The filmed scenes of the river dories splashing down the rapids of the Grand Canyon are absolutely spectacular, probably the best look you will ever get of a white-water run short of climbing aboard one of these frail wooden craft for your own Colorado River trip.

The all-too-brief sequence is a reenactment of John Wesley Powell's famed 1869 exploration of the canyon, and it's the highlight of a new 33-minute film, "Grand Canyon: The Hidden Secrets," making its Washington debut tonight on the giant screen of the National Air and Space Museum. The film is one of two short movies in the museum's special Imax Film Festival through Jan. 28.

The three-man dories, just a bit larger than standard rowboats, all but disappear beneath the rolling water of the Colorado and then suddenly leap free of its clutches, flying over the crests of the waves. For a moment, the movie puts the viewer into one of the boats for a taste of the thrill and the danger.

Unfortunately the rest of "Grand Canyon" fails to match this excitement. The movie purports to tell "the human history" of the canyon, dating back 2,000 years. Surely there are other tales as dramatic as Powell's, but instead we are shown foolish and tedious tableaux of ancient Indians around the campfire and armor-clad Spanish explorers gaping awestruck from their horses into the chasm.

Worst of all is the narration, overblown and uninformative. "What unknown forces caused the Colorado River to carve this great chasm?" the script asks typically. A geologist ought to have a good theory. In contrast are Powell's simple, beautiful words, quoted briefly from his Grand Canyon journal. "Always we wonder," he writes, "what we shall find tomorrow."

Ignore the story line, and see this movie for its white-water adventure and for the stunning views behind those costumed characters from history running all over the red rocks. Deep inside the canyon's walls, there is an enchanting paradise of sparkling pools and lovely waterfalls. You get only a glimpse, but it is unforgettable.

The second movie of this double feature is the 18-minute "Silent Sky," featuring three sailplanes in silent, graceful flight over the Northern California coastline. Produced in 1977, it has been shown previously at the museum. It is a beautiful film, without narration, so your thoughts fly as free as the planes.

The films will be shown daily at 5:30, 6:30 and 7:30 p.m. through Jan. 6, and only at 5:30 from Jan. 7 through Jan. 28. Adults, $3.50; children, $2.50.