What's above the water line has naturally seemed more obvious. There are Civil War battlefields, industrial cities fed by the river's easy transportation, farm communities nurtured by the rich soil of its flood plain. And a host of immigrant and indigenous peoples who either arrived via the river or have long been sustained by it.

When we think about the Mississippi River, it's the everyday culture of these people -- rather than the water's romantic lore -- that we tend to overlook. After all, they're literally Middle Americans. They live without the glories of either coast, away from the media spotlight trained on the entertainment and high-tech industries. They're part of the nation's breadbasket, the home of so-called Middle American values. And as such, they're often taken for granted as solid, stalwart types without terribly exciting lives -- the kind of folks who buy Garth Brooks records and really believe that pork is the other white meat.

The spirited four-part film The Mississippi: River of Song that premieres on PBS this Wednesday, January 6 (WGBH, Channel 2 at 10 p.m.), dispels the coastal bias about the people who live along Ol' Man River with a few essential truths and a lot of traveling. Over six years and at a cost of several million dollars, Waltham-based filmmaker John Junkerman -- joined four and a half years ago by Somerville musician and journalist Elijah Wald -- roamed the banks of the Mississippi finding and filming the people who make music along its muddy course.

What emerged after 250 hours of film and many miles of audiotape is a picture of the vital and varied cultures that truly make up Middle America. From Lake Itasca, Minnesota, at the Mississippi's headwaters, to Delacroix Island on the Gulf of Mexico, visits with music makers who uphold the tradition of everyone from their great-great-grandfathers to Chuck Berry explode the myth that culture in the heart of the country has become a colorless, homogenized affair.

After viewing the long but lively and entertaining River of Song, it's hard to take Middle Americans for granted. And if that attitude seems unlikely from our lofty Bostonian perch -- nestled smugly in a zone where research is a growth industry, just an hour's flight from the world's capital, New York City -- reconsider, baby, because that's what the filmmakers had to do.

"American music doesn't look or sound like what you think it does if you watch MTV and shop at Wal-Mart," says Wald. "It's so varied. For us that was a big surprise. We went out there expecting to be documenting a dying regional culture that was being destroyed by the homogeneousness of MTV and Wall Street. With a couple of exceptions, like the Delacroix Islanders, what we found wasn't dying at all. And it was plenty regional out there, which was really cool."

How regional? Well, up the Mississippi in Minnesota's north country, Junkerman and Wald (who conducted all the off-camera interviews) filmed Ojibwe Native Americans performing in a contemporary powwow drumming style. A hundred miles south of the Ojibwe, the Scandinavian fiddle orchestra called Skal Club Spelmanslag were caught in a homey hoedown. In Minneapolis, rockers Soul Asylum and Babes in Toyland document the city's pop-music scene. "Spider" John Koerner of the pioneering '60s white-blues outfit Koerner, Ray & Glover -- who's more recently adopted the American songster tradition -- is filmed at his local hangout, Palmer's, where he also tends bar. The film's most recent immigrants, the Hmong people of Laos, are captured passing thousand-year-old ritual music and dance to children who are barely school age. Then the Grammy-winning gospel group Sounds of Blackness perform, followed by German polka bands -- and that's just in one state.

The sequence with the Hmong is especially touching in the way it reaffirms the American ideal of tolerance. In their own land, the Hmong were forced out of their mountain homes by decades of murderous purges, their culture direly threatened. In Minneapolis, their beautiful traditions -- colorful costumes, delicate dance, and a truly unusual array of braying and chiming instruments -- flourish.

As the film continues downriver during its four one-hour episodes, the famous and obscure are regarded with equal respect. While trumpeter Manny Lopez, a Mexican-American jazz bandleader in Davenport, Iowa, is certainly a lesser musician than the chops-heavy young turks in the New Orleans brass bands of the final episode, Junkerman and Wald treat them no differently. The famous, like songwriter John Hartford and gospel/soul singer Fontella Bass, get their due, as do the St. Charles High School Band and Clarksdale, Mississippi bluesman Johnnie Billington (even if he is under the misimpression that one needs to wear a jacket and tie to play the blues). Obscure country musician Kenny Bill Stinson has his hour in the shade in Natchez, Mississippi, as does "the Cajun Hank Williams," D.L. Menard, who's shown hosting a party in his backyard.

