The Idaho Batholith underlay most of Boise National Forest , forming the forest 's Boise , Salmon River , and West mountain range ; the forest reach a maximum elevation of 9 @,@ 730 feet ( 2 @,@ 970 m ) on Steel Mountain . Common land cover include sagebrush steppe and spruce @-@ fir forests ; there are 9 @,@ 600 miles ( 15 @,@ 400 km ) of streams and rivers and 15 @,@ 400 acres ( 62 km2 ) of lakes and reservoirs . Boise National Forest contain 75 percent of the known populations of Sacajawea 's bitterroot , a flowering plant endemic to Idaho .