It's late on a Tuesday morning and — as it's done every day for decades — the Patrick Gannaway towboat pushes its two barges up the Mississippi River right through downtown Minneapolis.
To get its 2,400 tons of sand, gravel, and limestone past the river's only waterfall, the barges take a five-story vertical ride inside the Upper St. Anthony Falls lock.
Deckhands squeeze everything into the narrow chamber and use a winch to take up the slack in the boat's steel cables.
In a control room above, a lock operator closes the chamber's enormous gates before opening a valve and letting in 10 million gallons of rushing water.
The towboat and its fully-loaded barges rise quickly, 49 feet in just 10 minutes. Doors at the other end of the chamber open, and the Patrick Gannaway continues its journey upriver to a concrete plant.
But that was the boat's final trip through Minneapolis. Concerns about the spread of Asian carp led Congress to mandate the permanent shutdown of the Upper St. Anthony Falls Lock & Dam. It brought an end to 52 years of commercial barge navigation on the northernmost stretch of the Mississippi River.
Environmentalists and the tourism industry are hailing the move. Christine Goepfert with the National Parks Conservation Association says the carp — which can leap into boats --- pose a big threat to the food supplies of other fish.
"They have disastrous consequences," she says. "They out-compete our native fish populations like our prized walleye. They vacuum up everything in their path. So now we know that the waters north of that lock will be protected from that threat."
Goepfert concedes that invasive carp may still migrate upriver. The fish could bypass the lock entirely if a careless boater neglects to drain ballast water, or empties leftover fishing bait in an unaffected body of water.
One thing that is certain, though, is that the lock's closure means the end of river shipping through Minneapolis. Randy Gaworksi of Aggregate Industries — which owns the Patrick Gannaway — says it will now be much harder to get those construction materials to market.
"I think anybody in the transportation business understands that the most efficient way to move material is by barge," he says.
Gaworksi says each barge trip hauls the equivalent of 110 semis, trucks that he says are now driving on already congested freeways.
The lock opened in 1963 after a century-long civic effort to make Minneapolis the starting point for Mississippi River navigation.
But historian John Anfinson says by that time, the flour milling industry that built the city was long gone, and the lock never got the expected traffic. While Asian carp may have triggered the end of shipping here, he says it's been a long time coming.
"That lock and dam wouldn't have closed had Minneapolis thought it was significant to its economy," he says.
With commercial barges gone, city planners now hope to revitalize the down-at-the-heels industrial riverfront north of St. Anthony Falls, replacing an old barge terminal with housing, businesses and a park.
Anfinson says the end of navigation presents a once-in-a-century opportunity to rethink this city's relationship with the river. It's a conversation that's opening, now that the gates of the lock have finally closed.