Boise (i/b??si/) is the capital and most populous city of the U.S. state of Idaho, as well as the county seat of Ada County. Located on the Boise River in southwestern Idaho, the population of Boise at the 2010 Census was 205,671, the 99th largest in the nation. Its estimated population in 2013 was 214,237. The Boise-Nampa metropolitan area, also known as the Treasure Valley, includes five counties with a combined population of 664,422, the most populous metropolitan area in Idaho. It contains the states three largest cities; Boise, Nampa, and Meridian. Boise is the third most populous metropolitan area in the United States' Pacific Northwest region, behind Seattle and Portland. Accounts differ about the origin of the name. One account credits Capt. B.L.E. Bonneville of the U.S. Army as its source. After trekking for weeks through dry and rough terrain, his exploration party reached an overlook with a view of the Boise River Valley. The place where they stood is called Bonneville Point, located on the Oregon Trail east of the city. According to the story, a French-speaking guide, overwhelmed by the sight of the verdant river, yelled "Les bois! Les bois!" ("The wood! The wood!")—and the name stuck.