Mirim Lagoon

Mirim Lagoon, called Laguna Merín in Spanish, lay within the contested border country between Portuguese Brazil and Spain's southern American possessions. Its waters and tributaries formed part of a landscape where military routes, settlement and competing territorial claims met. Fortifications nearby reflected efforts to control that frontier: the Portuguese built San Miguel in 1737, while Santa Teresa changed hands during the struggles of the 1760s and 1770s.

The Treaty of San Ildefonso in 1777 placed both forts under Spain and defined neutral lands in the surrounding region. Surveying and proposals to use the lagoon's waterways accompanied these diplomatic arrangements. In 1779 the naval pilot Andrés Oyarbide described the potential benefits of developing the aquatic network and proposed a settlement on the shore. Such plans show the lagoon being considered as a route for regional development, while frontier conflicts and later independence struggles complicated their execution.