Three gauges, none of them in the Holy Waters
The number on this page comes from USGS 04136500, which sits a few hundred feet below Mio Dam and measures the release into the Trophy Water. Gates Au Sable Lodge labels it plainly on its stream-flow page as the main branch below Mio dam. The nearest gauge to the famous water above is 04136000, listed by USGS as near Red Oak but physically at Parmalee Bridge, below where the South and North branches have joined; Chris Izworski's Au Sable map notes there is no active gauge inside the Holy Waters at all. The third gauge, 04137500 near Au Sable, is down at Oscoda below Foote Dam and belongs to the steelhead reach. Wading between Grayling and Wakeley Bridge, watch the Red Oak reading, not this one.
Burton's Landing to Wakeley Bridge
Burton's Landing is where the Holy Waters begin, east of Grayling off M-72. From there to South Wakeley Bridge Road is 9.0 miles by the DNR's count, sand and gravel bottom, generally waist deep at the Louie's Landing access according to the DNR's Trout Trails sheet, and the same sheet says the reach holds wild brook trout to 12 inches and wild browns past 20. Stephan Bridge, with Gates Lodge on the road above it, is the landmark in the middle. Wakeley Bridge is the downstream line of the flies-only water and a good place to leave a car for a Stephan-to-Wakeley float.
Old Au Sable Fly Shop in Grayling describes the Holy Waters as fairly large but easy to navigate and wade. It is a different river during the Au Sable River Canoe Marathon, which starts in Grayling at 9 p.m. on a late-July Saturday and runs 120 miles to Oscoda through the night; the 2027 start is July 24.
Wakeley to Mio Pond, then the fifteen miles below the dam
Below Wakeley the river gets bigger and deeper. Old Au Sable calls wading between Wakeley and Mio Pond difficult in places and names the Whirlpool off Cherry Creek Road and Parmalee Bridge as the accesses, with Hexagenia limbata prolific through this stretch. McMasters Bridge marks the end of the artificials-only water; from there to the Rainbow Bend power line the mainstream is Type 4, and the North Branch comes in near Rainbow Bend.
Mio Dam backs the river up into Mio Pond, and what comes out the other side is the Trophy Water: from the Mio power line to Evans Road at McKinley Bridge, 15.0 miles that the shops split into two floats of roughly 7.5, Mio to Comins Flats and Comins Flats to McKinley. American Fly Fishing's Bob Linsenman writes that the DNR stocks thousands of 5-to-8-inch rainbows and browns here each spring at the Mio, Comins Flats and McKinley accesses. Michigan Streamside puts it at 100 to 150 feet wide and about 4 feet deep, and Old Au Sable says a boat is the best way to fish it. Michigan TU's account of the 2011 rule change notes the dam warms the water and trout start to suffer above about 70 degrees, which is why summer afternoons belong to the lakes and mornings to this reach.
Hendricksons at 10 a.m., Hex at midnight
Old Au Sable's hatch page names the Big Five: Hendrickson, Sulphur, Isonychia, Brown Drake and Hex. Hendricksons start sporadically after several days of water in the mid 40s and go big when it reaches the low 50s, size 12 to 14, with spinner falls that can happen in the morning or mid afternoon on a warm April or May evening. Sulphurs run late May through July in 14 to 20. Brown Drakes go late May to mid June in 8 to 12, triggered around 50 degrees, and a given stretch usually gets three to seven days of them.
Gates' chart adds the frame around those: early black stones and Blue-Winged Olives from April 1, Little Black Caddis in the last week of April, Light Cahill and Gray Drake from about May 22, Hex from June 15 to July 15, Isonychia from mid June through the end of August, Tricos from about June 22 into September, White Flies from mid August and flying ants at the end of it. Gates describes its own trip clock: a dry-fly trip leaves at 10 a.m. on May 1, at 2 p.m. by June 1, at 4 p.m. or later by the end of June with casts to rising fish at midnight, and by July 10 it is back to morning bugs.
Reading the Mio number
No shop publishes a wading range for the Mio gauge, so the 700 to 1,100 cfs band on this page is built from the USGS record itself, and it describes the Trophy Water below the dam, not Grayling. Monthly means at Mio since 1952 run about 910 cfs in January, 1,460 in April, 1,180 in May, 1,000 in June, 860 in July, 810 in August, 840 in September and 920 in October. At the Parmalee gauge upstream the same months are roughly 25 percent lower, 1,260 in April and 650 to 700 in July and August.
Flies only, artificials only, and the under-12 exception
The 2026 Michigan Fishing Regulations put the Au Sable mainstream in five different rule boxes. From Business Loop 75 in Grayling down to Burton's Landing it is Type 2: open the last Saturday in April through September 30, brook trout 10 inches, browns 12, five fish. Burton's Landing to South Wakeley Bridge Road is the Gear Restricted Holy Waters: open all year, artificial flies only, possession closed all year and zero trout, except that a child under 12 may keep one trout between 8 and 12 inches. Wakeley Bridge to McMasters Bridge Road is artificial lures only, open all year, with possession of brook and brown trout only from the last Saturday in April through September 30, two trout with no more than one 18 inches or longer, and minimums of 10 inches for brook and rainbow trout and 18 for browns.
McMasters Bridge to the Rainbow Bend power line and Mio Dam to the Mio power line are Type 4. The Trophy Water, Mio power line to Evans Road at McKinley Bridge, matches the Wakeley-to-McMasters rules except the brook trout minimum drops to 7 inches. McKinley Bridge to the 4001 Bridge is Type 2 again, Alcona Dam to the South Branch is Type 4, and from Foote Dam to Lake Huron the river is Type 3, open and harvestable all year with 15-inch brook and brown minimums and 10 inches for steelhead and salmon, with the water from Foote Dam to the downstream edge of its apron closed to fishing all year. On every gear restricted reach it is unlawful to possess live, dead or preserved bait on the water or the bank, and scent is banned on the flies-only water.