The mighty St. Lawrence flows from Lake Ontario to the Atlantic, slicing the United States from Canada. Ray and her two sons live near its eddies on the New York side, in a rustbound mobile home that shivers visibly on a subzero December morning.

Ray's trailer doesn't have sufficient insulation, nor does this working mother, who toils part-time in a dollar store, hoards her pennies, and between paychecks feeds her boys popcorn and Tang.

Frozen River, an urgent and incomparably moving first feature from Courtney Hunt, is a companion piece to last year's Oscar winner. Call this stirring story of survival, fortitude and unlikely deliverance No Country for Middle-Aged Moms. The film from Hunt, who wrote and directed, has the crackling pace of an action movie, the racking suspense of a thriller, and the unexpected resolution of an O. Henry tale.

Resembling an unvarnished Bonnie Raitt, Melissa Leo is Ray, whose husband has absconded with the down payment for a new trailer home. Ray has no time for self-pity, nor does Hunt's film, which runs purely on nerve and primal instinct.

When Ray sees her husband's car leave the parking lot of a bingo parlor operated by the Mohawk nation, she gives chase. The driver of the stolen vehicle is Lila Littlewolf (Misty Upham), a Mohawk so marginalized that, like Ray, her choices are of the lesser-of-two-evils variety.

As Ray pulls a pistol to get back the family car, she does not immediately recognize morose Lila as her mirror image. These two single mothers forge not a bond, but an uneasy partnership. To feed their sons, they will break U.S. law, smuggling illegal aliens across the border on a frozen river where the ice is about as reliable as men and money.

No one would mistake the saga of Ray and Lila for that of Thelma and Louise. For the women of Frozen River, the stakes are higher and the times are grimmer. When dependents are involved, defiance is a luxury. Immediate action is the only recourse.

Hunt is a natural storyteller who conveys her drama in the flinty body language and facial expressions of the women. When Ray pleads with her boss for fulltime work, her gimlet-eyed resignation at the coworker who gets those precious hours is practically Shakespearean in its resonance.

In what have to be the two least self-conscious performances in recent memory, Leo and Upham fuse with their characters to such a degree that I had to keep reminding myself that this wasn't a documentary. Also quite fine is Charlie McDermott as Ray's older son, T.J., who serves as a surrogate dad to his kid brother.

Like its heroines, every aspect of the film has a homespun poetry. Reed Moran's textured cinematography details the infinite variety of ice, from the gemlike droplets on leafless branches to the rusting tears on a frozen pipe. There is nothing sentimental or picturesque about the performances or imagery. The word that best describes both is elemental.

Movies about guys proving their manhood are commonplace. This is one of the few I've seen with gals proving their momhood. Hunt gives us a portrait of poverty showing that she is richest whose joys are cheapest.

Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, Flickgrrl, at http://www.philly.com/philly/blogs/flickgrrl/