[2] This lagoon is evidently the remains of an ancient depression in the coastline shut in by sand bars built up by the combined action of wind and current.

The shallow lagoon is located at sea level and its waters are affected by the tides, normally they are brackish only a short distance above the Rio Grande outlet,[2] but this can vary a lot.

[2] Top predators from ocean ecosystem, notably common bottlenose dolphins, can sometimes be seen at Lagoa dos Patos[7][8] and exceptionally southern right whales are found at the Rio Grande outlet.

It covers a surface of 10,100 km2 (3,900 sq mi),[2] slightly smaller than Jamaica, and occupies nearly one third the area of the Coastal Plain of the state of Rio Grande do Sul.

The lagoon roughly runs NE-SW, with an average depth of almost 6 m (20 ft), and mean tidal amplitude of 0.45 m. At its southern end, near Rio Grande city, the only inlet of Lagoa dos Patos has a mean discharge of 4,800 m3/s (170,000 cu ft/s).