)[2][3] From its southern terminus SH-39 heads westerly as a three-lane road (with a center left-turn lane) concurrently with Interstate 86 Business (I-86 BL) along Pocatello Avenue as it immediately passes by the southwestern edge of the American Falls Airport.

SH-39/I-86 BL then turns slightly north, with Pocatello Avenue (Old Highway 30) continuing straight to enter American Falls.

(From that intersection, I-86 BL heads briefly southeast along Idaho Street before turning south–southwesterly along Fort Hall Avenue toward it western junction with I-86/US 30.

SH-39 then narrows back to a two-lane road as it curves to head north along the southwestern edge of the reservoir in agricultural land.

Roughly 2.5 miles (4.0 km) east of Riverside, SH-39 has a slight jog to the north before reaching the unincorporated community of Collins.

SH-39 then turns northeast and promptly reaches its northern terminus at a T intersection with US 26, near the Snake River and the western edge of Blackfoot.