Lagoa do Peixe is barely knee-deep, and that is the whole point of it. A thin strip of sand and dune walls this forty-kilometre lagoon off the Atlantic on the coast of Rio Grande do Sul, and the water inside rarely rises past your shins. What it lays bare instead is mudflat — miles of it, packed with the small invertebrates that migrating birds eat. Up to a fifth of the world's rufa red knots stop here to feed. Each austral spring around nineteen species of Arctic shorebirds arrive after crossing the hemisphere, and one red knot tracked from this lagoon flew some 8,300 kilometres nonstop to Delaware Bay. Brazil made it a national park in 1986, and it now holds Ramsar status and international standing as one of the hemisphere's key shorebird stops. The water shifts between salt and fresh, depending on whether the sea is getting in across the bar.
Walk out at dawn in the southern autumn, when the northbound flocks are fattening for the long haul, and the flats come alive. Sandpipers scurry over the mud, Hudsonian godwits probe in tight groups, and Chilean flamingos wade in loose pink lines across the shallows. On the ocean side an all-but-empty beach runs for tens of kilometres toward the Farol da Solidão, the Lighthouse of Solitude. Behind it, burrowing owls stand sentinel at the mouths of their dune holes. This is remote country: soft sand tracks that swallow ordinary cars, and very few visitors. The balance here is fragile, and red knot numbers have fallen sharply since the 1980s, which is why access stays low-key and a guide who knows the tides and the roads is worth having. Stand with the surf at your back and the mudflats alive with wings, and it is obvious why so shallow a lagoon matters to a whole hemisphere.