The trip begins at Miandrivazo, the launching point for the roughly 160-kilometer drift down to Belo sur Tsiribihina. Most travelers take three days, sometimes a half-day more, drifting through landscapes that grow wilder by the hour. You can hire a motorized pirogue and shave off a day - fast, but loud enough to scatter every animal on the bank. The traditional paddled pirogue moves quietly, which is the point: it lets you slip up on the river's residents. Between stretches of water you pass the villages of the Sakalava people, who have farmed and fished this valley for generations and who watch the boats go by as they always have.

For much of its length the Tsiribihina meanders lazily across the rich alluvial soils of the Betsiriry Plain, some of the best farmland in western Madagascar. Then the river meets the Bemaraha Plateau - a great wall of limestone running north to south - and cuts straight through it. Here the water narrows into a steep, winding gorge, the canoe threading between rock faces while the current quickens. Waterfalls spill down the cliffs, and natural pools invite a swim in water surprisingly cool. Emerging on the far side, the river exhales, widens, and resumes its unhurried wandering toward the coast.

The Tsiribihina is one of the richest bird rivers in Madagascar, an island where the birds are like nowhere else on the planet. Eighty-two species have been recorded along the river and its delta, and twenty-two of them exist nowhere outside Madagascar. The critically endangered Madagascar fish eagle - one of the rarest raptors on Earth - keeps a few breeding pairs along these banks. Bernier's teal nests in the mangroves of the delta; Madagascar pratincoles gather on the river's edges in great numbers. Lemurs flit through the gallery forest, chameleons clutch the branches, and crocodiles bask where the channel slows.

What people remember most about the Tsiribihina is not a sight but a feeling - the rhythm of camp. Each afternoon the pirogues pull onto one of the immense sandbanks that split the channel, and the crew makes a bivouac on the bare sand. There are no lodges out here, no electricity, no signal. Dinner is cooked over a fire; the guide and the piroguier, often Sakalava, know every bend and every village. After dark the sky over western Madagascar opens into a depth of stars rarely seen in the modern world, and the only sounds are the river and the night birds. A few days of this strips away the noise of ordinary life. It is why travelers come, and why so many never forget it.

The journey ends in a vast mosaic of wetland. The lower Tsiribihina spreads into a delta of channels and mangroves protected within the Menabe Antimena reserve, and the whole delta is recognized as a wetland of international importance under the Ramsar Convention - and as an Important Bird Area by BirdLife International. It is a fitting finale: after the gorge and the villages and the long quiet days, the river loosens into a labyrinth of green where fresh water finally mixes with the Mozambique Channel. From here, most travelers carry on overland to the stone forests of Bemaraha, the river having delivered them to the threshold of one of Madagascar's strangest landscapes.