The Brazilian water agency has kept the Óbidos station since 1968, and the record has been reconstructed back to the early twentieth century. The station drains a basin of roughly 4.68 million square kilometres - most of the Amazon watershed, gathered into one measurable throat - which makes it, by catchment, the greatest hydrological station in the world. The long-term mean annual discharge works out to about 163,000 cubic metres per second. Written down, the figure is abstract. Standing on the Óbidos waterfront, it is not. The water moves the way water moves in a mountain river, except that it is brown, opaque and a mile and a half across. Survey boats drag acoustic Doppler profilers back and forth to read velocity at depth, and the crossings have to be repeated season after season, because the Amazon here rises and falls by many metres between the flood and the drought and rearranges its own bed while doing it.

The geometry that attracted hydrologists attracted soldiers first. In 1697 the Portuguese put a fort on the bank here - the Forte dos Pauxis, named for the Pauxi people whose territory it was built on and, in part, built to exploit. Manoel da Mota Siqueira raised it under the governor Antônio de Albuquerque Coelho de Carvalho, and its purpose was blunt: close the throat. Any Dutch, English or French vessel hoping to trade up the Amazon had to pass beneath it, and so did every canoe bringing cacao, cloves and sarsaparilla down from the interior. A customs post followed, and duties were taken from everything that squeezed through. The crown also expected the garrison to supply indigenous labour and forest products to Belém, which for the Pauxi and their neighbours meant something much closer to conscription than to commerce. In 1758 the settlement was raised to a vila and renamed Óbidos, after the walled Portuguese town it was thought to resemble when seen from the water.

Just off that same waterfront, in the small hours of 19 September 1981, the riverboat Sobral Santos II turned to come alongside and went over. She was on her weekly Santarém-to-Manaus run, carrying passengers in hammocks and cargo stacked wherever cargo would fit. Perhaps five hundred people were aboard; 178 were counted alive afterwards. Nobody has ever been able to say with confidence how many died, which is its own quiet verdict on how river travel was regulated at the time. It remains among the worst disasters in the history of Amazon navigation. The harbour where it happened is otherwise unremarkable - a working wharf, boats nosed into the bank, the current sliding past faster than it looks.

The municipality is enormous, more than 28,000 square kilometres for those fifty-odd thousand people, and most of it is forest that almost nobody visits. Its northern reaches take in a slice of the Grão-Pará Ecological Station, a reserve of some 4.2 million hectares described as the largest fully protected tropical forest conservation unit on the planet, along with a tenth of the Trombetas State Forest, created in 2006. Far up toward the Suriname border lies Missão Tiriyó, a village of the Tiriyó people. Back in town the streets climb from the waterfront in the Portuguese manner, and Óbidos is the seat of its own Catholic diocese. The critic and essayist José Veríssimo was born here in 1857 and went on to become a founding member of the Brazilian Academy of Letters - a reminder that the throat of the Amazon was a place of ideas as well as tolls.