Let me clear up this book's title right away. It's a refrain -- almost a signature -- supplied by a German guide with decades of Amazonian experience but a shaky command of English. "Wo" being the German word for "where," he keeps making the same mistake -- which only emphasizes the amount of time he and his clients spend lost on the great river's tributaries. Those clients are Tano and Zalis (the author), two thirtyish men who have never quite graduated from the open-air university that was America in the late '60s and early '70s.

Veterans of several trips together (both physical and pharmacological), the Americans set out to test a rumor that there are pyramids in the jungle northwest of Manaus, Brazil. Two previous expeditions had gone off with the same purpose. The leader of one was the secretary to Eric Von Danniken, author of "Chariots of the Gods," a book that ascribes nearly everything splashy built by primitive man to interstellar colonists. Upon his return the secretary reported that "he had found the pyramids and contacted the underground people from the stars."

A second, Brazilian expedition brought back "a blurred picture of a valley with triangular nubs." Tano and Zalis hope to do better. At the outset their prospects look rosy. They have enough time and money to support a 40-day excursion, which seems ample. Their guide, Kurt Glu ck, apparently knows the region well -- he filled the same role on an abortive earlier try.

But Gluck's peculiar English is not his only idiosyncrasy. As they motor upriver into the Amazonian wilderness, he still checks his watch every 15 minutes. He cooks paltry meals that leave the Americans hungry and peevish. He is never sure where -- or who, in his parlance -- they are, and though the tributary Rio Negro is braided with confusing channels, he is being paid, after all, to keep them on course. By the middle of the journey, his reliability has become depressingly suspect.

For a while Glu ck's disorientation is tolerable. The Amazon Basin is ecologically fascinating, and Tano and Zalis can reminisce. There is flashback after flashback -- to the May Day anti-Vietnam demonstration in Washington, to a previous South American trip, to a collective house whose routine they shattered with a binge of dish hurling. They have drunk a lot together, traveled a lot together and maintained a kind of marginal but reasonably comfortable existence. Their occupations peg them as members of what might be called the underestablished class: Tano describes himself as a "laborer," and Zalis is a stringer for a small newspaper.

All of which is told in a style that vacillates between colloquial eloquence and puerile awkwardness. At his best Zalis evokes the subtropical miasma in all its murky plenitude. "It might seem impossible to miss the mouth of a major river, but being on the Rio Negro was like walking along a string under a microscope, with thousands of hairs scattering in either direction. Rains and drought changed the shape of junctions, lifting islands up and down out of the water and fragmenting forks into deltas and backwaters."

Other times his writing hits ungrammatical notes that might have sounded true in the '60s but ring tinny now. " . . . The driver seemed like he was taking us somewhere by flashlight. He drove fast, but the ride to town seemed like it went on forever." Those "seemed likes" bespeak an author unsure of himself. Then, too, Zalis' memory lane-changing grows wearisome. He throws in digressions on a hunting trip and his baseball career that have no discernible tie-in with the pyramid hunt but do portray him as a sensitive guy. But sensitive guys don't flaunt.

Back on the Rio Negro, the boat runs out of gasoline a mere 17 kilometers shy of the place where the pyramids are supposed to be. This is what in the '60s we called a bummer -- and the kind of misfortune any journalist can sympathize with. To his credit, Zalis pulls the book out of the ensuing stall and ends it triumphantly. "Who Is the River" is an uneven book that a devoted editor might have helped transform into a consistent success.