Kennewick is a city in Benton County in the southeastern part of the US state of Washington, near the Hanford nuclear site. It is the most populous of the three cities collectively referred to as the Tri-Cities. Kennewick is located along the southwest bank of the Columbia River, opposite Pasco and just south of the confluence of the Columbia and Yakima rivers. The population was 73,917 at the 2010 census. On April 1, 2010.