Sediments in Lagoa Mirim and Lagoa dos Patos, Brazil and Uruguay32.7S 52.7W
Lagoa Mirim, as it is known in Brazil, or Laguna Merín in Uruguay, is a large estuarine lagoon which extends from the southern Rio Grande do Sul state in Brazil into eastern Uruguay. Lagoa Mirim is about 108 miles (174 km) long by 6 to 22 miles (35 km) wide.
Lagoa Mirim is separated from the Atlantic Ocean by a sandy, partially barren isthmus. 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. Here, Lagoa Mirim appears greyish brown, while its neighbor to the north shows a mix of dark brown, reddish tan and green.
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.