In fly fishing circles, few trout streams in the world are any more renowned than Michigan's Au Sable River. A state-designated Natural River as well as a Department of Natural Resources-dubbed "Blue Ribbon Trout Stream," the Au Sable is not only one of the eastern United States' most legendary streams, but it also is the birth place of Trout Unlimited, a national conservation group that soon will celebrate its 50th anniversary.

Yet within the Au Sable River system, which encompasses several branches and numerous stretches of varying character, there is one portion of the river that stands out from the rest -- a stretch known as "The Holy Water." An 8.7-mile reach on the mainstream that begins just east of the city of Grayling in Crawford County, the Holy Water meanders from Burton's Landing to Wakeley Bridge and is known for its wadable water, dependable insect hatches and quality trout fishing.

The name Holy Waters was coined in the early 1970s by the late Cal Gates, owner of Gates' Au Sable Lodge, a fly fishing mecca located on the south bank of the famed stream.

"My dad always said that God created the Au Sable River for fly fishing with its fabulous trout populations, its cold clear waters, its stable flows and stream depths and bottom ideal for wading," said Cal's son and present lodge owner, Rusty Gates.

The Au Sable River is unique among all rivers in the United States in one important aspect: it has the most stable flow of any stream in the country. The sand and gravel that was deposited by the last glacial retreat -- some 11,000 years ago -- allows precipitation to soak into the soil, creating massive ground water aquifers that produce streamside springs feeding the Au Sable River and its tributaries. These springs result in a constant flow of cold, clear and clean water -- the kind of habitat that not only allows trout to survive, but to thrive.

The numerous and plentiful fly hatches on the stream just add to the Au Sable's allure. Fly hatches begin occurring as soon as the days start to warm and the winter snow begins to melt, usually in late March and early April each year.

First, the black stoneflies come off, followed by the Hendricksons, sizeable mayflies that signal to most anglers that fly fishing season has truly arrived. In rapid succession, little black caddis, and mahogany and sulphur mayflies add to the smorgasbord.

June and July bring hatches of large and plentiful bugs -- brown drakes, Isonychia (white-gloved howdies), the infamous giant Michigan mayfly (Hexagenia limbata) as well as the tiny but prolific tricos.

August through October brings on the onslaught of terrestrial insects including grasshoppers, ants, inch worms, and beetles along with a variety of species of small mayflies known collectively as "blue-winged olives" (BWOs). "The Au Sable River has one of the most prolific and diverse insect communities of any Michigan river that I know of," said Andy Partlo, manager of the Old Au Sable Fly Shop in Grayling.

But it takes more than insects to make a trout stream.

"The trout population within this reach is second to none in the state," said Steve Sendek, a Department of Natural Resources fisheries biologist who works out of Grayling. "The Au Sable River boasts a magnificent population of brown trout and brook trout with a small resident population of rainbow trout mixed in."

Few trout streams anywhere have been studied as extensively at the Au Sable. The DNR maintains detailed data sets on the Holy Water, which shows how truly unique it is.

Population studies at Stephan Bridge -- about the midway point on the Holy Water -- show a long-term average of well more than 100 pounds of trout per acre. That's more than double the statewide median of 51 pounds per acre.

Unlike many streams in Michigan (or elsewhere), the Holy Water of the Au Sable is completely dependent on natural reproduction.

"We have not stocked that section of the Au Sable since 1965," Sendek said. "It is a totally self-sustaining trout population -- brook, brown and rainbow. Combined with our current no-kill, year-round flies-only fishing regulations, we're managing for quality."

Although the Au Sable's fish habitat is very good, Sendek said, it isn't as good as it could be. This summer and next, the DNR will take advantage of federal fisheries grants (from the Sport Fish Restoration Act) to improve the river by adding large woody debris in the Holy Water. The DNR will spend $400,000 over the next two years on this effort.

"We still haven't fully healed from the damage done during the logging era at the turn of the 20th century," Sendek said. "It takes a stream a long time to recover from being completely logged over. But with the maturation of the surrounding forests and the addition of whole trees into the river, we're getting there."