Why dose the amount of water in a river change during the year

Hello! 
The amount of water in a river varies for each particular area. Whether the area has been receiving rainfall or a drought. 
Seasons also play a role; if it's Spring, you may receive more rainfall and have a higher water levels. If it's Summer, you may experience a drought, causing lower water levels.
Although that was not very much information, I thought I'd provide a small bit of information. Hope it helps. :)

To make the answer very very simple its EVAPORATION