 The Tri-Cities is a mid-sized metropolitan area of approximately 296,224 people that also includes the cities of Kennewick and Richland in a 2018 estimate.Pasco's population was 59,781 at the 2010 census and 75,432 as of the July 1, 2019 Census Bureau estimate.On October 16, 1805, the Lewis and Clark Expedition camped in the Pasco area, at a site now commemorated by Sacajawea State Park.