HE RAFTED THE GRAND CANYON lying on a mattress.

Greg MacGillivray, 62, had concluded that this was the only way he could make his movie. He was suffering from a herniated disk and unable to endure the spinal compression that comes with sitting upright while boinging down Class IV and V rapids. So the veteran Imax director went to Home Depot and bought a waterproof chaise lounge mattress designed for the rigors of outdoor barbecues and mounted it in the bow of a guided raft, adding straps for his arms and legs so he wouldn't be bucked into the Colorado. On September 13, 2006, he put in to the river at Lees Ferry as the grinning figurehead of one of the largest expeditions in modern Grand Canyon history.

Forty-four team members in eight rafts, six kayaks, and two wooden dories started downriver that day. Led by guide Regan Dale, they were packing some 10,000 pounds of gear, including 20 cameras, 57 miles of 70mm film, 600 eggs, and 200 loaves of bread. Concealed in cameraman Doug Lavender's lens case were bottles of Grey Goose and Johnnie Walker. MacGillivray's four Imax cameras included two massive, boxy 3-D models, each worth a million dollars and weighing 350 pounds. It took at least four people to lift one.

The group included elite paddlers Steve Fisher, Anthony Yap, Rush Sturges, and other pros from the Teva Tribe (the shoe brand was a principal funder for the project), and a Park Service ranger who helped them avoid fragile areas. While they were granted a special permit to combine motorized and nonmotorized craft, they were forbidden to spend more than two nights at any campsite or to backtrack upriver, which meant they had to get their shots right the first time and move on. (Given that it costs $40 a second to shoot in 3-D, this wasn't an entirely unwelcome situation.)

MacGillivray's goal was to create an enthralling adventure story that would lure millions into those steep Imax theaters so he could terrify—and motivate—them with the stark realities of America's growing water crisis. To boost the film's pop appeal, he asked Dave Matthews Band bassist Stefan Lessard to compose the soundtrack (see "Rockin' the Boat," page 106) and signed up two charismatic environmental advocates: Waterkeeper Alliance chairman and NRDC senior attorney Robert F. Kennedy Jr. and anthropologist-explorer-author Wade Davis. Both experienced river runners—Kennedy, a kayaker, has made first descents of several remote rivers in South America, and Davis is a licensed whitewater guide in his native British Columbia—they are known for their passionate defenses of wild places. Their role was to articulate the ominous challenges facing rivers across the West, but especially the Colorado, a once vibrant waterway tamed by massive dams and endangered by thirsty crops and sprawling development. To add character appeal to the story (or maybe to keep the guys' orating habits in check), MacGillivray also invited their oldest daughters.

The result, Grand Canyon Adventure: River at Risk, rolls out in mid-March. In December, I sat down with MacGillivray, Kennedy, and Davis in Washington, D.C., to talk about the pleasure and pain of the trip, America's scary freshwater problems, and the solutions that are within our grasp—if we can just commit to them.