Deposition
To transport load a river needs to have energy so when a river loses energy it is forced to deposit its load. There’s several reasons why a river could lose energy. If the river’s discharge is reduced then the river will lose energy because it isn’t flowing as quickly anymore. This could happen because of a lack of precipitation or an increase in evaporation. Increased human use (abstraction) of a river could also reduce its discharge forcing it deposit its load.
If the gradient of the river’s course flattens out, the river will deposit its load because it will be travelling a lot slower. When a river meets the sea a river will deposit its load because the gradient is generally reduced at sea level and the sea will absorb a lot of energy.