Pará River in the context of "Amazon Delta"

Marajó (Brazilian Portuguese: [maɾaˈʒɔ] MARR-ə-ZHOH) is a large coastal island in Pará, Brazil. It is the main and largest of the islands in the Marajó Archipelago. Marajó Island is separated from the mainland by Marajó Bay, Pará River, smaller rivers (especially Macacos and Tajapuru), Companhia River, Jacaré Grande River, Vieira Grande Bay and the Atlantic Ocean.

From approximately 400 BC to 1600 AD, Marajó was the site of an advanced pre-Cabraline society called the Marajoara culture, which may have numbered more than 100,000 people at its peak. Today, the island is known for its large water buffalo population, as well as the pororoca tidal bore periodically exhibited by high tides overcoming the usual complex hydrodynamic interactions in the surrounding rivers. It is the second-largest island in South America, and the 35th largest island in the world.

Belém (Portuguese: [beˈlẽj]; Portuguese for Bethlehem; initially called Nossa Senhora de Belém do Grão-Pará, in English Our Lady of Bethlehem of Great Pará), often called Belém of Pará, is the capital and largest city of the state of Pará in the north of Brazil. It is the gateway to the Amazon River with a busy port, airport, and bus/coach station. Belém lies approximately 100 km (62.1 miles) upriver from the Atlantic Ocean, on the Pará River, which is part of the greater Amazon River system, separated from the larger part of the Amazon delta by Ilha de Marajó (Marajo Island). With an estimated population of 1,398,531 people — or 2,491,052, considering its metropolitan area — it is the 12th most populous city in Brazil, as well as the 16th by economic relevance. It is the second largest in the North Region, second only to Manaus, in the state of Amazonas.

Founded in 1616 by the Kingdom of Portugal, Belém was the first European colony on the Amazon but did not become part of Brazil until 1775. The newer part of the city has modern buildings and skyscrapers. The colonial portion retains the charm of tree-filled squares, churches and traditional blue tiles. The city has a rich history and architecture from colonial times. Recently, it witnessed a skyscraper boom.

Marajó Bay, the Marajoara Gulf or the Amazon Gulf, is a recessed body of water of the Brazilian coast located in the state of Pará. It is roughly 4,500 km (1,700 sq mi) in size, and is a receptacle for the waters of the Pará River distributary channel, the waters of the Tocantins basin and the waters of the Guajará Bay, serving as the eastern aquatic border of both the Marajó Island and the Marajó Archipelago. Marajó Bay is an estuary consisting of both salt and fresh water, resulting in the classification of an estuarine system. Marajó Bay also receives sediments from the Amazon River through the Breves Channel as well as the Pará River, causing the water to be turbid.

The Pará River and a small fraction of the Tocantins River are the main providers of water for Marajó Bay. The river is vital to the surrounding areas and makes different areas and ports very accessible for locals. The river also eventually clashes with salt water from the Atlantic Ocean in the Marajó Bay. A circulation process of salinity, temperature, and tidal range in the two different water types occurs in the bay as they meet. Salt water concentrations are also impacted by low water periods of the Amazon River.