One explanation that I found was rainfall but it is not apparent that rainfall could contribute to such volume of water.
In: Geology
When it rains on top of your roof you get a substantial flow collected by your gutters and down through your water spout.
A hill is similar to roof roof except that the water flows over/through soil and rock instead of across your roof and discharges out springs instead of a water spout.
There is obviously more to it than just that, but that is an ELI5 version.
For those interested, water will also move more slowly through rock/soil which means your peak flows will be lower during rainfall relative to a roof, but they will continue for much longer. This is why springs can continue flowing days, months, or even years after the last major rainfall event. Some water will also be lost due to evapotransipiration or will migrate to depth.
Source: Hydrogeologist, water flowing underground is my thing.
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