system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How do lakes that feed rivers never go dry

How do lakes I.e a lake in a mountain that feeds a river flowing down the mountain never go dry? There is so much water being moved away from it how does it maintain its mass?
gpt
Ever fill up an ice cube tray held at an angle under the tap?

The water starts down and fills one hole until its full, when full it spills over into the next. The relationship between the lake and river in your question is the same. 

The lake is filling from small streams, rivers and sometimes even springs that are under the water. Once the water has filled the hole it move down the line causing your river. When there is lots of water coming in the river rages (like spring thaw) in the summer it might slow to a trickle. Just like the ice cube tray, the water in and the water out have  a balance.