[2] The region is part of the geomorphological provinces of the Coastal Plain and the Uruguayo-South-Rio-Grande Shield, and in the Brazilian part of the region are Aceguá, Arroio do Padre, Arroio Grande, Bagé, Candiota, Canguçu, Capão do Leãu, Pedro Osório, Pelotas, Pinheiro Machado, Piratini, Rio Grande, Santa Vitória do Palmar and Turuçu.

The Lagoa Mirim and its surrounding wetland complex form one of the main transboundary river basins in South America, which is of great ecological and economic importance.

Two important environmental reserves, the Taim Ecological Station in Brazil and the Humedales del Este in Uruguay, are located on the shores of the basin.

Known worldwide as one of the most important nature reserves, it preserves marshes, lakes, fields, dunes and forests, and is home to a wide variety of plant and animal species.

The sluice dam was built in 1977 to prevent the inflow of salt water into the Lagoa Mirim, as a result of the development of the two countries between Uruguay and Brazil.