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Idaho State Highway 21: Map


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Idaho State Highway 21 is the Ponderosa Pine Scenic Byway, primarily a two-lane highway from Boisemarker to Stanleymarker. With two-thirds of its length in Boise Countymarker, it passes by historic Idaho Citymarker and the village of Lowmanmarker 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 Areamarker 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 Rivermarker and a tributary from Lowman to Banner Creek Summitmarker.

Route description

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 Airportmarker (and the 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 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, several miles below the base of the Lucky Peak Dammarker.

After the Highland Valley Summit above the Lucky Peak Dam, SH-21 ascends the Mores Creek tributary to the Boise Basin gold mining area centered around Idaho Citymarker, leaving the sagebrush to enter the Ponderosa pine forest of the Boise National Forestmarker. After Idaho City, the highway continues to climb with Mores Creek for another 13 miles (21 km) to the Mores Creek Summitmarker at 6117 feet (1864 m), and with Beaver Creek to the Beaver Creek Summit at 6041 feet (1841 m) ten miles (16 km) further. Following this summit, Highway 21 descends 2200 feet (670 m) in switchbacks to the village of Lowmanmarker at 3800 feet (1158 m) above sea level. SH-21 then ascends the South Fork of the Payette Rivermarker to Grandjean on the west side of the (Sawtooths, and climbs the Canyon Creek tributary to the Banner Creek Summitmarker at 7056 feet (2150 m), the route's maximum elevation, over 3200 vertical feet (975 m) above Lowman.

At the Banner Creek Summit, SH-21 enters Custer Countymarker and the Challis National Forestmarker, and descends northward with Banner Creek and Cape Horn Creek, tributaries of the Middle Fork of the Salmon River. At "Cape Horn" the highway turns southeast to gradually climb with Marsh Creek to the border of the Sawtooth National Recreation Areamarker, where SH-21 enters the drainage of the main Salmon River, and gently descends to Stanleymarker. About four miles (7 km) west of Stanley is the turnoff for the three mile (5 km) spur road to the photogenic Stanley Lakemarker at 6513 feet (1985 m), framed by the jagged McGown Peak at 9860 feet (3005 m).

In Stanley, SH-21 terminates at 6250 feet (1905 m) at the junction with SH-75, the Sawtooth Scenic Byway from Galena Summitmarker and Ketchummarker to the south, originating in Shoshonemarker. North of the intersection in Stanley, Highway 75 becomes the Salmon River Scenic Byway, and continues due north for a mile, then veers east with the twisty main Salmon River as it descends to Clayton, then north towards Challismarker, where the route terminates and rejoins US-93 just south of the city limits.

The upper elevations of Highway 21 are often closed during the winter months,
due to light traffic and heavy snowfall.


Portions of the road originated in the 1860s, connecting Boise with the gold mining areas near Idaho City.

Milepost log

County Location Mile Destinations Notes
Adamarker Boisemarker (southeast) - exit 57 - E. Gowen Rd. 0.00
Adamarker Warm Springs Ave. (old SH-21) 3.48
Adamarker Lucky Peak Dammarker 11.20
Adamarker Highland Valley Summit - 3782' (1153 m) 14.54
Boisemarker Idaho Citymarker 38.80
Boisemarker Mores Creek Summitmarker - 6117' (1864 m) 52.33
Boisemarker Beaver Creek Summit - 6041' (1841 m) 62.92
Boisemarker Lowmanmarker 72.16
Boisemarker Grandjean overlook 95.44
Custermarker Banner Creek Summitmarker - 7056' (2150 m) 105.52
Custermarker Stanleymarker 130.87


  1. Idaho Byways - Ponderosa Pine Scenic Byway
  2. Idaho Byways - Sawtooth Scenic Byway
  3. Idaho Byways - Salmon River Scenic Byway
  4. Idaho Transportation Department, State Highway 21 Milepoint Log

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