Region 4: The Columbia Plateau

The Columbia Plateau region forms an intermontane plateau bordered by the Northern Rocky Mountains to the east and the Cascade Range to the west. The plateau covers approximately 260,000 square kilometers (100,000 square miles) in Idaho, Oregon, and Washington and is one of the world’s largest accumulations of volcanic rock.

The Snake River Plain that stretches in a bow across southern Idaho was formed by volcanic eruptions starting 11 to 12 million years ago. The eastern Snake River Plain follows the path the North American plate has taken over the Yellowstone hot spot, which is now currently underneath Yellowstone National Park. This area is blocked by mountains on all sides, and therefore moisture is limited, leading to an accumulation of calcium carbonate and various salts and clays that form Aridisol soils (Figure 8.24). The Aridisols of the Snake River Plain have proven to be quite productive when heavily irrigated, and they compose much of Idaho’s agricultural land. This is possible because of the relatively flat nature of the high plain, making crop cultivation practical when water is available. Irrigation yields crops such as potatoes, corn, wheat, sugar beets, mint, alfalfa, and onions. When not irrigated, these Aridisols usually support a sagebrush steppe.

Figure 8.24: Aridisols of Idaho’s Snake River Plain, near Craters of the Moon National Monument.

Mollisols, with their dark surface horizon, are common on the periphery of the Snake River Plain and along the western Idaho border in central Idaho. Due to the semiarid conditions of the region, irrigation is still necessary to take advantage of the rich, well-developed soil. Unirrigated Mollisols in the area tend to support a sagebrush steppe environment as well. It is important to note, however, that northwestern Idaho has some of the richest dryland wheat and pulse production in the world. This level of productivity is possible because these particular Mollisols are formed on loess overlying the basalt.

In southwestern Idaho, the juniper-pinyon woodlands found on rocky or gravelly uplands host a concentration of Alfisols. These Alfisols have a subsurface horizon with accumulated clays. Alfisols can be productive soils, and agriculture here is practiced largely in the form of grazing where sufficient sagebrush steppe is available.

Entisols associated with floodwater and fluvial deposits are found scattered along the extent of the Snake River.