The Au Sable River in the northern Lower Peninsula of Michigan is the most important trout river in the Midwest and one of the most significant in the country. It's the home of the Holy Water, the Au Sable riverboat, the Hex hatch, and a tradition of fly fishing that is as deep and well-developed as anything the Catskills or Montana can claim. The river that flows through Grayling is where Michigan fly fishing was invented, and the best of it is still being fished.
The River System
The Au Sable is a groundwater-fed system. Unlike the Great Lakes rivers that run clear only in summer, the Au Sable — fed by the sandy glacial aquifer of the northern Michigan plateau — runs cold (52–58°F) and clear year-round. This makes it a spring-creek-like environment despite its size: a medium-width freestone river with exceptional water clarity and consistent temperatures that support both wild trout and the dense invertebrate communities those trout depend on.
The river has three distinct branches that converge in the lower system:
Main Stem — the primary trout water, running from its headwaters near Grayling through the Holy Water and the broader river below. The main stem carries the most history and the most pressure.
North Branch — flowing south through Kalkaska County, the North Branch is narrower and more intimate than the main stem, with excellent wild brook trout in its upper reaches and wild brown trout through its middle sections. Less pressure than the main stem and more technical fishing.
South Branch — the most remote of the three branches, flowing north through Crawford County. The South Branch is largely undeveloped — state forest land on both banks — and has wild brook and brown trout in a setting that requires commitment to reach and fish.
Species: Wild brown trout (dominant throughout), wild brook trout (upper North Branch and South Branch), rainbow trout. The Holy Water no-kill section has exceptional brown trout density — some of the highest counts on any Michigan river. The branches also have documented brook trout populations in a state that has lost many of its brook trout streams to warming and non-native competition.
The Holy Water
The Holy Water is the informal name for the catch-and-release, artificial-lures-only section of the Au Sable main stem near Grayling — running from Burton's Landing downstream several miles. This is the most famous and most pressured trout water in Michigan, and justifiably so. The fish density is exceptional, the hatch diversity is outstanding (Hendricksons, sulphurs, Brown Drakes, Hex, caddis, and more across the season), and the tradition of fishing here runs through multiple generations of Michigan fly fishers.
The Holy Water is best accessed by Au Sable riverboat — the low, flat-bottomed boats that are unique to this river system. The riverboats are poled by guides who stand at the stern with a long push pole, drifting the boat silently downstream while anglers fish from a seated position. The design was developed specifically for this river. Night fishing from a riverboat during the Hex hatch is the quintessential Au Sable experience.
Wading the Holy Water is possible — and some of the best dry fly fishing happens from the bank and from waded positions in specific sections — but the Au Sable riverboat tradition is worth experiencing at least once. Book a guided float several months in advance for the Hex period; the best guides fill quickly.
The Hatch Calendar
Opening through April: midges and early stoneflies
mid-April opening day
Michigan's trout season opens the last Saturday in April on most inland streams. The Au Sable opens with midges active and the Hendrickson hatch beginning in the last days of April on warm afternoons. Opening day on the Holy Water draws a crowd — for a more solitary experience, fish the branches or the main stem sections away from the primary access points.
Hendrickson: late April – early May
the first major hatch
The Hendrickson (Ephemerella subvaria) is the hatch that wakes the river after winter. It emerges mid-afternoon on warm spring days, bringing brown trout to the surface for the first dry fly feeding of the season. On the Au Sable, the Hendrickson signals that the real season has begun — it's celebrated the way the Green Drake is celebrated on Penns Creek.
Sulphurs and Brown Drake: May – June
the most reliable hatches
Sulphurs run through May and into June on the Au Sable — evening hatches that bring consistent surface feeding and are the most sustained productive dry fly window of the year. The Brown Drake (not to be confused with the Green Drake) emerges in late May, triggering explosive evening feeding that precedes the Hex season.
Hex hatch: mid-June – mid-July
the headline event
The Hexagenia limbata hatch on the Au Sable is covered in its own guide — see The Hex Hatch: Michigan's Greatest Night on the Water for the full breakdown. The short version: late June to early July, after dark, the largest mayfly in North America emerges and the largest brown trout in the river feed at the surface until midnight. It's the reason to plan a trip specifically around timing.
Caddis through summer: July – September
consistent evening action
After the Hex fades, caddis dominate the summer evenings on the Au Sable through August and into September. The Elk Hair Caddis in tan and olive, #14–#16, is the standard fly from July onward. Morning Trico spinner falls on the flat water of the Holy Water are productive in August for those willing to be on the water at dawn.
The Au Sable's Conservation History
The Au Sable was the river where Trout Unlimited was founded in 1959 — a response to Michigan's stocking-focused management approach and what a small group of Grayling-area anglers believed was a systematic degradation of wild trout habitat. The founding meeting happened in Grayling. The organization that grew from it now has chapters in all 50 states and has driven stream restoration work across the country.
That origin matters for understanding the river's management today. The Au Sable's no-kill sections, wild-trout focus, and restrictive artificial-only rules are the product of decades of advocacy by TU members who fished this water. The river fishes as well as it does in part because of what those anglers built in the 1950s and 1960s.
The Branches: Beyond the Holy Water
The North Branch and South Branch of the Au Sable are worth a trip separate from the main stem. The North Branch flows through the Kalkaska area with numerous public access sites and a wild brook trout population in the upper reaches that has survived and even recovered in recent years. The South Branch — accessible through Roscommon County and the Au Sable State Forest — requires more effort but delivers genuine wilderness-style brook trout fishing on water that sees a fraction of the Holy Water's pressure.
For anglers who value solitude and wild fish over setting and tradition, the branches often produce the best experiences the Au Sable system offers. See the Michigan regulations guide for current season dates, no-kill section boundaries, and the special rules that apply to the North Branch catch-and-release sections.