The ferry travels across the Lincoln County line to enter Ferry County, named after Elisha P. Ferry, the first Washington governor, southwest of Keller in the Colville Indian Reservation. Traveling northeast, between the Sanpoil River and the southern end of the Okanagan Highlands, SR 21 passes Keller and continues inland into the Columbia Mountains on the banks of the river. The river forms a canyon that the highway passes through and eventually both the river and road leave the Colville Indian Reservation. Shortly after leaving the reservation, the roadway enters Republic and becomes concurrent with SR 20 for before exiting the concurrency and Republic. Between Republic and the Canada-US border, an estimated daily average of 1,600 motorists used this segment of SR 21, making this section the busiest. The daily average has declined since 2006 and 2007, when a daily average of 1,700 motorists utilized the segment. Northeast of Republic, the highway passes Curlew Lake, the Curlew Lake State Park and the communities of Malo and Curlew. After passing through more dense forests, the roadway enters Danville, where SR 21 crosses the Canada-US border into British Columbia as Highway 41 (BC 41). BC 41 continues north to end at BC 3 southwest of Grand Forks, BC.