Why does the amount of water that flows in a river change during the year?

There are multiple factors that influence the amount of water that flows in a river.
1. A factor upstream like rain can cause the water level downstream to rise for a short period in time.
2. A blockage in a runoff stream can cause the main rivers water level to rise due to the lack of available runoff path for the water to take.
3. Also the melting of snow can cause the water level to rise as well.

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