In northern Crawford County, where the Kollke and Bradford Creeks converge, the Au Sable River begins its 138-mile journey to Lake Huron. The Anishnaabek called the river Mudaubeebetonagnce, meaning "flowing from the interior to the lake," and French explorers called it Rivière Au Sable, or "river to sand." During Michigan's white pine logging era from 1870 to 1920, the river's current carried logs to sawmills, and similar vessels were used to supply wanigans during log drives. In the 1870s, river guide Rube Babbit and likely others began customizing flat-bottomed Au Sable Riverboat Special boats for fishing; though they weigh 300 to 400 pounds, their flat bottoms allow easy navigation with a pole or punt stick, traditional construction includes cedar or pine planks, and they are still handmade and used in the Grayling area. The Au Sable River and its tributaries cross all parts of the county and have long been known for excellent trout fishing; the main stretch between Burton's Landing and Wakeley Bridge is often called the Holy Waters and is one of the United States' premier fly-fishing destinations. First held in 1947, the 120-mile Au Sable River Canoe Marathon runs nonstop from Grayling to Oscoda at Lake Huron, beginning with competitors carrying their canoes in a foot race to the river; it is the longest nonstop canoe-only race in North America, one of the three races in the Triple Crown of Canoe Racing, and draws elite racers from North and Central America. In 1959, 16 fishermen gathered at George Griffith's home on the banks of the Au Sable River near Grayling and founded Trout Unlimited to help wild and native trout prosper in their native habitat, and the organization later grew into a leading non-profit working to conserve, protect, and restore cold water fisheries and watersheds throughout the United States.