There are 15 US Post Offices in Clark County serving a total of 286,731 residents equating to 19,115 residents per post office. It's estimated that approximately 457,655 packages are handled across the 15 county post offices annually. 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. Its county seat and largest city is Vancouver. It was the first county to be established in 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. Clark County is included in the Portland-Vancouver-Hillsboro, OR-WA Metropolitan Statistical Area. It is located across the Columbia River from Portland.