The Lower Columbia River region encompasses the southern Cascade Mountains, the narrow Columbia Gorge, and the broad, fertile floodplains of the lower Columbia River and its major tributaries. The Cascades are especially rugged in this region. The bottomlands of the Columbia and its major tributaries are characterized by stands of black cottonwood and other hardwood trees, and thousands of acres of freshwater marshes.

Wildlife of the region include the black-tailed deer, Roosevelt elk, and other species typical of the Cascade or Coast ranges. The endangered Columbian white-tailed deer is now restricted to narrow, wooded habitat along the lower Columbia River. Spring chinook and other races of Pacific salmon migrate upstream to spawn in the upper reaches of the Columbia and Snake river systems. Many thousands of ducks, geese, swans, and other migratory birds stop, rest, or winter in the productive bottomlands of the lower Columbia River region.