Clark County, Washington

45°46′N 122°29′W / 45.77°N 122.48°W / 45.77; -122.48

Clark County is the southernmost county in the U.S. state of Washington. As of the 2020 census, the population was 503,311, making it Washington's fifth-most populous county. Its county seat and largest city is Vancouver. It was the first county in Washington, first named Vancouver County in 1845 before being renamed for William Clark of the Lewis and Clark Expedition in 1849. It was created by the Provisional Government of Oregon in Oregon Country on August 20, 1845, and at that time covered the entire present-day state. Clark County is the third-most-populous county in the Portland-Vancouver-Hillsboro Metropolitan Statistical Area, and is across the Columbia River from Portland, Oregon.

Clark County was created on August 20, 1845, as Vancouver District, named for its town, following the removal of the area from three other districts on July 27, 1844. It included all the land north of the Columbia River, west of the Rocky Mountains, and south of Alaska. On December 21, 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, Canada. 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.

In September 1902 the Yacolt Burn, the largest fire in state history, began in neighboring Skamania County and swept west along a 12-mile front to Yacolt, nearly engulfing the town. Salvaging the remaining timber was a lucrative industry for a time.^{[citation needed]}

According to the United States Census Bureau, the county has a total area of 656 square miles (1,700 km²), of which 629 square miles (1,630 km²) is land and 27 square miles (70 km²) (4.1%) is water. It is the fifth-smallest county in Washington by land area.

Clark County is surrounded 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.

Like most of Oregon and Washington south of Puget Sound into the Willamette Valley the landscape and climate of Clark County are determined by its placement between the volcanic Pacific Coast and Cascade Ranges, where glaciation helped form a U-shaped valley which meets the river valley of the Columbia River as it leaves the Columbia River Gorge. Volcanic andisol soils are common, with fertile mollisols in the lower areas. The central and southwest areas of the county are generally flat floodplains, sculpted by torrents of prehistoric Lake Missoula. A series of dramatic floods known as the Missoula Floods took place 15,000–13,000 years ago, as several ice dams melted, forming a series of low steps such as the "Heights", "Mill Plain", "Fourth Plain" and "Fifth Plain". Clark County's Köppen climate classification is "Csb".