The source of the Amazon is the lake, Lauricocha, in the Peruvian Andes. The river is known as the Maranon in its upper course in the Andes. The length of the Amazon is measured from the source of the Ucayali river, which joins with the Maranon to eventually form the Amazon.

The Pacific Princess on the Great Amazon River.

Northern Brazil is drained by the Rio Negro, which joins the Amazon to give it full strength before flowing into the Atlantic. The flow of the river has an average velocity of 1.5 miles per hour, which increases greatly at flood times. The powerful discharge at the mouth of the Amazon measures about eight trillion gallons a day, 60 times that of the Nile and eleven times that of the Mississippi. The annual average discharge is 6,350,000 cubic feet per second ("cusecs") into the Atlantic, rising to over 7,000,000 cusecs during a flood. The mouth of the Amazon is more than 250 miles wide.

Most of the Brazilian part of the river exceeds 150 feet in depth, although some parts near the mouth have recorded depths of as much as 300 feet. The width of the river ranges from one mile to 35 miles.