Grand Traverse Bay is a bay of Lake Michigan formed by the Leelanau Peninsula in the northwestern Lower Peninsula of Michigan. The bay is 32 miles (51 km) long, 10 miles (16 km) wide, and up to 620 feet (190 m) deep in spots. It is divided into two arms by the Old Mission Peninsula. The entire bay is conterminous with the Grand Traverse Bay Bottomland Preserve.