The river level increases when the discharge increases because a high discharge means there's more water in the channel. This means the factors that increase discharge can also cause flooding:
Prolonged Rainfall
After a long period of rain, the soil becomes saturated. Any further rainfall can't infiltrate, which increases runoff into rivers. This increases discharge quickly which can cause a flood.
Heavy Rainfall
This means there's a lot of runoff. This increases discharge quickly, which can cause a flood.
Snowmelt
When a lot of snow or ice melts it means that a lot of water goes into a river in a short space of time. This increases discharge quickly, which can cause a flood.
Relief
If a river is in a steep-sided valley, water will reach the river channel much faster becuase water flows more quickly on steeper slopes. This increases discharge quickly, which can cause a flood.
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