Rivers erode material from one location, and then deposit that material in another location.
Rivers are like conveyor belts in that sense, and its always on! Rivers begin life as mountain streams usually. As the water comes down from height and passes through different rock and minerals, it erodes these materials at different rates according to how hard and resistant the materials are.
The eroded particles are absorbed in the river (suspended) and washed away down stream where they will be deposited according to the geology of the area it happens in.
This will then form features of deposition. This is the life cycle of the river and you can tell the age of rivers by knowing about these process of erosion and deposition as you follow its course down to the sea which is where most rivers end, or they flow into another river.
Hope this helps!