Unlike many of Michigan's rivers, the Thunder Bay River drops considerably from its headwaters in southwestern Montmorency County to
Lake Huron. A hill near the headwaters northeast of
Lewiston is above sea level and above lake level. A former whitewater stretch northwest of Alpena, the "Long Rapids", carried the river down from the northern Michigan plateau to Thunder Bay. The rapids have since been drowned under
Lake Winyah (also known as Seven Mile Pond), a hydroelectric reservoir created by the Norway Dam (also known as Seven Mile Dam). Much of the middle and upper reaches of the Thunder Bay River flow through the
Mackinaw State Forest, which is a large swathe of northeastern Michigan that, after logging was completed in the 1910s, reverted to the state for unpaid
property taxes. The state forest contains large numbers of
aspen and
birch trees,
pulpwood trees typical of second-growth Michigan forests. The largest reservoir in the Thunder Bay River basin, the
Fletcher Pond (also called Fletcher Floodwaters) in western Alpena County, began to fill in 1932. It is noted for having the second largest population of
ospreys in the
Midwest. As of 2006, pond riparian property owners were actively trying to counter an exotic infestation of
Eurasian watermilfoil. ==Boardwalk==