Agricultural field patterns and the Columbia River are featured in this south-southwest-looking, low-oblique photograph of the vast Columbia Plateau. An important agricultural and grazing region, the plateau is underlaid by deposits of lava (mainly basalt) some 10 000 feet (3050 meters) thick interbedded with sedimentary rock and overlaid with volcanic ash. The Washington area of the plateau has scattered coulees (dry river canyons) and scablands (extensively eroded basalt surfaces) that were carved by glacial meltwaters. The Columbia River twists westward to the base of the Cascade Range and turns south. Grand Coulee Dam is barely discernible midway between the lower left and lower center of the photograph. Just south of Grand Coulee Dam is Banks Lake, which was part of the Columbia River channel during the ice age. Near the right center, Lake Chelan, the third deepest freshwater lake in the United States, is visible. Other lakes and dams dot the course of the Columbia River, making the plateau an important source of hydroelectric power.