Who Is the River : <i> by Paul Zalis (Atheneum: $19.95; 336 pp.) </i>

Considered opinion: Travel books are a drag. Reconsidered opinion: Well, not all travel books.

Surely not Paul Zalis’ free-flowing account of his trip up the Amazon, a river--and a story--with more ad-lib detours than an L.A. bus in a snowstorm.

The inevitable blurb touts the book as “a memoir destined to become the Woodstock Generation’s equivalent to ‘On the Road.’ ” It is a lot more.

“Who Is the River” is Dr. Scholl to the dwindling number of itchy-footed wanderers who travel for the sheer joy of it, destination out there in the mist and just enough money to make it to the next stop.

This one--the trip up the Amazon--is the big one, the one you take at the first intimation of slowing rhythms, encroaching responsibilities, suspended admission that you really wouldn’t mind a soft bed in the nearest Hilton.

Zalis begins the big one at Manaus, Brazil, the jungle capital of the Amazon. He is blessed in his choice of companions, Tano and Gluck. Tano is the indispensable intimate friend, the one to whom a nod conveys a volume, to whom an en-route disaster is only fuel for a lifelong memory. Gluck, their aging, quirky German guide, is a second ingredient indispensable to adventure: the local character, the eccentric one grows to love and hate, the crazy glue that binds the story.

Zalis also is blessed in choice of destination--the legendary pyramids that are supposed to lie in the jungle 800 miles upriver--and in degree of difficulty: virtually impossible. Their boat is Gluck’s little aluminum ketch, equipped with a 20-horsepower engine and precisely enough petrol for a voyage without incident. Their provisions: $70 worth of “canned swine,” powdered milk, rice and noodles.

All of which guarantees the third requisite of a first-rate travel yarn: hardship. (The perfect trip to Spain is preserved only on slides, but who will ever forget “The time we drove off the bridge in Andorra”?)

Finally, there is exotica (or why travel?), in which the vast and menacing Amazon basin abounds: primitive tribes with lethal blowguns; tapirs and toucans; the woman who raises three-toed sloths; Indians who drink a potent beer fermented by spittle and urine; parrots and piranhas; howling monkeys; rotting vegetation six feet thick. . . .

The trio gets lost, often, in the countless tributaries of a palpably foreboding river that changes character daily, rising, falling, snaking to its own perverse tempo. The rain becomes protagonist, “falling so hard it seems to stamp the air into the ground.”

Two weeks beyond the last traces of tin-can civilization, humping the ketch over fallen trees is an excruciating endeavor--stingrays menacing submerged legs, insects feasting on ripped flesh, garrulous Gluck threatening sanity with his catch-all appraisal of adversity: “Makes nuhzing.”

Lest the voyage become an unrelieved odyssey of jungle and river, Zalis leavens both tension and tedium with frequent inner excursions. Flashbacks can take him most anywhere: eating roast goat in Greece; bussing dishes in Maine; playing the ponies with his grandfather; fleeing the fuzz in Colombia.

Through it all run ribbons of bright color, the currency of the born griot . Feather palms are “giraffes with sombreros.” Night settles down “like a warm sponge.” A coyote is “a howl come alive.” A flirting girl in a cafe “handled the newspaper like a geisha with a fan.” A flying egret “looks like a white ghost rowing in the air.”

And in the end, there is the interior/exterior discovery that makes a journey worth the taking:

“The Amazon is a tangle of life fighting for a piece of the sky, a presence of green that forces you to react . . . an immense interaction, a composite of everyone’s stories . . . “ if only one could tell them.

Zalis can.