Lagoa dos Patos has an elongated shape that moves towards the SW-NE, parallel to the riograndense coast, from which is separated by a long sandbank of about 240 Km.

In the south, Lagoa dos Patos communicates with the sea between the towns of São José do Norte and Rio Grande, through Canal do Norte at the bar of Rio Grande, where two rock piers were built to improve the conditions to traverse the bar.

In the north, it has two ramifications - at the northeast, Lagoa do Casamento and at the northwest, Rio Guaíba, a passage between Lagoa dos Patos and Jacuí Delta, formed by Jacuí, Caí, dos Sinos and Gravataí rivers.

Rio Taquari is the main tributary of Jacuí. Lagoa dos Patos communicates with Lagoa Mirim through São Gonçalo channel.
Lagoa dos Patos area is around 9,800 Km², not including Guaíba.

It is 220 km long, has an average width of 33 km, and its maximum width reaches 56 km.

The access by sea to the port of Porto Alegre is made through 72 km of channels dragged in Lagoa dos Patos and in Rio Guaíba for a 5.18 m (17 feet) draft, after the passage of Rio Grande bar.