In the Colorado River guidebooks, it's identified as Mile 39. Not a very evocative name. At least not compared with Vasey's Paradise, the lush waterfall-fed garden of redbud and maidenhair seven miles upstream, named for George Vasey, the botanist who participated in the first full exploration of the Colorado by white men, in 1869. Or even compared with Bert's Canyon at Mile 411/2, named for Bert Loper, the noted river rat who died running a rapid in 1949. Maybe in a few decades they'll come up with a name for this spot that reflects its place in Western environmental and river history. But for now its name describes its location 39 miles downstream from Lees Ferry, where the Paria River enters the mighty Colorado, and where it is generally agreed that the Grand Canyon begins. If you're floating the Colorado in a raft, you generally hit Mile 39 the morning of your third day, and you can easily miss the spot as you slip by. But if you look up, above where the muav limestone is beginning to dominate the deepening canyon, you will see the ruins of one of the great environmental battles of the 1960s. Still bolted onto the redwall limestone are the skeletal remains of a wooden catwalk. Along its length are the drill holes left by federal Bureau of Reclamation engineers taking test borings for what was to have been the first of two dams in the Grand Canyon. Floating beneath the remains of that catwalk last summer and hearing about its history for the first time, my two children, 11 and 13, were incredulous. What kind of idiot would even think of damming the Colorado River in the Grand Canyon? they asked. In the tone of that question lies a march of years -- the rise of the environmental movement over the last four decades, changes in how Americans think about managing rivers and the West's natural wilderness, a decline of Sputnik-era enthusiasm for Big Dams as symbols of national progress. It's hard now to remember the early years of the environmental movement: its epic conflicts over dams, Alaska wilderness and landmark legislation such as the 1964 Wilderness Act and the 1973 Endangered Species Act. In those days, the fights were more dramatic and elemental, the leaders more inspirational, the activist groups leaner and more prone to wild ambition. Today, the big national environmental organizations operate out of suites on 17th Street or K Street in Washington. Their leaders fret about fund-raising and court the big foundations that keep them running. They conduct polling and focus groups, they worry about access to decision-makers, they bicker among themselves over turf and credit. The environmental movement has matured -- and become institutionalized. The underlying issues seem somehow diminished. It is not necessary to accept a Great Man theory of history to acknowledge that one major difference between then and now involves the role of David Brower, 84, and of the organization that he led to national prominence, the Sierra Club. Thirty years ago, they fought a monumental campaign against the Bureau of Reclamation, the water barons of the Colorado River basin states and their allies in Congress, all of whom were determined to put dams in the upper reaches of the Grand Canyon. Not only were such dams conceivable in those days, they were a near thing. Brower and the Sierra Club stopped them. Higher up in the Colorado River watershed, Brower also helped defeat the proposed Echo Park Dam. He was a driving force in the creation of great national parks such as Redwoods in California and North Cascades in Washington state. He fought for the national wilderness system that now spans more than 100 million acres. He founded the influential environmental group Friends of the Earth after the Sierra Club board of directors gave him the boot in 1969 for not being a team player and for pushing his conservation projects without regard to their cost. And he founded the Earth Island Institute after the board of Friends of the Earth gave him the boot in 1986 for pretty much the same reasons. As a courageous young mountaineer, Brower pioneered dozens of first ascents in the Sierras, and he led the expedition that first climbed the Shiprock monolith in New Mexico. He has twice been nominated for the Nobel Peace Prize. At this stage of his life, Brower ought to be savoring his triumphs and polishing up his memoirs. Instead, he is mapping out a last campaign -- epic in scale, quixotic in ambition, a campaign out of the grand old days -- to fix the one great regret of his life. As he wrote recently, "I like the idea of aiming high." The first time I ran across David Brower was in 1993, in his beloved Sierra Nevada. I was tagging along with Interior Secretary Bruce Babbitt on his first trip through the West after he joined the Clinton administration. Babbitt spent a night at a campground in Kings Canyon National Park and asked Brower to join him. My most vivid memory of that evening is of Brower seated across a picnic table from Babbitt, hectoring him long into the night, urging Babbitt to tear down the Hetch Hetchy Dam, the impoundment built originally to provide water to San Francisco, and which flooded the Sierra Nevada valley that was a twin of Yosemite. The next day we all hiked up to a lovely waterfall, and even at 80 Brower held his own for eight miles. Four years later, I pull up to his modest home below Grizzly Peak in Berkeley, Calif., not knowing what to expect. His car sports a "Free Al Gore" bumper sticker. On the street outside is a derelict milk truck, short a few windows and with a stout piece of cordwood under the left front wheel to hold it in place. Brower looks much the same -- he's aging better than the milk wagon (which turns out to have been left on the street