Groundwater:
About 18% of fresh water is in the ground.
So, are there underground rivers and lakes? No, it is more like wet dirt.
Where does that water come from?
When it rains, the water infiltrates through the soil until it hits something it cannot go through (called an aquitard).
The rate of infiltration is based on how porous and permeable the ground it.
Porosity is how holey the ground is.
Permeability is how easily water goes through it.
Usually, as porosity goes up, permeability does.
Water has to permeate the ground to get groundwater. When it infiltrates, it goes down until it hits an impermeability layer that it cannot get through. This could be rock or clay. The water pools up on top of this layer making an aquifer - an area of saturated soil. The ground it passed through stays wet due to capillary water - the water that sticks to things.
As you can see, one place may have multiple aquifers under it. Riverside has five under it.
All but the top aquifer are "confined," or capped, by an impermeable roof.
Those aquifers may be sloped and the aquifer flows slowly downhill.
There is an interesting formation (called an artisian formation) caused by a sloped and confined aquifer:
Some aquifers are capped. These are called confined and can lead to artesian formations where the water is pressurized and shoots up.
They also can lead to oases.
Or springs:
This is one of the two ways rivers are "born." The other is when erosion digs down to the groundwater.
The height of the water table is largely based on how shallow the aquitard is, how much it has rained, and how warm it has been.
Saltwater incursion:
Groundwater alters the landscape and has many effects. Here are some:
Geysers:
As water flows through the ground, it dissolves dissolveable things and deposits minerals it has been carrying. This makes fossils, petrified wood, caverns, stalactites, stalagmites, veins of ore, sinkholes, karts landforms, and hardwater / scale.
Caverns:
With stalactites and stalagmites. C for cieling and G for ground
Sinkholes:
Karst land:
Fossils:
Veins of ore:
Hard water is what we call water with lots of minerals in it.
As water goes through the ground, it deposits biological contaminant and gets purified.
So, groundwater alters the land it goes through significantly.