The length of the Amazon River is 6,575 km or 4,086 miles. Source: S. Liu, P. Lu, D. Liu, P. Jin, W. Wang, Pinpointing the sources and measuring the lengths of the principal rivers of the world,...
Amazon River is 11 million years old already and its not too wide and releases 300 000 cubic centimeter per second so it is a mature river base to its characteristics.. :-)