by his son). With his shock of white hair, piercing blue eyes and habit of sermonizing spontaneously, he creates the impression of an Episcopalian priest out to save as many souls as he can. At the same time, he loves to laugh and polish his arsenal of one-liners. He's 6 feet 3 inches and trim, with a ruddy outdoors look and a strong and broad chin that he likes to tip up at an angle, like FDR. His eyes still twinkle. He's still restless and dissatisfied with everything from world overpopulation to the design of automobile seat belts. Notwithstanding his age and a fondness for Tanqueray martinis, he has astonishing recall of names, books, dates and obscure facts such as the rate of water loss in Lake Powell attributable to "bank storage." When he has to grope for a name, he says, "There goes my switch." His switch doesn't go very often. Brower's most important role in the environmental community has always been as a visionary, an inspirer. "He flies solo," says one longtime environmentalist who has watched his struggles up close. "He has an enormous ego and it has to be constantly fed and stroked . . . He believes the Earth is sacred and that anything you do in the cause of protecting it is justified, including withholding information, twisting information and making outrageous claims." Brower doesn't advertise that he's a great manager. "Taking risks has been my trouble in the environmental movement," he concedes. But the movement today, he believes, has become too bureaucratic and timid, its leadership desperately short of charisma. "I think for the most part they are indentured, as the universities are, as the media are, to the principal funders and directors," Brower says. He describes the rough edges of his own approach as the virtues of conviction. As he put it in his autobiography, Let the Mountains Talk, Let the Rivers Run, "Compromise is often necessary, but it ought not to originate with the environmental leaders." That line may sound like a bold call to action. But it is also self-criticism, an expression of regret. Brower compromised once, over the Glen Canyon Dam -- 15 miles upriver from the Grand Canyon -- and now in the twilight of his years he wants to tear that compromise down. In his book about Brower, Encounters With the Archdruid, the writer John McPhee describes the special animus that environmental activists hold for dams. To them, there is something "disproportionately and metaphysically sinister" about dams, he wrote. "Conservationists who can hold themselves in reasonable check before new oil spills and fresh megalopolises mysteriously go insane at even the thought of a dam." Glen Canyon Dam looms as a colossus of the dam builder's art. Rising 710 feet above bedrock and stretching 1,560 feet between the towering Navajo sandstone cliffs near Page, Ariz., that funnel the Colorado toward Marble and Grand canyons, it is a wondrous thing to behold, an epic feat of engineering. Three weeks before he defeated Democrat Adlai Stevenson and won a second term in 1956, President Dwight D. Eisenhower telegraphed a signal from Washington that set off the first ceremonial blast inaugurating construction of the dam. Workers poured the first buckets of what would be 5 million cubic yards of concrete shortly before Sen. John F. Kennedy won the Democratic presidential nomination in 1960, and didn't finish their job until just two months before Kennedy was assassinated more than three years later. It took 17 years to fill the 186-mile-long reservoir behind Glen Canyon Dam -- the deep blue still water known today as Lake Powell. The lake is named for John Wesley Powell, the one-armed explorer who led two expeditions down the river in 1869 and 1871 and went on to head the U.S. Geological Survey. Here is how Powell described Glen Canyon when it was still above water: "On the walls and back many miles into the country, numbers of monument-shaped buttes are observed. So we have a curious ensemble of wonderful features -- carved walls, royal arches, glens, alcove gulches, mounds and monuments. From which of these features shall we select a name? We decided to call it Glen Canyon." In the shadow of the Henry Mountains, the last range to be discovered in the continental United States, Glen Canyon was one of the most remote and least accessible places in America. Few people ventured there even in the first half of the 20th century. But the names given to the side canyons and grottoes and other geologic features evoked a place of great magic: Cathedral Canyon, Moonlight Creek, Little Eden, Twilight Canyon, Music Temple, Dungeon Canyon. In The Place No One Knew, author and photographer Eliot Porter described its subtleties. "The big features, the massive walls and towers, the shimmering vistas, the enveloping light, are all hypnotizing, shutting out awareness of the particular," he wrote. "Later you begin to focus on the smaller, more familiar, more comprehensible objects . . . the velvety lawns of young tamarisks sprouting on the wet sand bars just vacated by the retreating flood . . . the festooned, evocative designs etched into the walls by water and lichens. It is an intimate canyon." David Brower never saw any of that until it was too late, until the waters of the Colorado were lapping up against the concrete behemoth of Glen Canyon Dam, built in part to convert the river's power into kilowatts for Los Angeles. It was a simple deal. Brower and the Sierra Club signed it in January 1956. After a prolonged battle with Colorado Rep. Wayne Aspinall and other Western proponents of the Colorado River Storage Project -- of which the Glen Canyon Dam was the linchpin -- the conservation community, including Brower, agreed to drop its opposition to the legislation. In