system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why aren't ground water reserves filling up when it is raining, doesn't the water just seep through the soil back into the reservoirs?

Does the rain coming from oceans resupply the water flowing into them from rivers and if not how ARE there still rivers to flow? Shouldn't they have drained?
gpt
Geotechnical engineer here:      
   
When it rains, some water is evaporated, and some of it seeps down into the ground, through the pore spaces in soil, and through cracks in rocks. It keeps going down until it hits a confining layer like clay or bedrock. At that point, the water starts to pool up.   It can still flow into the clay and into the cracks in the bedrock, but this is at a very slow rate. If the influx from rain and snowmelt is higher than the the amount that slowly seeps deeper into the ground, then the water table will rise.  If the water table rises above the surface, then water will start to pool on the surface.   
  
Notice how lakes are usually at a low point in elevation. This is because the surface of the ground is lower than the water table in that spot.     
  
If it is only raining in one spot, the water will spread out laterally to drier spots, like how water spreads out when you pour it onto a table.  Water spreads out a lot slower in soil than in air though, so if you have very intense rain in one spot, it can start to flood before it starts to spread out.   
The opposite effect can happen if pump too much water from a well in a short amount of time:  It can lower the water table in that spot, and it could take a while for the water to flow laterally and raise the water table back to where it was. 
  
  
There are a lot of other complicated processes that can come into play, but this is the basic principle.