High Quality Content by WIKIPEDIA articles! Clark County is a county located in the southwestern part of the U.S. state of Washington, across the Columbia River from Portland, Oregon. Clark County was the first county of Washington, named after William Clark of the Lewis and Clark Expedition. It was created by the provisional government of Oregon Territory on August 20, 1845, and at that time covered the entire present-day state. In July 1, 2008, Clark County's population was estimated to be 424,733. Its county seat is at Vancouver, which is also its largest city. Clark County began as the District of Vancouver on July 27, 1844. It included all the land north of the Columbia River, west of the Rocky Mountains, and south of Alaska. In 1845 the provisional government changed its name to Vancouver County. At that time it stretched from the Columbia River to 54 degrees 40 minutes North Latitude in British Columbia.