return, Congress abandoned plans for the Echo Park Dam. The deal also saved Dinosaur National Monument, and more important to the conservation community, it preserved the principle that dams should not be erected within national parks. But dams would go forward on the Gunnison River, in Flaming Gorge on the Green River, on the San Juan River -- and in Glen Canyon, the magical place so little known it had almost no constituency. Brower, then the Sierra Club's executive director, was staying at the Cosmos Club in Washington when he got the telegram from his board of directors telling him the fight was over. Author Russell Martin, who wrote a book about the struggle, reports that "Brower had been tempted to keep up the opposition" to the Western development legislation but folded his tent when he got the board's telegram. Forty years later, Brower says he made a terrible mistake, that without the compromise, environmentalists could have defeated the entire Colorado River Storage Project in the House of Representatives. Martin agrees, writing that Aspinall knew he couldn't have gotten the bill out of his own committee if it included Echo Park Dam. "Instead of going home and calling a special meeting of the board and explaining to them we've got this beat, it's a bad project, it violates our policy, I did nothing," Brower says. "And for years and years I've been quite unable to understand why I did nothing, why I sat on my duff and let this thing happen. I'm more responsible than anyone else for the building of the Glen Canyon Dam." Now David Brower has a plan to make amends, to restore something vital to the Desert Southwest and the Colorado Plateau. It is a simple idea. It is audacious. To many federal officials, it is madness. Brower wants to drain Lake Powell. He wants to drill through the capped diversion channels built during the early phases of Glen Canyon's construction and allow the Colorado River to run free again, around the dam. Brower has thought this through to the extent that he would save the dam structure itself for the anticipated time -- hundreds of years hence -- when Lake Mead, 279 miles downstream, silts up behind Hoover Dam. Glen Canyon Dam may be needed again at that point to manage the Colorado. No river in America has been the subject of more prolonged wrangling than the Colorado, a naturally rambunctious waterway that drains one-twelfth of the continental United States. The river shapes many aspects of life in the most arid quarter of the country. Allocation of the Colorado's waters is now governed by a complex of court decisions, treaties, legislation and administrative agreements known collectively as the Law of the River. Its centerpiece is the 1922 Colorado River Compact, which divided the watershed into an upper basin (Colorado, Wyoming, Utah and New Mexico) and lower basin (California, Arizona and Nevada) and gave each basin 7.5 million acre feet of the river's annual flow. Glen Canyon Dam is the cash register and faucet for the entire system. Its purpose is not to provide water for drinking or irrigation but to generate hydroelectric power, generate revenue for other dam projects and provide much of the storage needed to guarantee the upper basin's annual allocation. In the nearly two decades since Lake Powell filled up, it has also become the center of a huge recreational economy that attracts people from around the world to boat, water-ski, jet-ski, swim, fish for landlocked stripers and swill beer atop 60-foot houseboats ($4,000 and up a week, VCR included) while sweltering in 115-degree desert heat. To provide a sense of the startling dimensions of the place, Glen Canyon National Recreation Area superintendent Joe Alston agreed to take me on an aerial tour. As we fly north out of Marble Canyon and toward the mighty dam, the river comes into view, and with it, some of the 20,000 anglers a year who fish for trophy rainbow trout in the blue-ribbon tail waters below the dam. Just past the dam, with the sun rising strong over Navajo Mountain to the east, we pass over Wahweap Marina, with 1,000 boat slips, a lodge, an RV campground and boat repair facilities. At nearby Lone Rock Beach, it's not unusual to have 4,000 campers a night. Annual visitation at the recreation area runs more than 2.5 million people. "We're up there with Yellowstone-type numbers," Alston says. Our plane passes the southern terminus of the giant and forbidding Kaiparowits Plateau, shuddering in the grip of crosswinds racing down its face. We pass over Hole in the Rock, where early Mormon pioneers spent weeks blasting a path down to the river on their way from Escalante to Bluff, Utah. To our left is the heart of the 1.7 million acre Grand Staircase-Escalante National Monument created by fiat by President Clinton last fall, much to the outrage of many Utah politicians. Ahead is Bullfrog, a mini-city created by the National Park Service to cater to boaters and other recreationists. They pump some $500 million a year into the local economy surrounding Lake Powell. A dozen traffic lanes feed into the boat ramps. On a busy summer day, there might be 4,000 boats on Lake Powell, including several hundred $150,000 houseboats rented out by the Park Service concessioner. "The demand for those things is insatiable," says Alston. "You have to order them a year in advance." Working for the Park Service, where the ethic is to keep things natural, Alston is not unsympathetic to Brower's wish to restore the Colorado to what it was. But he's also a practical man. "Once these things are done, it becomes almost impossible to reverse these developments," he says. Brower, of course, does not blink at the thought of dismantling so much modernity. "There's lots of flat water," he