came as a result of the Missoula Floods. At the end of the last glacial maximum, an ice dam blocked the Clark Fork River in Montana. The pressure from the resulting lake would periodically build to the point that the dam would fail sending massive amounts of water cascading to the Pacific Ocean. The flood's movement was impeded by the Horse Heaven Hills, creating a temporary lake known as Lake Lewis This abrupt halt in flow allowed the floodwater to drop a significant amount of sediment before passing through Wallula Gap toward Hermiston. During the largest floods, the water's surface reached above sea level This completely covered all of the land within Kennewick's city limits.