M-13 is a north–south state trunkline highway that runs through the Saginaw Bay region of the US state of Michigan. It runs from Interstate 69 (I-69) south of Lennon to US Highway 23 (US 23) near Standish. The southern section of the trunkline runs along a pair of county lines in a rural area dominated by farm fields. The highway directly connects the downtown areas of both Saginaw and Bay City. North of the latter city, the Lake Huron Circle Tour follows M-13 along the Saginaw Bay. The original 1919 version of M-13 ran along the west side of the state, roughly replaced by what is now US 131 in 1926. The current highway was first designated by 1932 with the M-13 moniker. After a series of extensions in the 1930s, the highway ran from Lennon to Saginaw. Another set of expansions in the 1960s added roadways that previously carried US 23 all the way north to the Standish area. A related section of highway is numbered Connector M-13, a connector route that was once part of US 23 in the Kawkawlin area. Route description M-13 starts at its junction with I-69 near the village of Lennon along the Genesee–Shiawassee county line. The highway follows County Line Road north into the village where it follows Sheridan Avenue through town, including a crossing of the Huron and Eastern Railway North of Lennon, the trunkline intersects M-21 and continues along the county line through fields. Northeast of New Lothrop, M-13 moves from the Genesee–Shiawassee county line to the Genesee–Saginaw county line. Near Montrose, the highway intersects M-57 before passing into woodlands while crossing fully into Saginaw County. M-13 angles to the northwest briefly before returning to a due north course along East Road. The highway crosses the Flint River in another area of farm fields. As the road approaches Saginaw, it crosses the Cass River and runs to the east of it as it merges with the Shiawassee River and the Tittabawassee River to form the Saginaw River. M-13 follows Washington Avenue in the city of Saginaw inland from, but parallel to, the Saginaw River through downtown. The trunkline intersects M-46, which follows Rust Avenue, and passes Hoyt Park. Washington Avenue runs by the campus of St. Mary's of Michigan Medical Center. As the highway passes out of downtown, it crosses or runs alongside lines of the Saginaw Bay Southern and Lake