The valley became a destination in the early eighteenth century, when the bandeirantes - paulista adventurers pushing inland from the coast - discovered gold. Sebastião Leme do Prado located veins in the region of Minas Novas, and the rush was on. Settlements sprang up at river junctions, most of them mere clusters of huts at first. Berilo, known then as Água Suja ('dirty water'), grew at the confluence of the Araçuaí and the Água Suja stream. The gold lasted about three decades. By the 1730s the placers were exhausted, and the people who stayed behind turned to corn, cotton, and cattle to feed themselves - the usual second act after a gold rush empties out.

But gold was not the only treasure locked in these hills. The Araçuaí Valley sits within one of four districts in northeast Minas Gerais that contain the largest pegmatite source in Brazil - coarse-grained igneous rock that grows enormous crystals over millions of years. Aquamarine came out of the Coronel Murta mine. Tourmaline from Rubelita. More rough gems from Itinga and Taquaral. The world's largest pegmatite deposits, rich with emeralds and topaz and all the stones that make jewelers breathe quietly, run through this land. Most of the primary and secondary deposits have already been worked out. What they left behind was a paradox - one of the gemstone capitals of the world, still one of the poorest rural regions in Brazil, often described as having social and economic similarities to the semi-arid northeast.

The river does not only give. In 1928 it took. Severe flooding swept through the valley that year, drowning most of the buildings and mansions lining the banks. Mud and water erased what ranchers and merchants had spent decades building. A few monuments survived the torrent - the Nossa Senhora do Rosário Church and the old mansions of Domingos de Abreu Vieira in Araçuaí town, structures built with enough mass to resist what the river had become. Walking past these buildings today is walking past survivors. Every other face on the street belongs to something younger, rebuilt, replaced after the water went down.

Scientists have been cataloging the river's creatures since at least 1874, when Steindachnerina elegans was documented in its waters. A survey of the Jequitinhonha basin identified nineteen fish species using the Araçuaí and its tributaries - an arcane list of armored catfish and characins and tetras with Latin names longer than most of the fish themselves. Hoplias malabaricus, the wolf fish with its fearsome teeth. Rhamdia jequitinhonha, named for the larger river it calls home. Guppies and tilapia and eels share the water with species found nowhere else. Above the banks, the 498-hectare Mata dos Ausentes Ecological Station - a small but fully protected reserve - harbors what remains of the original forest cover in a valley otherwise much reshaped by human hands.

The river is now a working system. In 1990 two generators were installed at the Santa Rita Dam for 75 megawatts of hydroelectric capacity - modest by Brazilian standards, meaningful here. Starting in November 2002, a water management program backed by the World Bank began extending supply and sanitation to Araçuaí and Carbonita, a $198 million project meant to reach about 47,000 people in two cities that had grown faster than their infrastructure. The Araçuaí that today's residents drink and wash in and irrigate with is not quite the wild river the paulistas poled their canoes up three centuries ago. It is a negotiated river, managed and dammed and regulated - still flowing toward the Jequitinhonha, still draining a landscape where some of the world's most beautiful stones lie waiting below the soil.