Chase Bridge, where the walk starts
Park on the south side of Chase Bridge. The DNR's site sheet for it describes ample parking, a canoe and riverboat launch and a pit toilet, reached from Roscommon by taking M-18 northeast 2.5 miles to Chase Bridge Road and turning left for 2 miles, or from Grayling by driving 9.2 miles east on M-72 and following Chase Bridge Road south for 7.5 miles. The same sheet warns that the land upstream of the bridge is mostly private, so above it you stay in the river.
Downstream is the reason to come. Below Chase Bridge the South Branch enters the Mason Tract, and the Mason Tract Pathway follows the river north toward M-72 so you can walk in, fish, and walk out. Old Au Sable Fly Shop sums up the branch as flowing north past Roscommon to meet the mainstream a short way below Wakeley Bridge, with brook, brown and some rainbow trout.
George Mason's 1954 gift and what it bought
The DNR describes the Mason Tract as a 4,493-acre special management area built around a 1,500-acre gift from the George Mason family in 1954, later enlarged through land exchanges with the Forest Service and private owners. The gift came with conditions: a permanent game preserve, never sold or developed, and no camping for 25 years. The state kept the camping ban, so the only campsites are at Canoe Harbor State Forest Campground at the north end of the tract. The Mason family built the small Mason Chapel on the river in 1960 as a place for fishermen.
What that means on the water is miles of river with no cabins and few roads. Access points inside the tract are two-tracks and small lots: the DNR's Lelands Lot sheet describes room for two or three vehicles about 2.2 miles down South Branch Road from M-72, past Canoe Harbor, and Lower High Banks is 4.1 miles down the same road, bearing left onto a two-track past the west entrance to Durant's Castle, then just under a mile to a sandy, narrow lot.
Chase Bridge to Lower High Banks: zero trout
The 2026 Michigan Fishing Regulations split the river at Lower High Banks, T25N R2W Section 13. From Chase Bridge down to it, 4.3 miles, the South Branch is open to fishing all year with artificial flies only and a possession limit of zero trout; the only exception lets children under 12 keep one trout between 8 and 12 inches. From Lower High Banks to the confluence with the mainstream, 11.5 miles, it stays flies-only and open all year, but you may keep 2 trout, no more than one of them 18 inches or larger, with a 10-inch minimum on brook and rainbow trout and 18 inches on browns. Brook and brown trout may only be possessed from the last Saturday in April through September 30; rainbows year-round.
In practice nearly everyone releases everything on both reaches. The DNR's site sheets say brook trout up to ten inches are common throughout, and brown trout reach 18 inches or larger at Chase Bridge and twenty inches or larger at Lelands Lot and Lower High Banks.
Daisy Bend, the Castle and the other deep spots
Gates Au Sable Lodge breaks the river into eleven cleanup beats every September, and the list doubles as a map of the named water: Chase to Forest Rest, Forest Rest to Daisy Bend, Daisy Bend to Castle, Castle to Highbanks, Highbanks to Lower Highbanks, then Icebox, Baldwins, Downeys, Dogtown, Canoe Harbor and finally Smith Bridge. The substrate at the DNR sites is cobble, gravel and sand.
Wading here is not casual. Gates' own line from May 2025 was that the South Branch is wadeable if you already know it, and in September 2026, at 250 cfs, the lodge called the river perfect for fishing but flagged Daisy Bend, the Castle, the water below Lower High Banks and the stretch above Canoe Harbor as places to take care. A first trip is best done with someone who has walked it, or by floating from Chase Bridge.
Brown drakes, yellow sallies and noon ants
Gates' dated reports give the South Branch its own calendar. In May 2026 the river ran in the mid-sixties with a few March Browns showing, and a week later the lodge expected stronger Sulphur hatches with the drakes to follow. In June 2025 the brown drakes had pushed through the South Branch by the middle of the month. The June 2026 report credits yellow stoneflies fished upstream through the riffles on a long leader to 5X for one of the writer's best South Branch evenings in years, with Isonychia arriving soon after. In early September 2026 the same writer hit a rare noon flight of flying ants on this branch.
The Gates hatch chart for the Au Sable system and the Fly Shack's Au Sable chart fill in the rest and agree closely: Hendricksons from late April into May, Sulphurs and Brown Drakes from late May into June, Tricos in late summer and a fall run of small olives. Gates also charts Hex from mid June into July; the Fly Shack chart leaves it off, and no report says it is heavy on this branch, so treat it as a bonus rather than a plan. Gates also notes that its dry-dropper technique works least well on the South Branch.
The Smith Bridge gauge and what 250 means
USGS 04135700 sits where Old M-72 crosses the river near Smith Bridge, the bottom of Gates' last beat, and measures 401 square miles of drainage with a record from 1966. Gates links it as the South Branch at Smith Bridge. Monthly means over that record are about 258 cfs in March, 384 in April, 285 in May, 208 in June, 154 in July, 137 in August, 153 in September and 192 in October.
No shop publishes a formal range, so the 130 to 260 cfs band here pairs the summer lows in the USGS record with what Gates has written: 250 cfs perfect for fishing with care in the deep bends, 260 in a steady July, the mid-300s manageable in May, and over 400 cfs leaving little for the wader. At the low end the river is easy to cross but the fish in the flats are hard to approach.