I think they mean it's the furthest inland you can travel from the ocean/sea just by boat (and from the title, maybe specifically steamboats). You can take the Mississippi up from New Orleans to St Louis and then the Missouri goes all the way up to this part of Montana.
Josef_The_Red t1_j13929k wrote
Reply to comment by Obiwan_Salami in When this bridge in Fort Benton, Montana, USA was built 1888 it was required to have a swing span to allow steamboats to navigate. It was considered the furthest navigable point on Earth, more than 2,700 miles from the Gulf of Mexico. by triviafrenzy
I think they mean it's the furthest inland you can travel from the ocean/sea just by boat (and from the title, maybe specifically steamboats). You can take the Mississippi up from New Orleans to St Louis and then the Missouri goes all the way up to this part of Montana.