Three Swing Bridges over Saginaw River in Bay City, MI

Going downstream:
1890-1976 Lost/Third Street: (Bridge Hunter; Satellite)
1905 CM/MC + CN/GTW: (Bridge Hunter; Historic Bridges; Satellite) CM = Central Michigan
1892 LSRC/Detroit & Mackinac: (Bridge Hunter; Historic Bridges; Satellite) LSRC= Lake State Railway

You can tell that shipping is active on the Saginaw River because the railroad bridges are normally open.

Lost Third Street Bridge

JA Garfield posted two images with the comment: "Third Street Bridge 1889-1976, Bay City, Michigan. It opened in the middle of the night for a ship, and collapsed. Drawing by Scott Shaver."

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JA Garfield posted seven photos with the comment:

More pictures of the Third Street Bridge, over the Saginaw River, that connected the east and west side of Bay City, Michigan. It was built in 1872, and collapsed June 18, 1976 as it opened for a ship at 3:10am. It had been struck by another ship passing trough the day before, which probably did structural damage.

Dan Mathers What is happening to the bridge today?
JA Garfield Dan Mathers Unfortunately they pulled it down, and built a typical DOT type drawbridge downstream to replace this, and another two lane swing bridge in town. Both new bridges have been nothing but trouble, and are down for repairs, more than they are open to traffic.
[I could not find a two lane swing bridge.]
JA Garfield When I was a kid, when we would hear a ship blowing for the bridge downstream, we would ride our bikes to this bridge, and ride the span when it swung open. We'd get a great close up view of the ships as they passed through a couple feet away.

[Bridge Hunter has newspaper clipping concerning the collapse.]

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JA Garfield posted three photos.

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Central Michigan/Michigan Central and Canadian National/Grand Trunk Western

Also known as the Bay City Bridge.

Street View

C Hanchey Flickr, Jun 2012, License: Creative Commons Attribution-NonCommercial (CC BY-NC) via Bridge Hunter
Central Michigan Railway Saginaw River Bridge (Bay City, Michigan)
Through truss swing bridge on the Central Michigan Railway over the Saginaw River in Bay City, Michigan.

Fred Sibert, Aug 2019
[This is obviously the MC Swing Bridge. It was mislabeled the Liberty Bridge. The Liberty Bridge is on the right in this photo.]

Marty Benard shared
Central Michigan RR Excursion with U23B 8904 built as Missouri Pacific 669 in Jan. 1973 crossing the Saginaw River in Michigan in 1992, Karl Miller photos.

Dennis DeBruler commented on Marty's share
Judging from photos and satellite images, this bridge is normally open.
https://bridgehunter.com/mi/bay/bh52118/
I found a Jun 2016 Google Earth that shows the bridge operating. It is almost in the closed position.

Lake State Railway/Detroit & Mackinaw

Street View

JA Garfield commented on a post
It was abandoned for many years and has recently been rehabilitated for use by the Lake State Railway, which runs on some of the old Michigan Central, and New York Central tracks in mid Michigan. A picture from September [2018]:

2 of 5 photos posted by Ben VanOchten with the comment: "The Dorothy Ann / Pathfinder was back in town over the weekend. She headed to Saginaw and then headed outbound towards Stoneport. Got some neat pictures of her going through the train bridge!"

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I found several piles of aggregates on both sides of the river in Saginaw, MI. 

Satellite

Todd Shorkey posted
John J. Boland inbound on the Saginaw River at the Lake State Railway Bridge, headed to the GM dock in Saginaw to unload coal. Olive L. Moore/Menominee is in the background, tied up at the Port Fisher Terminals Dock in Bay City. October 1, 2022
[The GM plant is a "metal casting operation." (How many aluminum castings does an eletric car need?) Casting needs a lot of sand. That plant used to have its own slip. But now it must share the "GM Stone Dock" [boatnerd] that is across the river. A satellite image shows that all of the piles are shades of grey. But coal is black. In this day and age of closing power plants, who would be trucking coal from this dock?]