The Baía de São Marcos is a bay of the Atlantic Ocean

Atlantic Ocean

The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...

 in Maranhão

Maranhão

Maranhão is a northeastern state of Brazil. To the north lies the Atlantic Ocean. Maranhão is neighbored by the states of Piauí, Tocantins and Pará. The people of Maranhão have a distinctive accent...

 state of northeastern Brazil

Brazil

Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

.

The bay is an estuary approximately 100 kilometers (60 mi) long and up to 16 kilometers (10 mi) wide. It receives several rivers, including the Grajaú

Grajaú River

-Brazil:* Grajaú River* Grajaú River...

, Mearim

Mearim River

The Mearim River is a river in Maranhão state of northern Brazil. The river originates in the southern part of Maranhão, and drains north into the Baía de São Marcos, an estuary that also receives the Pindaré and Grajaú rivers, which are sometimes considered tributaries of the Mearim. The lower...

, and Pindaré

Pindaré River

The Pindaré River is a river in Maranhão state of north-central Brazil. The Pindaré rises in the low hills which separate its basin from that of the Tocantins River to the south, and flows north into the Baía de São Marcos...

. The Mearim is known for its pororoca

Pororoca

The pororoca is a tidal bore, with waves up to 4 metres high that travel as much as 13 km inland upstream on the Amazon River and adjacent rivers. Its name comes from the indigenous Tupi language, where it translates into "great destructive noise". It occurs at the mouth of the river where river...

, or tidal bore

Tidal bore

A tidal bore is a tidal phenomenon in which the leading edge of the incoming tide forms a wave of water that travel up a river or narrow bay against the direction of the river or bay's current...

.

São Luís Island

São Luís Island

São Luís Island is an island in Brazil with an area of 1,410 km² , located between the Baía de São Marcos and the Baía de São José in Maranhão state. There are 4 cities located in the island: São Luís, after which the island is named, São José de Ribamar, Paço do Lumiar, and Raposa. São Luís is the...

, also known as Maranhão Island, separates the Baía de São Marcos from the Baía de São Jose

Baía de São José

The Baía de São José is a bay of the Atlantic Ocean in Maranhão state of northeastern Brazil.The bay is an estuary which receives several rivers, including the Itapecuru and Munim....

 just to the east. São Luís Island is home to São Luís

São Luís, Maranhão

São Luís is the capital and largest city of the Brazilian state of Maranhão. The city is located on Ilha de São Luís in the Baía de São Marcos , an extension of the Atlantic Ocean which forms the estuary of Pindaré, Mearim, Itapecuru and other rivers. Its coordinates are 2.53° south, 44.30° west...

, Maranhão's capital.