I was on the Mississippi Riverboat and we approached this beside Pocahontas Indian Village.
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Looks like it's this:
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When the water around BTM was frozen, this was serving as a bridge from the "mainland" to the island...
It's an emergency bridge which is used to evacuate BTM whenever the tunnels aren't accessible (e.g. a fire or flooding in the tunnels). Previously this role fell to the Keelboats. When the Rivers freeze as seen above it's put in place as a precaution, shutting down any river traffic which might otherwise be able to beak through thin layers of ice.