Flowing generally west from the glacier, it passes south of McNeil Point, which is on the stream's right. It then flows between Yocum Ridge, which is to the left, and Bald Mountain to the right before being crossed by the Pacific Crest Trail near the eastern end of Old Maid Flat. The Sandy River enters Old Maid Flat to the left of Muddy Fork, and the two run roughly parallel across the flat until joining near Last Chance Mountain, which is on the right. The confluence is upstream of Fred McNeil Campground and Lolo Pass Road (Forest Road 18). Muddy Fork has no named tributaries.