The Columbia Plateau is a wide basalt plateau between the Cascade Range and the Rocky Mountains that covers parts of Washington, Oregon, and Idaho (fig. 1). The climate over much of the Columbia Plateau is semiarid with precipitation ranging from 7 to 15 in/yr in the central part (Vaccaro and others, 2015), yet the area supports a $6 billion per year agricultural industry, including the production of apples, corn, grapes, hops, mint, potatoes, stone fruit, and wheat. Groundwater pumpage and surface-water diversions supply water to irrigated croplands that account for about 5 percent of the Nation’s irrigated lands. Groundwater also is the primary source of drinking water for about 1.3 million people living on the plateau.