Above the Lucky Peak Dam, SH-21 climbs over the Highland Valley Summit (), then descends to just above the reservoir. It then crosses the Mores Creek tributary and ascends with it to the Boise Basin gold mining area centered around Idaho City, leaving the sagebrush to enter the Ponderosa pine forest of the Boise National Forest. After Idaho City, the highway continues to climb with Mores Creek for another to the Mores Creek Summit at , and with Beaver Creek to the Beaver Creek Summit at further. Following this summit, SH-21 descends in switchbacks to the village of Lowman at above sea level.