The lagoon has average length and width of and , respectively. It covers a surface of , 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 , 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 . Sea water penetrates northwards into the lagoon up to 200 km during exceptional conditions favored by southern winds, low water levels in the lagoon, and spring tides. The lagoon receives freshwater from about , mostly from the catchment basin of the
Guaíba River system, whose mouth is located in
Porto Alegre - the
Jacuí Delta - at the northwestern end of the lagoon. The coastal plain bordering the lagoon typically has elevations that reach up to 6 m, mostly consisting of sandy deposits interrupted by small inlets. Those spits rise in average 1 m above the mean water level and on the west margin of the lagoon their submerged part extends about 15 km into the water body. ==Tributaries==