Region 2: The Columbia Plateau

This region’s geology, and therefore its soil, is dominated by the Columbia River Flood Basalts. Beginning in the Miocene (17 million years ago) and leading up to the Pleistocene, flood basalts erupted in numerous locations throughout the region and flowed over the landscape, leaving few places untouched. These effusive lavas resulted from a hot spot. This hot spot now rests under Yellowstone National Park, the site of considerable hydrothermal activity. Wind-transported sediments known as loess are also important to the region. The Palouse Loess, for example, is famous for supporting productive agricultural lands.

The Columbia Plateau is mostly covered by grasslands and some forests, and is composed primarily of Mollisols. These soils tend to be dry in the summer and are later re-moistened by the fall and winter rains. They typically rest on top of gently sloped surfaces, in this case loess-covered flood basalts. Mafic minerals, a result of past volcanism, are common in the loess that these soils are derived from. This region experiences slower erosion than the more steeply sloped and continuously changing mountainous regions nearby. This allows the soils ample time to develop a rich and dark topsoil horizon, one that is perfect for supporting grasslands and farming. Agriculture is widespread in this region and is supported by irrigation from several rivers flowing through the area, which help to soak the summer-dried soils. Some of Washington’s best apples are grown on the Columbia Plateau.

Some Aridisols can be found in the Channeled Scablands of eastern Washington (Figure 8.13)—a barren, eroded area scoured clean by the Missoula Floods—as well as in the southeast corner of this region.

Figure 8.13: Aridisols in the Channeled Scablands near Wenatchee, Washington.