Idaho State Highway 21
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| Route information | |
| Maintained by ITD | |
| Length: | 130.869 mi (210.613 km) |
| Tourist routes: |
 Ponderosa Pine Scenic Byway |
| Major junctions | |
| South end: |
 I-84 in Boise at Exit 57 |
| North end: |
 SH 75 in Stanley
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| Highway system | |
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Idaho State Highway 21 is the Ponderosa Pine Scenic Byway, primarily a two-lane highway from Boise to Stanley. With two-thirds of its length in Boise County, it passes by historic Idaho City and the village of Lowman to the western edge of the Sawtooth Mountains, then along their northern boundary to Stanley.
The road is designated as one of Idaho's scenic byways and provides access to Sawtooth National Recreation Area from Boise and the Treasure Valley. It primarily follows the Boise River and its tributary Mores Creek to the Boise Basin and beyond, and then the upper South Fork of the Payette River and a tributary from Lowman to Banner Creek Summit.
State Highway 21 begins at exit 57 of Interstate 84 southeast of Boise, exactly 3000 feet (914 m) above sea level. The highway is an eastern extension of Gowen Road, which runs along the southern and western perimeter of the Boise Airport (and the Air National Guard's Gowen Field) to connect with Orchard Road at exit 52 of I-84.
After three miles (5 km) eastbound from I-84, Highway 21 descends and crosses the Boise River to connect with Warm Springs Avenue, old SH-21, starting in east Boise. The highway heads upstream, briefly as a four-lane divided highway, along the north bank of the river in a basalt canyon. The route passes the century-old Boise River Diversion Dam and several miles later the Lucky Peak Dam, completed 62 years ago in 1955.
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