At the end of the last ice age, the Columbia River basin experienced one of the most catastrophic floods. The ancient ice-dammed lake Misula, periodically breaking through the ice dam, flooded the plains of the modern states of Washington and Oregon about 15 thousand years ago (Mystery 5:10-5:30). The Missoula floods formed the area known as the Channel Scablands, a complex network of canyon-like channels. Although the canals may appear dark brown, they are dry with no signs of moisture. They are distinguished by a dark brown tint due to the light surroundings. Moreover, their color is given by volcanic basalt, which appeared during the destruction of layers of sedimentary rocks. Even though scientists have concluded that the landscape was formed by prehistoric floods, this process is not fully understood. The Channel Scablands are the original trail of huge, high-gradient glacial flows.
Among towering cliffs and descending depressions, boulders are scattered across the landscape, bedrock traversing the bends of the channels. For geologists, this region, called Channeled Scablands, is valuable because it can tell a lot about past extreme floods. As the edge of the Cordillera Ice Sheet moved south, it formed an ice dam rising 600 meters along the Clark Fork River (Mystery 18:37-19:44). Glacial Lake Missoula grew behind an ice dam and eventually accumulated as much water as Lake Erie and Lake Ontario combined. When the dam collapsed, more than 2,000 cubic kilometers of water spilled across the region at about 130 km/h (Mystery 34:18-36:23). A flood of gigantic proportions during the Ice Age cut through three parallel sections of flood channels. For this period, they were united in a large temporary lake. On its way, flood waters formed several hundred waterfalls and canyons. Thus, the channel was formed by powerful repeated floods.
Work Cited
Mystery of the Megaflood. Directed by Ben Fox, Joe Kennedy, PBS Nova Documentaries, 2005.