South Branch Au Sable River

The South Branch is the wild, walkable half of the Au Sable's holy trinity of trout branches, and for a lot of people who fish this system it's the one they keep coming back to. It runs north out of the Roscommon lowlands through the George Mason Tract — roughly 14 miles of protected, undeveloped public frontage that auto executive George Mason bought up through the 1930s and willed to the state on his death in 1954. The crown of it is the river itself: a gravel-and-sand bottom, cedar sweepers and sunken logs, and a flies-only catch-and-release run from Chase Bridge down to Lower High Banks that holds wild brown and brook trout and gets a fraction of the foot traffic the mainstem Holy Water sees in Hex week. A fieldstone chapel sits in the woods along the bank, and the Mason Tract Pathway shadows the river the whole way, so you can walk in, fish a bend, and walk to the next one.

It fishes small and intimate up top and opens into slow, sandy float water below. Through the tract you can wade the gravel bars in most flows — summer baseflow sits around 200 CFS at the Luzerne gauge — but the river is a tangle of soft sand, deep undercut bends, and logjams, so it wades harder than it looks, and a lot of the best water is reached on foot from the pathway or floated in an Au Sable riverboat. Because it's groundwater-fed it doesn't blow out with rain the way a freestone does; the honest limiter here is summer heat and low water — the gauge can push toward the low 70s°F in a heat wave, and that's when you back off and fish the cool ends of the day. The signature event is the Hexagenia limbata hatch, the giant Hex mayfly that comes off after dark from mid-June into early July, and the South Branch is the place to fish it if you want the big-fish drama without the mainstem crowd. Bring a headlamp, fish by ear, and expect to hook the biggest browns in the river in total darkness.

The trade-offs are real: the upper tract is narrow and buggy, the sand can be grabby, and the Hex is a night game that not everyone loves. Access is genuinely public and generous — Chase Bridge, Canoe Harbor, Downey's, and the High Banks all have parking and trail access — but there are no services on the river itself. You stage out of Grayling (about 25 minutes north) or Roscommon to the south, and the fly shops that stock this water are the same historic Grayling shops that serve the mainstem. If the South Branch is warm or you want a change, the North Branch, the mainstem Holy Water, and the trophy tailwater below Mio Dam are all short drives.