It included all the land north of the Columbia River, west of the Rocky Mountains, and south of Alaska.

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.

At this time it included all of present-day Washington and continued to be divided and subdivided until reaching its present area in 1880.

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.

The central and southwest areas of the county are generally flat floodplains, sculpted by torrents of prehistoric Lake Missoula.

Spring thaws can often swell county waterways, with two of the more destructive floods being those of the Columbia River in June 1894 and May 1948.

The 1948 Memorial Day flood almost topped the Interstate Bridge's support piers and completely destroyed nearby Vanport, Oregon.

Construction of The Dalles Dam and destruction of Celilo Falls are credited with a decrease in such floods.

In addition to a wide variety of birds including great blue heron, raptors such as barred owl, osprey, red-tailed hawk and bald eagle, corvids (raven, crow, California scrub and Steller's jay) and others, the native streams are home to various species of salmon and the Vancouver Trout Hatchery.

Larger mammals include black-tailed deer, coyote, raccoon, skunk and invasive opossum; with sightings of lynx, bobcat, black bear, cougar and elk not uncommon, especially in the northern parts of the county.

Common foods used by the indigenous people such as the Klickitat tribe and Chinook included salmon, huckleberry and Camassia quamash (after which the city of Camas, Washington is named).

Clark County's largest industries include health care, professional and business services, and retail.

[9] Clark County residents also cross the Columbia River to shop without sales taxes, which Oregon also lacks; this phenomenon caused up to $5.9 million in estimated lost sales tax revenue for the county government in 2022.

Clark County is religiously diverse, with no single group comprising 10% of the population.

[20] The area is also home to the nation's largest population of the Old Apostolic Lutheran Church with between 8,000 and 12,000 members living in the county.

The sheriff's office was established in 1849 and is the oldest law enforcement organization in the state of Washington.

The Clark County Council was created in 2014 and has five seats elected by districts of proportional size that are redrawn every 10 years.

[26] The county manager is the chief executive officer appointed by the council and oversees the administrative departments of the government.