DOWN AROUND CAIRO, at the southern end of Illinois, is the greatest juncture of the greatest river on the North American continent. It is there that the Ohio pours its water into the upper Mississippi, forming a river much greater than either, the Mississippi of legend destined to roll from Cairo down to New Orleans and the gulf.

Rivermen since before the time of Mark Twain have observed that it is not an easy joining. In those days before pollution, the waters of the Ohio were clear and blue, while the Mississippi tended to the muddy, brown, and opaque. Long after joining at Cairo, the two rivers seemed unwilling to mix and mingle. Instead, rivermen observed, they would flow southward side-by-side, each holding itself apart, the blue water by the eastern bank and the brown along the western.

The lower Mississippi, Cairo to New Orleans, is the site of most of the action in John Brunner's big, historical The Great Steamboat Race. And like the river itself, Brunner's novel seems at times to be an uneasy joining of two distinct currents, one clear and brisk, the other more than a bit muddied.

The most famous steamboat race of all time was run from New Orleans to St. Louis back in 1870, when the Natchez took on the Robert E. Lee in a showdown that generated as much hype, and as much betting interest, as a 19th-century equivalent of the Super Bowl. The Lee took an early lead and never gave it up; in fact, the two boats weren't even in sight of each other for most of the run.

The Great Steamboat Race is based very loosely on the race between the Lee and the Natchez, but Brunner has the good writer's canny sense of the dramatic, and in fictionalizing the event he has also improved on it markedly. In Brunner's 1870, the two steamers are the Atchafalaya and the Nonpareil, and the race they run over that same Independence Day weekend has everything the real-life race did not: a lead that changes hands again and again and again, sly tricks, lightning piloting, dastardly plots, and death, destruction, and fire lurking upriver for the losers.

The principals in the confrontation are two characters worthy of Twain, and of real-life rivermen like Mike Fink, Thomas P. Leathers, and Roaring Jack Russell. There is Miles Parbury, the driving force behind the creation of the Nonpareil, a superlative pilot blinded during the Civil War when his previous steamer was sunk while in service to the Confederacy. His great antagonist is Hosea Drew, master of the Atchafalaya, who laid up his old boat during the war rather than risk losing her, and whose sympathies were with the Union anyway. Loathed by Parbury, the cunning, hard-headed, stubbornly honorable Drew nonetheless comes to dominate the postwar river trade with his newly-built Atchafalaya, fifth of that name, and holder of all the horns for speed--until Parbury finally gets the backing to build his dream boat, and the race is on.

It's a helluva race, and Brunner's telling of it makes a helluva story. I think Sam Clemens, who loved fast steamboats and good storytelling, would have approved heartily . . . up to a point.

Unfortunately, the race between the Atchafalaya and the Nonpareil is not the only story that Brunner attempts to tell in The Great Steamboat Race. It's a massive novel, some 568 pages long in trade paperback, and the race, as enthralling as it is, doesn't even begin to get seriously underway until page 233. Prior to that, we have some necessary scene-setting, some interesting narrative of the war years, and an awful lot of backing-and-filling as the novel takes on wood in the person of a whole host of secondary and tertiary characters. Alas, Mark Twain is not the only literary ghost striding along the steamer promenade; Arthur Hailey is aboard with him, his spirit invoked by Ballantine's blurb writer, and there are times when The Great Steamboat Race seems perilously close to reading like some unknown Hailey opus, no doubt titled "Steamboat."

It's the Hailey influence that muddies the clear, swift storytelling of the Twain here. John Brunner has handled large casts splendidly in science fiction novels like Stand on Zanzibar and The Sheep Look Up, both rightly considered genre classics, but in this case one has to wish that he had concentrated on Drew and Parbury and the story of their confrontation on the river.

Instead, we are given character upon character. Ballantine has obligingly given us a list of them up front, to help us keep them all straight, but no list could serve to make them more interesting. All of them have stories, none of them even a fraction as compelling as the main plot, the one that must necessarily get put on hold while Brunner attends to this welter of subplots.

To be sure, a few of the minor characters are quite well done. Fernand Lamenthe, the young Creole pilot Drew takes aboard the Atchafalaya, is nearly as interesting a character as the two antagonists, and the ex-slave, Caesar Predulac, who destroyed Parbury's old boat and winds up engineering on the new one, is quite believable, even if his presence on the Nonpareil is not.

We could have lived without the embittered French conductor who finds music in the great river, however. And without the volatile Mexican band-leader. And without the two rival voodoo queens, one for each boat. The selfish young cad would not be missed, nor the spunky wife he so mistreats, nor her decadent brother, dying of tuberculosis. The Electric Doctor grates, although not nearly so much as the Scottish mystery financier, whose secret is painfully transparent about a hundred pages before it is finally revealed. Nonpareil, Atchafalaya, and novel would all haveemoved along a lot more quickly had Parbury, Drew, and Brunner chucked this motley lot into the river before setting out.

Still, like the Mississippi it invokes so well, The Great Steamboat Race may at times be a bit swollen and muddy, but it has a real power nonetheless. John Brunner, best known for his science fiction, demonstrates here that he can do a solid historical novel as well, and the fact of his being British does not seem to have interfered in the least in a splendid depiction of life on the Mississippi in the waning days of steam power. Brunner is to be commended for a fine job of research, and a marvelous evocation of time and place. Though we might wish it was somewhat leaner and more to the point, The Great Steamboat Race still gives us two memorable characters, and one heller of a race.