A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake . A mature river is fed by many tributaries and has more discharge than a youthful river. Its channels erode wider rather than deeper. Examples.
A mature river is somewhere inbetween a youthful river and an old river. The bottom of youthful rivers tends to be a sharper V-shape, while mature See full.
According to About.com, mature rivers are bodies of water that flow over wider valleys and snake along flat plains. The meandering streams then create larger.
Mature river is a river with a gradient that is less steep than those of youthful rivers and flow.