Craig Blacklock was 14 years old in 1968 when the St. Croix and Namekagon rivers were included among the initial eight U.S. waterways protected by the National Wild and Scenic Rivers Act.

Though barely a teenager, Blacklock had already logged countless miles with his father, Les, a renowned photographer and environmentalist.

The elder Blacklock was a pioneer in the art of large-format color nature photography, and his images, including those in "The Hidden Forest," which he created with pre-eminent Minnesota wilderness advocate and writer Sigurd Olson, resonated deeply with readers in Minnesota and beyond.

Intrigued as Craig Blacklock was about the subjects of his father's lenses, whether birds or bogs, lilies or lakes, he was equally fascinated by the intricacies of photography itself: How images are composed, framed and lighted.

Now, demonstrating he was indeed paying close attention during those early expeditions, and in fact in the years since has elevated the art form of nature photography to new heights, Blacklock, 63, has published, "St. Croix and Namekagon Rivers — The Enduring Gift." It's the 18th book he has authored or co-authored.

The book's arrival and a companion hourlong video with music by Stillwater's Peter Mayer coincide with this year's 50th anniversary of the National Wild and Scenic Rivers Act. The book includes an introductory essay by former Vice President Walter Mondale, who labels Blacklock's tome "a glorious book."

As a U.S. senator, Mondale helped secure passage of the National Wild and Scenic Rivers Act, which was signed by President Lyndon Johnson Oct. 2, 1968.

Blacklock's research in advance of photographing the St. Croix and Namakagon rivers was reminiscent of the preparatory work he undertook for his 1993 book, "The Lake Superior Images," during which he circumnavigated Lake Superior in a kayak.

"In 2015, I took my first exploratory paddle of the St. Croix to determine its photographic value," said Blacklock, who lives near Moose Lake, Minn. "I followed that in 2016 by paddling every mile of each of the rivers, and later revisiting areas I thought had the most photographic potential."

In places, shorelines of the two rivers are studded with statuesque white pines and sheer sandstone cliffs. Originating in northwest Wisconsin, the watery corridors are alive as well with wildlife, including sandhill cranes, swans and bald eagles. All would be prime fodder for Blacklock's 50-megapixel Canon digital cameras — Ferraris, in many ways, compared to the Model T 4-by-5 film outfits his father utilized a half-century ago.

The rivers also would prove fertile ground for drone photography. But the book's compressed production timeline required deployment of a drone pilot with more advanced aeronautic skills than Blacklock possessed. So he hired his friend and fellow nature photographer Jon Smithers to do the flying, while Blacklock directed the photography.

"Out of the 277 images in the book, about 50 were made with the drone," Blacklock said. "Drones offer opportunities to photograph from 400 feet up, or from a few feet above a river. And the technology has evolved so far in recent years that I was able to use a 21-megapixel camera with the drone. By comparison, in 2007, when I shot my book, 'Minnesota's North Shore,' I did it with a 16-megapixel camera because that was the best available at the time."

Included also in the "The Enduring Gift" are musings from people who regularly swim, paddle, boat and camp on the rivers, and in some cases live on them. One is from self-described river bum Greg Seitz of Stillwater, who summed up his love of the St. Croix, saying, "To know the river's little secrets is also to know its true grandeur."

"Inclusion in the book of comments by these river users on the importance to them of the St. Croix and Namekagon gives insight into the rivers' value," Blacklock said.

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The book's subtitle — "The Enduring Gift" — has a double meaning. One reference is to the donation to the government in 1968 by Northern States Power Co. (now Xcel Energy) of more than 20,000 acres along the Namekagon and St. Croix rivers, without which the wild and scenic designation wouldn't have been possible.

"The second reference," Blacklock said, "is to the gift we've given to ourselves by protecting these rivers."

It's doubtful such a sweeping law defending the nation's most beautiful rivers could be enacted today. Across the U.S. in the 1960s, the environmental movement gained widespread momentum, lending support to the proposal in Congress to protect the relatively pristine nature of the nation's remaining wild and scenic waterways.

It helped also that the damming of rivers for electric power was increasingly impractical economically and socially.

Idaho Sen. Frank Church and Wisconsin Sen. Gaylord Nelson played key roles along with Mondale in proposing protection for the initial group of eight rivers under the river legislation (the law today safeguards more than 200 rivers.)

Waterways in this exclusive club, in addition to the St. Croix and Namekagon (counted for the law's purpose as one), were the Clearwater (Idaho); Eleven Point (Missouri), Feather (California); Rio Grande (New Mexico originally, and later Texas); Rogue (Oregon); Salmon (Idaho); and Wolf (Wisconsin).

"The way environmentalists and people in general spoke of the need to protect these rivers was much more strident than is the case today," Blacklock said. "They realized if these rivers weren't protected, soon enough they'd look like the highly developed rivers along the East Coast — like strip cities."

At times in summer, the Namekagon can be a busy place, with rafts of canoeists and tubers adrift on its alternately lazy and swift currents. Similarly, on weekends in June, July and August on the St. Croix near Stillwater, powerboats rule.

Yet these 50 years later, the two rivers remain largely devoid of shoreline development.

What's more — as Blacklock learned during his long paddles of discovery — on weekdays, even near the Twin Cities, the St. Croix and Namekagon retain the character of true wilderness.

Mondale is among those who escape to the St. Croix whenever he can. In 1992, he and his family bought a cabin near Scandia, Minn., overlooking the river, and in his introductory to "The Enduring Gift," he professes appreciation for the getaway and its riverine setting:

"I sit on my deck and listen for the sound of the wooden canoe paddles hitting the metal gunwales in the river valley. Songbirds, sandhill cranes and trumpeter swans join with the laughter of young and old enjoying this river … 'Save our St. Croix' was a rallying cry more than a half century ago and still resonates with me today, as does the honor of playing my part in legislation to protect the St. Croix, Namekagon and so many rivers throughout our country."

Dennis Anderson • 612-673-4424