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.
A well-tested explanation for a set of observations or experimental results is a theory. When we're talking about theories, we're talking about a set of hypothesis about a certain topic which have been shown to work and be correct but aren't completely proven up to the point where we could say they always hold true.
