system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why does there continue to be water in a river, even though it doesn't rain all the time in the upper parts?
gpt
It's called base flow.

Basically there's a bunch of water underground. It seeps into the rivers. So, usually it's a combination of the water cycle and base flow. But when the water cycle is interrupted (drought), the base flow is still running. 

So you get lower water levels, but still running.