An unhappily married couple (Meryl Streep and David Strathairn) take their 10-year-old son (Joseph Mazzello) on a white-water rafting trip in the Northwest, hoping it will patch up their family problems. Downriver they pick up two stranded, shady characters (Kevin Bacon and John C. Reilly) who–no surprise–turn out to be gun-toting desperadoes. But Mom has weapons of her own. She used to be a river guide, and thus is the only one who can get everyone down perilous rapids alive. She also knows sign language, being a teacher of the deaf. The River Wild is the sort of thriller in which every detail that gets planted gets used. In spite of the fact that everything turns out exactly as you think it will, director Curtis (“The Hand That Rocks the Cradle”) Hanson’s movie, written by Denis O’Neill, is a tense, satisfying entertainment. In an era of overproduced, hyperventilating action films, it has a becoming modesty; Hanson is wise enough to let the river, and the splendid scenery of Montana and Oregon, provide their own natural-born special effects. “The River Wild” also demonstrates that the right actors can freshen up the oldest formulas (basically, it’s a floating version of “The Desperate Hours,” with a whiff of”Deliverance”). It takes a while to adjust to seeing Streep in an outdoorsy, can-do role. She invests so much emotion in the simplest expository scenes you think she’s overqualified for the job. But once she hits the water, oars in hand, she takes command of the movie. We haven’t seen this sturdy, physical, macho Streep before, yet she seems totally in her element. The fun, and the power, of the movie come from Streep’s newly robust heart: she’s a fiercely convincing matriarchal action hero. It was also clever to cast the boyish Bacon against type as the villain. He’s a plausible, life-size nemesis. It makes sense that Streep’s son would be drawn to his spontaneity, and Streep to his sexuality; Dad’s a dour workaholic. But if the father is a Yuppie cliche, the formidable Strathairn plays him with poker-faced finesse, revealing subtle currents of pride and rage under Dad’s pinched surface. All the actors stay within themselves, and the same could be said of the movie. It knows what it is, doesn’t try to be more, and gets the job done. It must nave been a fiendishly difficult movie to shoot, but the filmmakers make it look easy.