Launched a mere seven years ago on the banks of a quiet Mississippi River channel in Winona, the Minnesota Marine Art Museum always hinted that it wanted to be more than just a small-town showcase for seascapes.

That ambition took full sail this weekend with the private unveiling Sunday of seven new pictures by marquee names — including a $4.5 million painting by J.M.W. Turner and a rare Gauguin — plus a $1.1 million gallery in which to show them.

The new works join what is already a blockbuster collection of Monets, Cézannes and a Van Gogh. Leading art scholars are finding their way to Winona and declaring the improbably grand collection “world-class.”

“It’s amazing,” said Patrick Noon, paintings curator at the Minneapolis Institute of Arts. “They’ve built this really great collection with basically their own vision and learned expertise. Their collection is small but growing every year, and it’s becoming a very important museum in this region.”

The new gallery and paintings will open to the public on Sept. 29.

The paintings are on loan from the collection of the museum’s chief backers, Mary Burrichter and her husband, Robert “Bob” Kierlin, whose $700 million fortune derives from Fastenal, his $15 billion Winona-based hardware-supply company that Bloomberg BusinessWeek last year called “the top-performing stock since the crash of ‘87.”

Typically the couple buys quietly from art dealers and private collectors and the prices remain private, which they prefer. Auctions, however, are a matter of public record as was the case of the Turner, a famous 1841 watercolor called “Heidelberg With a Rainbow,” purchased for $4.5 million in January at Sotheby’s in New York.

On Saturday, the day before the latest unveiling, Kierlin — a former Minnesota state senator — emphasized the public intentions behind their acquisitions. The couple has already given many pieces of art to the MMAM and work closely with the museum’s five-person staff to develop exhibitions.

“It’s something we’re having fun doing,” Kierlin said. “It’s good for the community and a way to share success.”

As for the eye-popping price, “good art does not come cheaply,” said Burrichter, who is finance director for the city of Winona.

The fact that the Turner painting is a legend among the British artist’s work guaranteed attention. Though a mere 14 inches tall and about 21 inches wide, it is large for a watercolor and in splendid condition, its delicate tints still fresh after more than 170 years. It is a romantic scene of washerwomen and townspeople on a riverbank opposite a distant town dominated by a castle wrapped in a rainbowed mist.

Price aside, it is the quality of the Kierlin-Burrichter art collection, which mostly hangs at the museum now, that is garnering attention. The couple began buying art about 13 years ago when they married, moved into a new Winona house, and had big white walls to fill. They started with traditional marine art.

Soon their collection outgrew their home, and they enlisted the help of other Winonans in launching the MMAM, which now has a staff of five, an annual budget of $650,000 and about 1,300 members. The museum has its own collection, but considers Kierlin and Burrichter to be “collecting partners,” said Andy Maus, executive director.

As the couple’s collection expanded, so did their definition of marine art.

“Our theme now is that a painting has to have a body of water big enough to float a boat,” Burrichter said.

Their collection includes stellar examples of 19th-century American paintings from the Hudson River School and Luminist group and important Impressionist and 20th-century European pictures.

Their top-notch American pictures alone are a who’s who of talent, starting with Thomas Cole and his compatriots Frederic Church, Jasper Cropsey, Winslow Homer and Thomas Moran and such later luminaries as Andrew Wyeth and his son Jamie.

Their best European holdings are similarly distinguished, with two striking water vistas by Claude Monet, a major J.B.C. Corot, an unusual Cézanne river scene, and a historic Van Gogh, a moody 1882 seascape that was one of the Dutch artists first finished paintings. The Van Gogh museum in Amsterdam wanted to borrow the latter for a recent exhibition, but the MMAM turned down the request so that the picture can be featured in the new gallery.

A second Turner, a Gauguin

The museum in Winona will remain open for the next two weeks but the Masterworks collection, which contains the major American and European pictures, will be closed. It will reopen in two galleries Sept. 29.

The pictures unveiled Sunday include a second Turner watercolor depicting Hampton Court Palace near London; Gauguin’s “Still Life with Onions” (whose water connection is a Japanese print tacked to the wall behind the vegetables); and a lively 1905 watercolor of nude bathers by André Derain.

The two American paintings are a prime landscape by Asher B. Durand and a stunning 1868 view of the Mississippi from a bluff near Winona by Alfred Thompson Bricher.

“The paintings are fantastic,” said Annette Blaugrund, former director of the National Academy Museum in New York and an expert on 19th- and early 20th-century European and American art. After meeting the collectors and seeing their art, she agreed to edit a book about the collection and rounded up 10 leading scholars to write on specific pictures.

“They are world-class and I think it [the museum] is going to be a surprise to anyone,” said Blaugrund.

Meanwhile, Burrichter and Kierlin continue having a ball as they seek out masterpieces for Winona.

“We take vacations where we visit art museums and Fastenal stores,” said Kierlin. This summer alone they visited 6 art museums, 19 Fastenal stores and drove 3,000 miles.