North of Pentwater, US 31 crosses into Mason County and passes Bass Lake and the Ludington Pumped Storage Power Plant, which uses a reservoir next to the freeway to generate electricity. Just north of that reservoir, the freeway turns to the northeast and Ludington's business spur runs off to the northwest. US 31 curves around the east side of Ludington, crossing the Pere Marquette River. Due east of downtown, the freeway ends and US 31 turns east to merge with US 10. The concurrent highways follow a four-lane roadway to Scottville. On the west side of that town, US 31 separates from US 10, turning northward to bypass Scottville.