Boise River, c.75 mi/121 km long, Idaho and Oregon; rising in SW Idaho, Middle and South forks join at Arrow Rock Reservoir, on the boundary of Boise and Elmore cos., 20 mi/32 km E of Boise, and flowing W through Lucky Peak Reservoir, through city of Boise, past Middleton and Parma, joining the Snake R., 2 mi/3.2 km S of Nyssa (Oregon), at the Oregon state line. South Fork Boise R., rises in N Camac co., 60 mi/97 km E of Boise, flows c.100 mi/161 km generally SW through Anderson Ranch Reservoir then NW to Arrow Rock Reservoir; Middle Fork Boise R., rises in NW Elmore co., Sawtooth Range, 60 mi/97 km ENE of Boise, flows c.70 mi/113 km SW, joining South Fork at Arrow Rock Reservoir. North Fork Boise R. rises in E Boise co., in Sawtooth Range, 60 mi/97 km NE of Boise, flows c.50 mi/80 km SW, joining Middle Fork 15 mi/24 km ENE of Arrow Rock Dam, forms most of boundary bet. Boise and Elmore cos. In 1811 the Boise R., originally called Reed’s R., was explored by an expedition financed by John Jacob Astor (1763–1848), an Amer. merchant. Irrigation, hydroelectric power, public water consumption and flood control are part of the Boise Project.
