The numbers strain the imagination. The lagoon covers roughly ten thousand square kilometers, occupying close to a third of the entire coastal plain of Rio Grande do Sul. It runs northeast to southwest behind a sandbar about five miles wide that separates it from the open Atlantic. For all that size, it is strikingly shallow, averaging only a few meters deep, with a gentle tidal range of less than half a meter. The great Guaíba river system pours fresh water into its northwestern end near Porto Alegre, through the maze of channels called the Jacuí Delta, while the Camaquã River feeds it from the west. This is not quite lake and not quite sea. It is something in between, and that betweenness defines everything that lives in it.

The lagoon's character is a tug-of-war between river and ocean. Most of the time fresh water dominates, but under the right conditions, when southern winds blow hard, lagoon levels drop, and spring tides surge, salt water can push astonishingly far inland, penetrating up to two hundred kilometers north from the single inlet near Rio Grande city. That shifting boundary makes the water brackish and biologically rich, a nursery for fish and a haven for birds. Black-necked swans and coscoroba swans glide its margins, and Chilean flamingos wade the shallows in their improbable pink. The lagoon is fed and flushed in slow rhythm, an estuary on a scale that few places on Earth can match.

At the narrow mouth near Rio Grande, the wildlife turns spectacular. The Lagoa dos Patos estuary hosts the largest known population of Lahille's bottlenose dolphin, a southern lineage of the species, and these dolphins patrol the inlet where lagoon water trades places with the sea. Watch the channel long enough and dark fins break the surface, working the same currents that fishing boats have used for centuries. On rare occasions the spectacle grows larger still: southern right whales, ocean giants that breed off the South American coast, have wandered through the Rio Grande outlet and into the lagoon's edge, lost leviathans in an unexpected place.

People have always gathered at this water. The lagoon's largest and most fertile island, the Ilha dos Marinheiros, the Sailors' Island, sits near the eastern shore and belongs to the municipality of Rio Grande, its piers reaching out into the shallows. Ringing the lagoon are towns whose lives turn on it, fishing communities and ports strung along the coast: Rio Grande and Pelotas at the south, then São Lourenço do Sul, Tapes, Arambaré, Mostardas, Tavares, and São José do Norte tracing its long shoreline. Near Pelotas, the navigable São Gonçalo Channel links the Lagoa dos Patos to its smaller sister, the Lagoa Mirim, threading the two great waters of Brazil's far south into one connected system.