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Mackinac Trail / Carp River

Mackinac Trail / Carp River


County: Mackinac

City/Township: St. Ignace Twp.

Location: Mackinac Trail / Carp River

Year Built: 1920

About this Bridge:
Located some eleven miles north of St. Ignace, this graceful concrete arch bridge spans the Carp River on the Mackinac Trail. The crossing is situated immediately west of I-75, about a mile from the river’s mouth at St. Martin Bay. The structure is comprised of a 60-foot, filled spandrel arch, with an elliptical profile. The arch springs from massive concrete abutments, which feature battered backwalls and angled wingwalls. Cobblestone retaining walls are built above the wingwalls at the approaches. The tapered arch ring of the Carp River Bridge is corbeled from the spandrel on each side. This, and the MSHD standard guardrails, originally provided the bridge’s architectural expression. The original guardrails have since been removed, however, and the deck widened by cantilevering slightly over the spandrel walls to form concrete stringcourses. The existing guardrails are also MSHD standard, this time with ornamental steel rails and concrete bulkheads. This alteration marks the most serious loss of physical integrity for the bridge. The only other change in minor: the bridge plate that was bolted to the guardrail when the bridge was inventoried in 1982 has since been stolen.

Soon after the legislature passed the State Trunk Line Act of 1913, authorizing the formation of a trunk line system of roads, a route began to form through Mackinac and Chippewa counties that would link the Straits of Mackinac at the southern tip of the Upper Peninsula with Sault Ste. Marie on the Canadian border. Improvement work began first in 1913-1914 on the southernmost segment, from St. Ignace north to a point just beyond Rogers Park, south of the Carp River crossing. A major Upper Peninsula artery, the Mackinac Trail was extended to Sault Ste. Marie in the 1910s. Its construction entailed both roadway grading and bridge construction. One of these latter structures spanned the Carp River in Mackinac County about midway between the Straits and the Chippewa County line. For this crossing engineers of the Michigan State Highway Department delineated a 60-foot concrete arch in 1919. MSHD hired Flint contractor A.S. Decker to build the bridge. “The 60-foot-span arch over the Carp River in St. Ignace Township of Mackinac County has just been completed,” MSHD reported in the summer of 1920. Total cost for the bridge and adjacent roadway grading: $41,223.53. Later incorporated into US-2, the structure carried increasingly heavy traffic, prompting MSHD to widen its deck by five feet and install new guardrails in the 1929-1930 biennium. Decorative cobblestone retaining walls were built at the approaches at that time. This segment of the Mackinac Trail is now a county road, superseded by I-75. Still, the Carp River Bridge carries local vehicular traffic without further alteration.