Overall, 41 artists or groups are featured, from nearly all points of America's cultural compass. Thirty-five also appear on the two-CD set The Mississippi: River of Song -- A Musical Journey Down the Mississippi (Smithsonian Folkways), a companion effort produced by Wald. And the entire adventure is recapitulated with plenty of oral history from featured performers in a book, River of Song -- A Musical Journey Down the Mississippi (352 pages, $32.50), just published by St. Martin's Press and authored by Wald and Junkerman, with an introduction by Ani DiFranco.

"For me, it all started with my first trip to New Orleans about 10 years ago," director/producer Junkerman explains. "We went out to Cajun country to listen to music as well, and my wife and I really enjoyed ourselves. I thought, 'There's got to be a way to get back down here and spend some more time around this music.' So I started toying with the idea of doing a series about American music. There have been so many films done about Louisiana music; there's all the Les Blank films. And a number had been done about blues, like Robert Mugge's Deep Blues. I didn't want to duplicate what had already been out there. I wanted to capture something of what Blank and Mugge had done, but in a broader sense."

Although Junkerman now divides his time between the Boston area and Japan, where he's made a number of acclaimed films documenting aspects of the traditional Japanese way of life and the impact that the atomic bombing of Japan has had on its culture, he originally hails from Wisconsin. "I knew Bob Dylan grew up not too far from the River in the north," he continues. "I began to think that tracing the River would be an interesting way to do it. I began to flesh out the idea -- at first with [noted music historian and Elvis Presley biographer] Peter Guralnick, who helped with the original proposal for the project. Then with Elijah when Peter became unavailable. By following the River from north to south, it puts a lot of different types of music in interesting juxtaposition from one to the next.

"That achieved for us what is the bigger idea of the series: a wide variety of music that continues to be performed and appreciated by audiences in this country exists side-by-side regardless of genre or commercial viability. So we ended up with a sense of American music that isn't defined in the normal ways -- via the commercial marketplace by genre, or by ethnomusicological categories -- many of which are arbitrary. We achieved something that reflects the vitality and ever-changing character of music in a way that hasn't been achieved previously."

Wald echoes that. "I don't think there has ever been a music program done that didn't file music by category. To me and to almost all of the musicians involved, the thing that was coolest about this project is that it acts as if genre doesn't exist. Musicians have always understood this; they listen to music, not categories. Genre has only existed as long as record stores have existed. It's a way to stock record stores. It has no relevance outside of that.

"What I like most after that, besides the thrill of going to all these places, was the thrill of having the budget to do it right," Wald continues. "For the record, I want to state that we paid everybody well. Nobody who was a front man featured in this, who had a speaking part, got less than a thousand dollars. A few people got more, and the sidemen got a couple hundred dollars. That was one of the things we decided to do in the very beginning. We paid the players. I think it's despicable to walk into a place like the Delta and except everybody to 'perform' for you without paying. If you can buy a goddamned camera, you can pay the musicians."

But financing was a struggle. The entire project was underwritten by the Smithsonian Institution, making it the largest multimedia project they had ever undertaken. Yet money came in spurts that occasionally stalled the filmmaking process, and corporate donations and sponsorships will float the broadcast presentation.

Despite its Smithsonian wellspring, there's nothing "institutional" about River of Song. The dialogue's informal as well as informed: real conversations with real folks. (Note the authentic can of beer in polka-man Karl Hartwich's hand as he speaks.) The camera does its job without artistic ambitions. And the film evades generic narration by drafting self-made pop star DiFranco as its piloting voice.

DiFranco's narration assures The Mississippi: River of Song at least some interest from teens and twentysomethings who might otherwise steer clear of a feature with the Smithsonian/PBS brand. "The main reason we chose Ani was because of her strong commitment to independent music, through her own career," says Junkerman. "But she's also been a big supporter of other kinds of independent music. Last spring she had the Re-Birth Brass Band from New Orleans open for her on tour. So she was a natural along those lines. Plus, she has a wonderful voice, a great presence to contribute.

"The other piece is, we wanted to break out of the niche this kind of music is placed in as traditional or folk music, and Ani sort of straddles the folk and rock and pop worlds and appeals to a younger audience. In one of our early demo takes, [folk legend] Odetta did the voice-over. She did a wonderful job, but to stick with Odetta would have perpetuated that ghettoization of this kind of music. One of the things we wanted to establish is that there's no real border between pop music and folk and traditional musics."

