BY BOB STRAUSS >FILM CRITIC

"Frozen River," this year's Sundance Film Festival Grand Jury Prize winner, has a reputation as an unremitting downer. As it turns out, it is pretty grim. But Courtney Hunt's immigrant smuggling story is also grippingly suspenseful, wonderfully acted and, though it pushes maternal buttons like they could ever go out of style, manages to do that with a good deal of class.

All in all, pretty entertaining for something so gloomy.

Melissa Leo, best-known for playing Detective Howard on "Homicide: Life on the Streets," gives a bold, deglamorized performance as Ray Eddy. An upstate New York trailer mom, she's devastated and enraged when her chronic-gambler husband disappears with the balloon payment for the double-wide she's been dreaming of moving into with her two sons. And just a few days before Christmas, no less.

Contrary to the set-up, this is not redneck rodeo theater. Though tattooed and withered by worry, Ray can be enterprising and sly.

When she tracks the deadbeat's car to a little two-wheeler on the nearby reservation, shots are fired, but a solution to her money problem is soon negotiated. Seems the Mohawk who lives there, Lila Littlewolf (Misty Upham), has some connections on the Canadian side of the res.

All she needs is a car with a big trunk - like the beat-up Dodge Spirit Ray is driving - that can cross the iced-over St. Lawrence, pack in a couple of undocumented Chinese or South Asian immigrants, then bring them back to the States for a handsome price.

Ray and Lila initially hate each other, but hey, the money's good.

And Lila's got a big parenting issue, too, so that's something to bond over. Fortunately, they're both pretty hard-core, difficult personalities, and sentimentality is usually squelched by their cussed, pragmatic attitudes and, in Ray's case, a thoughtless prejudice that almost leads to unforgivable tragedy.

Writer-director Hunt, who expanded the feature version of "Frozen River" from her earlier short film, has virtually nothing to say about the rights and wrongs of human smuggling. That's a good thing; with no preaching or griping to do, she can concentrate on building tension and examining the hardscrabble lives of these women to whom illegals are mainly just cargo. Going more or less unsaid is a subtext about who has the right to this land; it subtly registers anyway.

Ray's oldest boy, TJ (Charlie McDermott), is perceptively drawn, too.

Well aware of their financial situation (if not Mom's new business), the sullen adolescent's drift toward criminality is nicely balanced by the responsibility he takes for his little brother.

Sometimes, Upham gets a bit carried away. Multiple Christmas Eve melodramas almost inspire snickering, but she pulls them back from the edge quite impressively. The film was also beautifully shot, in subzero weather, on a Sony Vericam whose crisp winter images chill to the bone - and put the muddled visuals of the similarly chilly, much more costly "X-Files" movie to shame.

Bob Strauss, (818) 713-3670

bob.strauss@dailynews.com

review>

FROZEN RIVER

>R: language, violence, children in peril.

>Starring: Melissa Leo, Misty Upham, Charlie McDermott.

>Director: Courtney Hunt.

>Running time: 1 hr. 37 min.

>Playing: Town Center 5, Encino; Playhouse 7, Pasadena; Sunset 5, West Hollywood; Landmark, West L.A.; Edwards Westpark 8, Irvine.

>In a nutshell: Grim but highly suspenseful and well-acted Sundance favorite about two poor moms who smuggle illegal immigrants in from Canada.