"There are approximately 450 miles of groomed snowmobile trails within the Proclamation Boundaries of the Ottawa National Forest. These trails are maintained through a cooperative effort between the Michigan Department of Natural Resources, numerous local snowmobile clubs and the U.S. Forest Service. Generally, the snowmobile season runs for 16 weeks, from December 1st through March 30th on groomed snowmobile trails and un-plowed Ottawa National Forest Roads. After March 1st, each spring, the area of the Ottawa National Forest, which lies south of M-28 and east of Highway M-64, is closed to cross-country snowmobile use to prevent disturbance of nesting eagles. Snowmobiles are permitted to use any designated trails, or Forest Service roads within this area, which have not been snowplowed, signed, gated, or otherwise closed to such use. The remainder of the forest (north of M-28 and west of M-64) is open to cross-country, designated trail use, or operation on unplowed Forest Service roads which have not been closed to such use. Snowmobiling is prohibited in Federally Designated Wildernesses (McCormick, Sturgeon River Gorge and Sylvania Wildernesses) and is permitted only on designated trails within Semi-primitive Non-motorized Management Areas and Wild Segments of Federally Designated Wild and Scenic Rivers Corridors."
http://www.fs.usda.gov/activity/ottawa/recreation/wintersports/?recid=12303&actid=92