says. "There's only one Glen Canyon." As he once wrote, "What is reasonable and what is not is all a matter of perspective." Edward Abbey, the writer who inspired a generation of Southwestern environmentalists, fantasized about blowing up Glen Canyon Dam in his books Desert Solitaire and The Monkey Wrench Gang. In the former, Abbey envisioned "the loveliest explosion ever seen by man, reducing the great dam to a heap of rubble in the path of the river." It was no accident that the founders of the radical environmental group EarthFirst! chose for their debut in 1981 to drape a 300-foot sheet of plastic shaped like a giant crack down the face of Glen Canyon Dam. "It was sort of a statement that it's not enough to save what is left, but we have to reclaim some things," says Dave Foreman, the group's co-founder. Near the beginning of the first Clinton administration, Babbitt said he wanted to be the first secretary of the interior to preside over the deconstruction of a major dam. Babbitt came to regret that bit of conservationist braggadocio as his name became reviled throughout most of the West for his initial fervor in pushing more restrictive mining, grazing, water development and logging policies. Now, even though his department is inching ever so slowly toward the dismantling of a salmon-killing dam on the Elwha River on the Olympic Peninsula in Washington state, Babbitt doesn't want to be anywhere near a discussion of Brower and the Glen Canyon Dam. The department's position, says an Interior spokesman, is that the proposal to drain Lake Powell is "totally unrealistic." In March 1996, Babbitt wasn't so reluctant to talk about the Colorado, the dam and what the administration was doing to help restore the Grand Canyon to a more natural state. Standing at the base of the dam, following instructions written by Bureau of Reclamation officials on three yellow Post-It notes, Babbitt opened four giant jet tubes and sent 29,000 cubic feet of water per second cascading downstream. The seven-day flood Babbitt started that morning was the culmination of a decade of study by scientists looking for ways to manage the dam that would do less harm to the Grand Canyon. The flood was designed to mimic the annual spring runoff floods that used to stir up sand in the canyon, creating beaches and backwaters as well as prime habitat for fish and wildlife. Brower considers such restoration programs far too modest. "I want the diversion tunnels opened up," he says. "We'd have a free-flowing river again." At the Bureau of Reclamation's Upper Colorado River basin office in Salt Lake City, officials have drawn up detailed position papers rebutting Brower and his arguments that the dam and its electrical power are simply not needed. Their case is that Lake Powell's critical role in flood control and water storage simply cannot be met by Lake Mead, which would have to bear the burden if Lake Powell disappeared. The larger importance of Brower's proposal, says David Wegner, the scientist who designed the test flood begun by Babbitt and who now works as an environmental restoration consultant, is that it opens up a public debate on managing the great rivers of the West. "We're at a pivotal point in how we manage the river systems of the western United States," he says. "It's not just the Colorado, it's the Columbia and the Trinity and the Sacramento, all the rivers we control by dams." Against all odds, the old warrior is making progress. He has an important regional ally in the Glen Canyon Institute. The Sierra Club board unanimously endorsed his idea last fall. And even some old adversaries are saying that perhaps Brower's notion is not so crazy after all. One of these is Stewart Udall, the Kennedy administration interior secretary who, along with his congressman brother Mo, was a powerful advocate for the Colorado River Storage Project in the days when Glen Canyon Dam was built. Stewart Udall remembers with great clarity the day in January 1963 when Brower came to his office to plead one last time for Glen Canyon. The time had come to plug up the diversion channels and begin filling up Lake Powell. Brower pleaded with Udall not to close the gates. Udall knew that if he tried to stop the dam, "I would have had the congressional delegations from six or seven states storming my office," he recalls. "I think I probably said softly to him, Dave, you're asking me to do the impossible.' " It was a different time, says Udall. People did not look at rivers and canyons the way they do today. Glen Canyon Dam, says Udall now, was "a tragic mistake." It is a natural human impulse to want to tie up loose ends in the twilight of life, to get one's affairs in order, to repair emotional connections. It is possible to dismiss Brower's grandiose scheme as a hopeless longing to make amends for his one big regret in life -- a way to tidy up his activist attic. Brower impatiently dismisses that suggestion. "No, no, no," he says, sitting in his study as the fog swirls over San Francisco Bay far below. "I'm just thinking of the opportunity." Even if Brower's quest is in some way self-centered, it has made an undeniable impact -- just as Brower always has done, and in the same growling, threatening, insistent way. In his most recent book, Brower did suggest that he had one last game of environmental brinkmanship in mind -- one last deal before yielding to the next generation of environmentalists. "Give us back Hetch Hetchy and Glen Canyon," he wrote, "and I'll go quietly." Tom Kenworthy is The Post's Denver correspondent. CAPTION: Glen Canyon Dam under construction on the Colorado in the early '60s. CAPTION: Boaters and other recreationists pump $500 million a year into the economy of the Lake Powell area.