No one had to twist Robert Redford's arm to get him to narrate an IMAX film about a Grand Canyon river trip.

Redford, a lifelong environmentalist, previously navigated the Colorado River and has a boat on Lake Powell, a manmade reservoir on the Arizona-Utah border created by a dam. He worries about water conservation as a drought threatens the region — and, visibly, the Colorado.

"(The river is) endangered now on a number of fronts," Redford said by phone Monday from his office in Park City, Utah, where he holds his Sundance Film Festival every year.

"Its volume is shrinking because of a megadrought cycle that's now facing the Southwest, and some scientists predict that it could last into the next century," said Redford, who referred to a recent study forecasting that lakes Powell and Mead — another vital storage reservoir on the Colorado — could dry up in the next 13 years amid climate change and increasing demand for water.

The issues affecting the river, a source of water for some 27 million people in seven states, are addressed in a viewer-friendly way in "Grand Canyon Adventure: River at Risk 3D," which opens nationwide Friday.

The film, directed by Greg MacGillivray, follows environmental activist Robert F. Kennedy, Jr., and anthropologist Wade Davis — each accompanied by their respective daughters, Kick Kennedy and Tara Davis — as they explore the river with the expertise of Native American guide Shana Watahomigie. It features music by the Dave Matthews Band.

Redford praised the movie's "generational" narrative.

"(Kennedy and Davis are) passing something on (to their daughters), as I try to do with my kids (and) say `ok, you guys, we have an obligation to future generations to protect something,'" he said.