From today's Mullan townsite, the Mullan Road continued southeast up Willow Creek to cross the Idaho-Montana border at today's St. Regis Pass (). It was formerly Sohon Pass, named by Mullan for artist Gustavus Sohon, whose explorations found the gap, about a mile west of Lookout Pass. After the strenuous project was completed in 1860, floods wiped out substantial stretches of the road, and the road was re-routed in 1861. Floods again damaged the road, and ultimately, no provision for maintenance was provided.