Ah, the Mississippi. Ol' Man River jest keeps rollin', inspiring songs, shows and enough folklore to fill several serious museums. He's called "Big River" in the musical that opened a month's run Wednesday at the Kennedy Center, a tributary that started flowing in a regional theater in California and ended up with a raftful of Tony awards last year in New York.

Based on "The Adventures of Huckleberry Finn," the show opens with the posted admonition that "persons attempting to find a plot will be shot by order of the author," which is misleading to say the least. "Big River" has enough plot for several musicals, as Huck and the runaway slave Jim set off on their raft and encounter danger, horror, misery and cruelty at every turn.

But the main attraction of "Big River" is a broad, unabashed, skilled theatricality that fills every inch of the stage. The Mississippi stretches out behind the players in a stunning panorama, stars twinkle, and the hardy little raft sways along in the river's currents with appealing verisimilitude.

It might be best to forget your mental image of Mark Twain's characters before coming to this musical; this is Broadway, after all, and not a reading room. Most of "Big River's" inhabitants, especially as played here, border on cartoons, lacking only signs labeled "Good" or "Evil."

The main exception is the character of Jim, played here as on Broadway by Ron Richardson, who won a well-deserved Tony for his portrayal. Richardson has a big, beautiful voice that commands attention, and a dignity that lends the character its moral force. For Jim is the counterpoint to all the hullabaloo exploding on the rest of the stage, the quiet reminder that all is not well even as the white folks are singing and dancing, that their troubles seem trivial compared with his loss of freedom and family.

Huck, as played by Brian Lane Green, is less successful a creation. Green has a splendid voice for Roger Miller's melodies, but his acting is limited to the realm of children's theater. His innocence is that of a pasted-on happy face, his emotions as broad as a plank and as thin as veneer. In the smaller part of Tom Sawyer, Roger Bart has a similar problem. His Tom is a wimpy and obnoxious jerk, all actor's energy and no substance.

But Richard Levine is a wonderful hambone as the itinerant fleabag performer, the Duke, and was well partnered Wednesday by understudy Graham Pollock, who stood in for Michael McCarty as the King. And the entire population of Bricktown, Ark. (played by members of the company) looked convincingly like the products of inbreeding.

Miller, who won a Tony for his music and lyrics, produced a score of surprising variety and interest, although echoes of his familiar rhythmic patter songs can be heard in a few numbers. Richardson ignites the evening's main show stopper, "Muddy River," and it's a thundering ballad of impressive volume.

"Big River" presents a version of early America generations of interpreters have pounded into a cultural cliche', where people talk in broad hillbilly accents and slap their knees, and the primary social interchange seems to be roughhousing. If you can handle this kind of corn pone, packaged as it is in an elegantly evocative set (which won designer-producer Heidi Landesman a Tony), "Big River" is a good value. Director Des McAnuff, touted as one of the wonder boys of the current theater scene, has proved here he can orchestrate a Broadway product, which has come to be as dependable a commercial commodity as Ivory Soap.

Big River, music and lyrics by Roger Miller, book by William Hauptman, directed by Des McAnuff, sets by Heidi Landesman, costumes by Patricia McGourty, lighting by Richard Riddell, fights by B.H. Barry, choreography by Janet Watson. With Ron Richardson, Brian Lane Green, Gordon G. Jones, Laurie Franks, Karen Looze, Roger Bart, Jase Draper, Jennifer Naimo, Sal Biagini, Jon Ehrlich, T.J. Meyers, Bill Buell, Mary Denise Bentley, Jody Gelb. At the Opera House through May 24.