The Luapula belongs to the Congo River watershed, but it belongs more intimately to a question: where does the Congo actually start? The Chambeshi River flows into Lake Bangweulu through a maze of shifting channels, lagoons, and swamps with no single clear outlet. South of the lake, the Luapula emerges - somehow, somewhere - and arcs west, then northwest, then north. It marks the border between Zambia and the Democratic Republic of the Congo for hundreds of kilometers. Near the main road, the river drops over Mambilima Falls in a set of well-known cataracts. The geography that flummoxed nineteenth-century explorers has not become much tidier in the satellite age. Some of the world's most important water moves through landscape that still will not hold still for a map.

For 300 kilometers between the Chembe Bridge and Lake Mweru, the Luapula Valley cuts 500 meters deep into the surrounding plateau. Unlike the plateau above, the valley is thickly populated - villages strung along the river like beads. North of Mambilima Falls, this stretch picks up its nickname: Mwapoleni Road. It comes from the Chibemba greeting called out whenever people pass one another on the path. The valley is not really a road at all, but a river corridor where the name of the road is simply the word for hello. Few places announce their social character so plainly. You travel by Mwapoleni Road and the geography itself teaches you how to greet a stranger.

The last 100 kilometers of the Luapula dissolve into a delta that covers 2,500 square kilometers before emptying into Lake Mweru. The swamps run 30 kilometers wide for most of that length. Four inhabited islands rise from the delta on the DR Congo side, including the largest in the system, which becomes accessible overland when the dry season drains the surrounding channels. Three more inhabited islands sit on the Zambian side, including Chisenga Island. Mofwe Lagoon is the largest of many lagoons threading through the wetland. In the 1930s the river port of Kasenga grew prosperous supplying fish from Lake Mweru to Elizabethville - now Lubumbashi - and the towns of the Katanga Copperbelt. Fish caught on the lake came up the Luapula by boat, iced in local plants, and trucked inland along the first motor road to reach the valley. The fish catch has declined, and better Zambian roads now carry most of the traffic. Kasenga remains the river's only port.

For most of the colonial and post-colonial era, crossing the Luapula meant taking a pontoon ferry. The first vehicle ferries ran at Kasenga, Kapalala, and Shiniama near Matanda. None of those remain, though a passenger ferry still connects Kasenga with Kashiba on the Zambian side. Around 1950 the Chembe Ferry opened on the Congo Pedicle road, eventually becoming the main vehicle crossing. At that point the river is 400 meters wide - but in a heavy rainy season, it can balloon to a full kilometer. In 1983, engineers finally spanned the river with the 2.5 km Luapula Bridge on the Samfya-Serenje road. The approaches alone include nearly 20 km of elevated causeway over wetlands, plus another 40 km of embankment across the floodplain. The second bridge, the 320-meter Chembe Bridge, opened in 2008 and retired the old ferry. It took 125 years after Livingstone's death to make the river routinely crossable by car.

Most borders are political lines drawn across landscape. The Luapula is the opposite - a landscape that happens to be a border. It separates two countries while tying dozens of fishing villages into a single watery economy. People move across it to sell fish, greet family, attend markets. The Chibemba greeting Mwapoleni is spoken on both sides, by people whose communities were divided by a colonial map but who still pass one another on the river. The Luapula lends its name to an entire Zambian province. It feeds Lake Mweru and, depending on which geographer you ask, may be part of the true source of the Congo. Livingstone died with his question unanswered. The river kept flowing.