Pasco is a city in and the county seat of Franklin County, Washington, United States. It is one of three cities that make up the Tri-Cities region of the state of Washington. The Tri-Cities is a mid-sized metropolitan area of approximately 230,000 people that also includes the cities of Kennewick and Richland. The population was 59,781 at the 2010 census. On October 16, 1805, the Lewis and Clark Expedition camped in the Pasco area, at a site now commemorated by Sacajawea State Park. The area was frequented by fur trappers and gold traders. In the 1880s, the Northern Pacific Railway was built near the Columbia River, bringing many settlers to the area. Pasco was officially incorporated on September 3, 1891. It was named by Virgil Bogue, a construction engineer for the Northern Pacific Railway after Cerro de Pasco, a city in the Peruvian Andes, where he had helped build a railroad. In its early years, it was a small railroad town, but the completion of the Grand Coulee Dam in 1941 brought irrigation and agriculture to the area.