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.. :-)
The word Niagara (modern English pronunciation nai-æg-era) is believed to be derived from the Mohawk word "ohnyákara", meaning 'neck of land (between the lakes)
A mature river is on a shallower slope and it doesn't have rapids or waterfalls.
(Waterfalls: steep fall of water, as of a stream, from a height.
Rapids: part of a river where the current is swift.)