 A notoriously hilly section of the highway at Maple Creek was replaced by the Knapp's Hill Tunnel, which opened in 1936.The development of hydroelectric dams on the Columbia River began in the 1930s and expanded after the end of World War II.The planned reservoir created by the Rocky Reach Dam, located near Orondo, would inundate of shoreline, including the city of Entiat and sections of PSH 10 and the adjacent Great Northern Railway.