Kelso is a ceety in soothwast Washington State, Unitit States, an is the coonty seat o Cowlitz Coonty. The population o the 2008 estimate wis 11,895. Kelso is pairt o the Longview, Washington Metropolitan Statistical Aurie, which haes a population o 101,254. Kelso shares its lang wastren border wi Longview, Washington. It is locatit near Mount St. Helens.