The Channeled Scablands are a dry and mostly soil-free area with many old flood channels, coulees, cataracts, and flat basalt layers left after huge floods. This region is in the southeastern part of Washington state. Over the last two million years, the area was shaped by more than 40 massive floods during the Last Glacial Maximum and many older floods. These floods happened when large glacial lakes broke through ice dams, sending water across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. The most recent of these floods occurred between 18,200 and 14,000 years ago. These floods eroded the land, forming the Palouse loess.

In the 1920s, geologist J Harlen Bretz described "scablands" as low areas with many irregular channels and rock basins carved into basalt. Flood waters removed the loess layer, creating large interconnected channels that exposed bare basalt and formed butte-and-basin landscapes. The buttes range in height from 30 to 230 meters (98 to 755 feet), and the rock basins vary in size, with Rock Lake being 11 kilometers (7 miles) long and 30 meters (100 feet) deep. Bretz noted, "The channels run uphill and downhill, they unite and divide, they begin on slopes and cut through peaks; their design is irregular and complex."

For four decades, scientists debated the origin of the Scablands, making it one of the most significant controversies in earth science history. The Scablands are also important to planetary scientists because they may be the best example on Earth of the Martian outflow channels.

History

Bretz studied the Channeled Scablands and wrote many scientific papers in the 1920s. He believed the area was shaped by sudden, huge floods, but he could not explain how these floods happened. Many geologists at the time disagreed with Bretz’s ideas, preferring to explain the landforms using processes that happen slowly and continue today.

In 1925, J. T. Pardee told Bretz that the sudden draining of a large glacial lake might explain the massive floods. Over the next 30 years, Pardee gathered and studied evidence that eventually showed Lake Missoula was the source of the floods, now called the Missoula floods, which created the Channeled Scablands.

Bretz and Pardee’s theories were accepted only after many years of careful research and strong scientific arguments. Studies on water movement in open channels during the 1970s supported Bretz’s ideas. In 1979, Bretz was awarded the Penrose Medal by the Geological Society of America, recognizing his work as one of the most important discoveries in earth science.

Geology

Distinct landform features include coulees, dry falls, streamlined hills and islands of leftover loess, gravel fans and bars, and giant current ripples.

The term scabland describes an area where river erosion has removed loess and other soils, leaving the land bare. Rivers create V-shaped valleys through erosion, while glaciers form U-shaped valleys. The Channeled Scablands have a rectangular shape with flat plateaus and steep canyon sides, covering large areas of eastern Washington. The landform pattern of the scablands is butte-and-basin. The area of the Scablands is estimated to be between 1,500 and 2,000 square miles (3,900 and 5,200 square kilometers), though these estimates may be too low.

The region shows a unique drainage pattern with an entrance in the northeast and an exit in the southwest. The Cordilleran ice sheet blocked Glacial Lake Missoula at the Purcell Trench Lobe. A series of floods between 18,000 and 13,000 years ago covered the landscape when the ice dam broke. The eroded channels have a braided, or interconnected, appearance.

Evidence of Middle and Early Pleistocene Missoula flood deposits has been found in the Channeled Scablands and other parts of the Columbia Basin, such as the Othello Channels, Columbia River Gorge, Quincy Basin, Pasco Basin, and Walla Walla Valley. Based on findings like interglacial calcretes, magnetostratigraphy, optically stimulated luminescence dating, and unconformity-truncated clastic dikes, the oldest megafloods in the Channeled Scablands likely occurred before 1.5 million years ago. Older glaciofluvial deposits have been largely removed by later Missoula floods, making it difficult to estimate the exact number of ancient floods. As many as 100 separate Ice Age floods may have occurred during the last glaciation. At least 17 complete interglacial-glacial cycles have occurred since about 1.77 million years ago, and possibly as many as 44 since the start of the Pleistocene around 2.58 million years ago. Assuming a dozen or more floods occurred with each glaciation, the total number of cataclysmic Missoula floods through the Channeled Scablands during the entire Pleistocene could reach hundreds, possibly over a thousand.

The Scablands also contain large potholes and ripple marks much bigger than those found in typical rivers. When these features were first studied, no known theories could explain their origin. Giant current ripples are between 3 and 49 feet (1 and 15 meters) high and are regularly spaced, forming uniform hills. Vast amounts of water would be needed to create such large ripple marks, as they are larger versions of smaller ripples found in streams. Large potholes were formed by swirling water movements called kolks, which eroded and removed bedrock.

The Scablands are covered with large boulders called glacial erratics. These boulders were carried by glaciers and deposited during glacial outburst floods. The rock type of erratics often does not match the surrounding area, as they were transported far from their original location.