The Purari does not start as a river so much as assemble itself from others. The Kaugel comes down from near Mount Giluwe. The Erave drains south from Mendi country. The Lai, the Tua, the Pio all join at different points along the descent. The thing we call the Purari really begins south of Mount Karimui, where the Tua and Pio merge and plunge into a spectacular gorge. From that gorge the river breaks out onto lowland and flattens into delta country before the Gulf of Papua takes it. Mount Wilhelm, the high point of the whole catchment at 4,510 meters, feeds meltwater and monsoon runoff into tributaries that feed the main stem. Upstream, the water runs clear over rocks. Downstream, it is thick brown, the color of milk chocolate, laden with silt stripped from the mountains by rain that falls two to eight meters a year depending on where you stand.

Mick Leahy was a gold prospector. Australian, ambitious, the kind of man who kept going when sensible men turned back. In 1930 he and his brother-in-arms Mick Dwyer set out to cross unexplored country in search of gold, following the headwaters of the Purari upstream from the coast and then working back down. The expedition charted the river system itself, but that was incidental. The real news came from the highlands they crossed: valleys packed with villages, terraced gardens, sophisticated irrigation. Accurate estimates suggested hundreds of thousands of people lived in country outsiders had assumed was uninhabited. The Leahys triggered a wave of official patrols through the 1930s. The highlands had been hidden behind the Purari's gorges and the mountain walls for as long as the world had been mapping itself. The river was the way in.

Rivers are usually understood as water. The Purari is more honestly understood as sediment. Each year, roughly 105 cubic kilometers of water leave this delta. Inside that water ride 88.6 million cubic meters of mud, sand, and gravel - mountain matter in transit. The Gulf of Papua's floor is being paved, kilometer by kilometer, with ground-up highlands. That sediment, along with what the Fly and Kikori rivers contribute, creates one of the great deltaic complexes on Earth. Banks shift. Channels braid and re-braid. What looks on a map like a permanent river mouth is really a temporary arrangement between flood cycles. Dugout canoes can work the lower river from Wabo to the sea; upstream, the water is too white and too fast, except for people who grew up on it.

For decades, planners have looked at the Purari and seen voltage. The river's altitude drop and its reliable monster discharge make it one of the more obvious hydroelectric candidates in the tropics. The Wabo Dam has been studied, debated, and deferred for half a century. In 2010, the governments of Papua New Guinea and Queensland signed a Memorandum of Cooperation with PNG Energy Developments Ltd and Origin Energy to progress the project. The idea is to dam the river and export power to Australia via submarine cable. Every engineering study shows it is technically feasible. Every environmental and social review shows it would transform the valley and displace communities whose ancestors have lived along this river since long before the concept of a kilowatt existed. The dam remains unbuilt. The river remains the third-largest in Papua New Guinea, running brown to the sea.

Population along the Purari is sparse. Most of the basin sits under heavy tropical jungle - high rainfall forest that supports enormous bird populations, cuscus, tree kangaroos, and birds of paradise at altitude. Human settlements are mostly subsistence villages strung along the riverbanks, garden plots cut into the forest edge. From Wabo downstream, dugout canoes appear on the water, log-hulled boats carved from single trees. Populations thicken as the river nears the delta, where villages can support themselves on fish and sago palm and the occasional catch of wild boar. The long, quiet middle of the river belongs to the forest, the floods, and the few clans whose territories run along its banks. Flown over from altitude, it reads as a brown thread winding through solid green, bending back on itself in lazy meanders where the highlands finally give up their weight and let the river slow.