Lagoa Mirim or Laguna Merín is a large estuarine lagoon which extends from southern Rio Grande do Sul state in Brazil into eastern Uruguay. Lagoa Mirim is separated from the Atlantic Ocean by a sandy, partially barren isthmus.
Lagoa Mirim is about 108 miles (174 km) long by 6 to 22 miles (35 km) wide. It is more irregular in outline than its larger neighbor to the north, Lagoa dos Patos, and discharges into the latter through São Gonçalo Channel, which is navigable by small boats. Lagoa Mirim has no direct connection to the Atlantic, but the Rio Grande, a tidal channel about 24 miles (39 km) long which connects Lagoa dos Patos to the Atlantic, affords an entrance to the navigable inland waters of both lagoons and several small ports. The southern part of the lake lies in Uruguayan territory, but its navigation, as determined by treaty, belongs exclusively to Brazil.
Both lagoons are the remains of an ancient depression in the coastline shut in by sand beaches built up by the combined action of wind and oceanic currents. They are at the same level as the ocean, but their waters are affected by the tides and are brackish only a short distance above the Rio Grande outlet.
The Jaguarão/Yaguarón River, which forms part of the Brazil-Uruguay boundary line, empties into Lagoa Mirim, and is navigable 26 miles (42 km) up to and beyond the twin towns of Jaguarão (Brazil) and Rio Branco (Uruguay).