More than four decades after it became law, a little-known federal act safeguards hundreds of primordial waterways.

This story appears in the November 2011 issue of National Geographic magazine.

The Middle Fork of the Salmon is not so much a river as an exuberant expression of water at play. It tumbles and turns and trips over itself for a hundred miles through the largest unbroken wilderness in the lower 48, the 2.3-million-acre Frank Church–River of No Return Wilderness, named for the pristine Salmon River gorge and the Idaho senator who made sure most of its vast watershed would stay that way. No dams temper its flow. No roads line its banks. It dances down its canyon much as it has since the glaciers receded 10,000 years ago—in spring as a raging, tree-felling torrent, in late summer as a spare, crystalline rivulet.

Today it is one of the ultimate white-water experiences in the United States, drawing thousands of visitors each year. But 60 years ago its future—and that of hundreds of other rivers across the country—looked very different. For much of the 20th century, the federal government seemed determined to dam virtually all the major rivers in the country, harnessing their power for electricity, irrigation, navigation, water supply, and flood control. The dam binge was particularly acute in the arid West, where even the Grand Canyon was slated for flooding. The Army Corps of Engineers evaluated five prospective dam sites on the Middle Fork alone. The river would have morphed into a chain of man-made lakes if two brothers hadn't helped stem the tide of concrete.

John Craighead, now 95, is legendary in the field of wildlife biology, famous with his twin brother, the late Frank Craighead, for pioneering studies of grizzly bears in Yellowstone National Park and for numerous articles and documentaries published by National Geographic. Their groundbreaking work inspired efforts to save the species from extinction in the lower 48. Yet the proudest achievement of John Craighead's long and storied life, he says, is the passage of the Wild and Scenic Rivers Act.

It took a decade of reports, lectures, and political wrangling, but when President Lyndon Johnson signed the Wild and Scenic Rivers Act in 1968, much of its language came from the Craigheads. The initial act spared eight rivers and narrow buffer zones around them from dams and development. Today the list has grown to more than 200 rivers in 39 states and Puerto Rico.

Craighead's memory fades in and out these days, but if you ask him which river inspired him most, his answer is quick and clear: the Middle Fork of the Salmon. My son, Sam, and I were headed there, but we'd stopped to visit Craighead at his Missoula, Montana, home on our way out to paddle that river. Before we left, Craighead gave Sam a dozen spider imitations tied just for the Middle Fork's native cutthroat trout. "You know, you can't buy that fly in a store," he said, as he shook Sam's hand and gave him a knowing smile.

It took two attempts before our back­country pilot could penetrate the fog nestled in the deep valleys of the Frank Church, whose endless ridges bearded with whitebark pine keep the modern world at bay. But by midday our party of 20 was gathered by the roaring river to listen to Diana Yupe, a Shoshone-Bannock archaeologist, tell us about her people. The Sheep Eaters lived in the river corridor for thousands of years before the U.S. Cavalry drove them out. She asked us to respect the old campsites that occupy nearly every river terrace, as well as the many pictographs, including child-size red handprints, that adorn the canyon walls. Then she sent us off with a Shoshone blessing for safe travel on the river and a safe journey through life.

The day was raw and gray, the big, dry rafts inviting. Sam nonetheless picked a pair of inflatable kayaks, because nothing makes you feel more 11 than bouncing down a river in an oversize inner tube. He'd never been in white water before, and he soon discovered that paddling the little kayaks, called duckies, was hard work. We struggled with headwinds, grounded on rocks, and paddled hard to keep up with the rafts. Yet tired as we were, Sam came off the river almost skipping.

That night the Milky Way choked the sky, and we couldn't find the Big Dipper in the twinkling throng. Sam turned in early, so I went down to the water to listen to the river's simple symphony. Something splashed at my feet, and when I flicked on my headlamp, I beheld a tiny fish darting around the shallows: a native chinook salmon, offspring of the big shadows we'd seen lurking in the deeper pools. Chinook fed the Sheep Eaters for millennia. Once tens of thousands of them came to spawn annually in the Middle Fork; now, eight major dams on the Snake and Columbia Rivers have exacted a toll on the fish in their 900-mile journey to the sea—one of the greatest migrations in nature.

A wild and scenic river designation is no guarantee that a river will remain truly wild. In fact, several of the nation's most cherished waterways have landed on the annual Most Endangered Rivers list produced by the advocacy group American Rivers. They include southern Oregon's Chetco, where gold miners plan to suction-dredge some of the best salmon spawning grounds in the state. Maine's legendary Allagash, the river that taught Henry David Thoreau the meaning of wilderness, has long been mired in controversy over bridges and additional access points in its protected corridor. And former Vice President Walter Mondale, a co-sponsor of the Wild and Scenic Rivers Act, says of the treasured St. Croix, which runs by his Minnesota summer home: "If this river is ever destroyed, it'll die of nicks and cuts. A bridge here, a power line there. These threats are everywhere," he adds, "and they have to be fought everywhere. Just go to one of the unprotected rivers in the Northeast or South and see how polluted they are."

The stream of my youth, North Carolina's aptly named Tar, is one such river, though my friends and I were too young to know the difference then. We caught bass and bluegills from beneath the rafts of old soda and bleach bottles that floated at each logjam. We shot the ducks that exploded from the quiet bends where discarded washing machines and tires lay. We waded when the water dropped to knee-deep in summer and carried a faint whiff of the sewage treatment plant upstream. Though I caught countless fish from the Tar's waters, I released them to their turbid home. My parents drew the line at eating them.

Such threats seemed many miles and moons from the clear, clean water of central Idaho. The next day the sun rose white-hot above the ridgeline, turning the Middle Fork into an undulating strand of emeralds. A herd of bighorn sheep joined us for breakfast. Bald and golden eagles glared at us from their perches as American dippers flitted from rock to rock. The guides filled our water jugs from springs we passed while the anglers among us hooked hungry trout on what seemed like every other cast. It was a living page from America's past, when every river was clean, potable, and full of life.

After lunch on a gravel bar I sat in the shade and watched Sam struggle with the fly rod as most beginners do, flailing it like a whip instead of achieving that "art … performed on a four-count rhythm between ten and two o'clock," as Norman Maclean wrote in A River Runs Through It. But gradually he checked himself and stopped the rod close to ten. The line uncurled on the water like a prayer, dropping the Craighead spider fly into an alluring eddy. He was too pleased with himself to notice the shimmering torpedo emerge from the depths. Only when he tried to back-cast did he find himself hooked into a living, breathing dynamo. This was no video game, no virtual walleye of Wii. This was barefoot boy against bantamweight pisces, and the age-old fight was on. As the two splashed in the cool, green water, whoops rose from the bank. The bronze bomber skittered onto shore, the same westslope cutthroat with its jaunty red sash that so delighted Lewis and Clark.

Sam was beaming, caught deep in Craighead's web. I once asked Craighead why wild rivers were such a crucial issue for him, thinking he would wax philosophical about the need for wild things in an increasingly man-made world. He shrugged. "I just loved rivers," he said.

It was enough. Because he and others loved moving, living, untarnished waters, we now have some left to cherish. To help us think more like a river, less like a dam.