watercourse, southwestern Idaho, U.S., formed by the confluence of the Middle Fork and North Fork branches, southeast of Idaho City in Boise National Forest. It flows generally westward through Arrowrock and Lucky Peak reservoirs and through the city of Boise to join the Snake River at the Oregon line after a course of about 95 miles (150 km). Dams along the river include Lucky Peak, Arrowrock, and Anderson Ranch (on the South Fork). The name of the river, like the name of Idaho's capital, is from the French boisé, meaning “wooded.” Before 1863 the river was called Reed (or Reeds) River.