It covers 668000 km 2 or 258000 sq mi. The drainage area for a river basin is measured in a horizontal plane. Columbia River Drainage Basin Wikipedia Use this interactive map to learn about the columbia river system including reservoir levels dams reservoirs and hydro generation. Columbia river drainage basin map . By volume the columbia is the fourth largest river in north america. Interactive columbia river system map. The river itself flows from its headwaters in british columbia canada through only two states forming part of the. The columbia basin province occupies the entire south central portion of the state. In its 1200 mile course to the ocean the river flows. Fascinating new map shows every river basin on the globe with. It is a wide arid lowland area between the okanogan highlands the southern cascade range the idaho rockies and continues on through much of eastern oregon and northern nevada. To th...