''The Milagro Beanfield War,'' Robert Redford's first film as a director since ''Ordinary People,'' is a populist fable set in some beautiful New Mexican landscapes about one stubborn Chicano's battle against an uncaring system. It's as full of good intentions as a campaign to get out the vote.

For reasons not entirely clear, the decent, picturesque Chicanos of the Milagro Valley have been denied the right to irrigate their farms with water now being channeled to a huge development called the Miracle Valley Recreation Area. Instead of farms, the valley will soon be dotted with condominiums, golf courses, tennis courts and swimming pools.

When, by accident, an irrigation canal begins to drain onto his property, Joe, a handyman, comes to a fateful decision. He decides to borrow some of the water that once was his (and his father's, and his father's father's), to plant a small patch of beans in his side yard. Says Ruby Archuleta, who runs the local garage, ''I always knew Joe couldn't go through life without attempting one great thing.''

The word spreads through the Chicano community like wildfire, as gringos say. Ladd Devine, the head of the Miracle Valley project, is outraged. He imports a hired gun to restore order. Charlie Bloom, once active as a lawyer for political activists, knows that the Miracle Valley development will bring higher taxes, which will force the Chicanos to leave their homesteads. He throws in with Joe, Ruby and the others to protect their rights.

If ''The Milagro Beanfield War'' were more sharply focused, it might have had some of the primitive appeal of an old cattlemen-versus-the railroad western. However, the movie can't quite bring itself to be so crude and, in avoiding cliches, it avoids telling any particular story whatsoever.

The screenplay, by David Ward and John Nichols, based on Mr. Nichols's novel, is jammed with underdeveloped, would-be colorful characters, including a philisophical Chicano angel, who face a succession of fearful confrontations with the law that come to nothing. The narrative is a veritable fiesta of anticlimaxes, from the time the sun sets at the beginning of the film until it sets, yet again, behind the closing credits.

The film is very big on sunsets and sunrises. It also has a touristy appreciation for all manner of things folkloric. What it doesn't have is dramatic coherence or backbone. Even the villains are spineless.

The more prominent members of the cast include Chick Vennera as Joe, Sonia Braga of Brazil as Ruby, John Heard as Charlie Bloom, Christopher Walken as the hired gun, Daniel Stern as a New York University sociology student, Ruben Blades as the Chicano sheriff, Richard Bradford as Ladd Devine and Melanie Griffith as his wife. None of them have a great deal to do except look the part. The most riveting creature in the movie is a great white pig that does tricks on cue that would tax Lassie.

''The Milagro Beanfield War,'' which opens today at the Beekman Theater, is not Mr. Redford's finest hour. RIPARIAN RIGHTS AND WRONGS - THE MILAGRO BEANFIELD WAR, directed by Robert Redford; screenplay by David Ward and John Nichols, based on the novel by Mr. Nichols; director of photography, Robbie Greenberg; edited by Dede Allen and Jim Miller; music by Dave Grusin; art director, Joe Aubel; produced by Mr. Redford and Moctesuma Esparza; released by Universal Pictures. At Beekman, Second Avenue at 65th Street. Running time: 115 minutes. This film is rated R. Joe Mandragon...Chick Vennera Sheriff Bernabe Montoya...Ruben Blades Ladd Devine...Richard Bradford Ruby Archuleta...Sonia Braga Herbie Platt...Daniel Stern Kyril Montana...Christopher Walken Nancy Mondragon...Julie Carmen Horsethief Shorty...James Gammon Flossie Devine...Melanie Griffith Charlie Bloom...John Heard Amarante Cordova...Carlos Riquelme Coyote Angel...Roberto Carricart