Clark County is a county located in the southwestern part of the U.S. state of Washington. As of the 2010 census, the population was 425,363, making it the fifth-most populous county in Washington. The county seat and largest city is Vancouver.
Clark County was the first county to be established in Washington, and 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 of Washington.
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 what is now British Columbia. On June 15, 1846 the United States Senate approved the present boundary between the U.S. and Canada at the 49th Parallel.
On August 13, 1848, President James K. Polk signed an act creating the entire region as the Oregon Territory. On September 3, 1849, the Oregon Territorial Legislature modified the borders again and changed its name to Clarke County in honor of explorer William Clark. At this time it included all of present-day Washington and continued to be divided and subdivided until reaching its present area in 1880. It was not until 1925 that the spelling was corrected to its present form.
According to the U.S. Census Bureau, the county has a total area of 656 square miles, of which 629 square miles is land and 27 square miles (4.1%) is water. It is the fifth-smallest county in Washington by land area.
Clark County is bordered on two sides by the Columbia River and on the north by the North Fork of the Lewis River. The East Fork of the Lewis River and the Washougal River cut across the county. The largest stream arising solely within the county is Salmon Creek, which terminates at Vancouver Lake before eventually flowing into the Columbia River.