With a single toot that reverberated through the cavernous Upper St. Anthony Falls on Monday afternoon, where minutes before 10 million gallons of water floated her 50 feet higher, the towboat Becky Sue nudged twin barges filled with 2.400 tons of scrap steel downriver.

In that same moment, Becky Sue ushered out a historic era of Minneapolis river navigation.

The commercial lock closed for good Tuesday at midnight, marking the end to 150-year-old dreams of creating a port that would rival St. Paul. But the closure now brings new hopes of transforming the city’s upper river with more parks, housing and office buildings.

“Personally, I understand why it’s being done and support the validity of that, but it feels bittersweet, poignant because it is the end of an era,” said Ann Calvert, the city’s point person on river issues for decades.

Congress mandated the shutdown in hopes of blocking the migration of invasive carp, which experts warned could tag along in the lock and then continue north and threaten sport fish populations in Minnesota’s lake country.

Lee Nelson, who said he tried and failed to negotiate a compromise between politicians and the barge industry, took Becky Sue’s helm for its final lockage. He has been a towboat captain since 1983, but he is now president of Upper River Services, which shuttles most of the barges on Twin Cities waterways.

His administrative duties keep him off the river most of the time, but he successfully nosed the 400 feet of barges strung out in front of him into the lock.

“He’s not standing up, so he’s not nervous,” quipped Tom Fleming, who piloted the rest of the trip, a journey he estimates he’s made a couple hundred times. Nelson feigned wiping his brow, then added: “Our job is to make it look easy.”

The lock never achieved the level of tonnage that advocates predicted when it opened. The lock handled just 711,000 tons of cargo in 2014, far below the 3 million tons the corps projected in the 1950s.

When the first barge cleared the locks and entered the upper harbor in 1963, it was mostly for show because there was no place to unload. The tow returned to Savage to offload its 24-inch water pipes. Tuesday’s scrap cargo came from Northern Metal Recycling’s North Side yard, bound for mills in the South.

The closure polarized advocates of revitalizing the city’s upper river and businesses that make their living on the river.

“We did something big here,” mayoral aide Peter Wagenius said Friday as a coalition gathered to celebrate the closure of the lock. “We fought on the biggest possible stage of the industrial want versus the environmental need.”

Shippers saw it differently, successfully lobbying the Corps of Engineers to keep the lock open until the last possible day allowed by Congress.

“This thing got closed under an ulterior motive,” said Nelson, whose firm operates six towboats that shuttle barges among Twin Cities area terminals. “I think the carp is an emotional hook.”

“We never believed in the Asian carp invasion. … This is a great way to shut down the barge industry,” said Bob Schmitz, who runs kayak trips in Minneapolis and St. Paul. “Our fish expert politicians made the decision.”

Schmitz ran several trips of kayakers through the lock on Tuesday, including a planned 9 p.m. run of more than 60 people who represented the last noncommercial lockage.

Between them, Nelson and Fleming have more than 60 years of river experience. They’ve watched five bridges on the Minneapolis riverfront be replaced.

They recall rushing upriver in 2007 after the collapse of the Interstate 35W bridge, supplying barges for divers to use in the search for bodies. They remember in the 1980s when the hydro plant at Lower St. Anthony dam collapsed, emptying the pool, leaving a barge high and dry at the former General Mills barge terminal. They know the tricky portions of the river, like Monkey Rudder Bend below the Ford dam, or how the power turbine at the Lower St. Anthony changes the current.

The battle over closing the lock, except for flood control purposes, has been hard-fought and dates to a 1999 plan that called for a gradual phaseout of river shipping, forcing businesses to turn to rail or trucks to haul their loads.

The lock closing is expected to add trucks to Twin Cities freeways, and increase the price of concrete by 5 to 9 percent.

It also will have unknown impacts on the flow of the river itself.

The Minneapolis River Partnership, University of Minnesota’s St. Anthony Falls Laboratory and Mississippi Water Management Organization are working to establish baselines in areas such as the riverbed’s contours, water quality and biological markers such as mussel populations, and hope to study changes for five to 10 years.

Former corps historian John Anfinson, now superintendent of the Mississippi National River & Recreation Area, said nobody has analyzed whether the cost of operating the locks was justified by the economic benefit.

“There was a baked-in inefficiency,” he said, because the three upper locks can handle only two barges at a time, compared to nine lower on the river.

It took 26 years between the congressional authorization to build the lock and the passage of those first barges. During that time, there were important transportation changes. Nationally, the interstate highway system was under construction, favoring truck transport. On a regional level, the St. Lawrence Seaway opened a competing route. Locally, another new barge route siphoned much of the grain shipping the Upper Harbor had been expected to capture. Cargill Inc. began shipping grain from a terminal in Savage.

The month before the first barge came through the lock, Life magazine excoriated the project as a “glaring example of pork.”

But it was important to the two remaining shippers, Northern Metal and Aggregate Industries, which has stockpiled sand and gravel with twice-daily runs since the shipping season opened this spring.

The scrapyard has one of the finest terminals on the river for handling barges, Nelson noted as Fleming deftly reversed the order of two barges for better handling on the downriver run.

“Nice working with you,” Fleming shouted to a trio of workers who watched as the last barges were readied. “Hopefully, they’ll reopen it some day.”