[This is based on the preliminary 3-second DEM. Publication has been on hold since September, 2003 because Rolf would like to use other means of checking the boundary where the DEM disagrees with other maps.]
There are various ways to define the basin. The full tri-colored basin includes the southern nagivation channel, the city of Belém, and the Tocantins River. This basin has an area of 6,903,034 square kilometers.
A more popular definition of the basin (red plus green) has an area of 5,973,851 square kilometers.
A third definition would be the area upstream of the city of Óbidos, the highest point of tidal influence. It has an area of 4,692,200 square kilometers.