Wald reiterates: "That's something all the musicians were very excited about. For example, Lori Barbero, the drummer for Babes in Toyland, was really excited about being in a movie with the Ojibwe and the Hmong."

Indeed, one of the joys of the film is that it depicts everyday people, from bluegrassers the Bob Lewis Family to Canary Island descendants Irvan and Alan Perez, deriving sheer enjoyment from making music. There's an "I love this, and you can do it, too" sensibility to virtually every live performance, even if bluesman Big Jack Johnson's vibrato and string-bending on acoustic guitar does seem a bit superhuman.

"A lot of times when we see people on-stage performing in front of big audiences, we lose sight of the fact that they are ordinary people and that they had ordinary lives up until the time that they became famous," Junkerman adds. "The Minneapolis scene was very refreshing in that way, because both Soul Asylum and Babes in Toyland are quite well known internationally, but were such down-home folks it really helped us to illustrate that. They continue to maintain ordinary friendships and go bowling with their buddies even when they become famous.

"One of the wonderful things we found is that musicians have marvelous stories to tell," Junkerman continues. "There are a couple of narrative experiments we did. The first, of course, was following the Mississippi. The other is that we committed ourselves to just interviewing musicians and not ethnomusicologists or historians. This was something of a risk because musicians express themselves through their music more than their words, and some of them are not wonderfully articulate in a traditional sense. But we found that the stories they told about their music and what it meant to them, what part it plays in their lives and their communities, was more articulate than anything the ethnomusicologists would have said."

Obviously, The Mississippi: River of Song has many highs, from its opening Ojibwe powwow to a rehearsal-room version of Soul Asylum's "Misery" to Fontella and Martha Bass wailing gospel to a reunion of the Jelly Roll Kings -- who Wald calls "the greatest Delta-blues band of the past 30 years" -- in the now-fire-destroyed Bobo Grocery store in the musical heart of the Mississippi Delta. (Blues fans should note that an entire Jelly Roll Kings set was filmed and will be archived in the Smithsonian, along with the rest of the unused footage. "Usually we set up and filmed an entire set of everyone who appears," says Wald. "In the case of Geno DeLafose" -- a young zydeco artist caught performing in Lafayette, Louisiana's Y-Ki-Ki Club -- "we filmed all three sets he played that night.")

Asked separately to pick their personal peak experience making the film, Junkerman and Wald both zoomed in first on an impromptu session with soul singer Ann Peebles and the Memphis Horns in a Memphis recording studio. "The rapport between Wayne Jackson and Andrew Love of the Memphis Horns was just amazing to witness," says Junkerman. "It goes beyond speech. Although one is white and the other African-American, they just seem to complete each other in an amazing way that somehow comes through in their music -- the sound they make together.

"I had seen the Memphis Horns backing [bluesman] Robert Cray, and at the end they both took solos. I thought, 'My God, Wayne Jackson is a Louis Armstrong nut.' So when we did the session, I thought, 'Let's pick something for Ann Peebles to sing so Wayne can play obbligato like on an old Bessie Smith record.' [Armstrong was a frequent Smith accompanist.] So we picked 'St. Louis Blues.' And I got to produce the session -- just the horns with Ann and her pianist. And there is no greater high than producing a Memphis Horns and Ann Peebles session. Afterwards, they asked for a tape of it, so they could try to find somebody to sell it to as a concept. For me, that was the high point because I don't know if I'll ever be able to produce another session with musicians of that caliber in my life."

Certainly The Mississippi: River of Song has an abundance of joyous moments and delightful performances. But Junkerman chose wisely and naturally to close with the Perez brothers of Delacroix Island, a point where the Mississippi is dispersed into the Gulf.

Irvan and Alan Perez live in the Louisiana backwaters, in a remote community where shrimping and muskrat trapping still provide a living -- much as it did for their fathers and grandfathers. Delacroix was settled by a group of immigrants from the Canary Islands, and an ever-decreasing population of their descendants keeps their Spanish culture alive. Part of the old way of life is enshrined in beautiful, rough Spanish ballads called decimas, which are sung a cappella -- as they have been by toiling fishermen for hundreds of years. The decimas tell of the travails of fighting to earn a living from the sea, being forced to hunt crabs in high waters to feed starving families.