Re: why don't rivers just drain into the ground like water poured on the ground

Hi Miles

You asked why rivers don't drain into the ground like water from a hose.  
Good question, something has to be holding it up, right?
The answer is so close to your question, I bet you know it already.

Where you're from, there's a whole lot of water already there in the 
ground.  It's called, simply enough, "ground water."  (I say "where you're 
from..." because out here in the desert and many other places in the 
world, some rivers actually do just run into the ground)

Ground water is where we get most of our city water from, and household 
wells tap right into it.  It's what makes basements damp, it's why water 
will show up in a deep hole, even if it hasn't rained in a while, and it's 
also why rivers (and lakes and big mud puddles) don't seep into the ground.

Out here in New Mexico, the rivers often do just run into the ground.  
They start in the mountains where there is enough water to fill the spaces 
in the dirt (we say the soil is "saturated"), and the excess finds a low 
spot and forms a river.  The other way that rivers can form is when water 
flows faster than the ground can soak it up.  

Water is usually pretty predictable, but soil can have different 
properties.  When there's a lot of clay in the soil, water will fill a few 
of the spaces between the clay and make a layer that's really hard to pull 
water through.  Sand, on the other hand, usually lets water pass right 
through.  

Here's the Answer: The water doesn't soak down into the ground because 
there's already water there underneath the river. 

To make an experiment out of it, find a slope in your yard, even if it's 
just a small one, and on a nice, clear dry day, turn the hose on one turn 
(or whatever's convenient) and measure how long your river is from the 
hose opening to where you can no longer see water sitting on the surface 
of the soil.  Then, just after it stops raining some day (when the soil is 
well saturated), do the same thing and make sure you use the same hose, 
same slope, things like that, and measure how far the water goes before 
you can't see it on the surface.  

A useful question to ask when you get done with the experiment is how long 
you have to water the lawn or the plants for the water to "soak in" deep 
enough for the roots to get to it.  And which is more efficient at your 
house, to dump a gallon of water on your plants, or sprinkle it on like 
rain?  You might be able to find out from someone in your house where 
the "water table" is.  That's the top of the saturated zone that we know 
as ground water.  If you were to dig down to just above the water table, 
you'd be digging through moist or mucky soil.  If you dug into the water 
table, your hole would fill up with mud-colored water.  

The study of water flow (yup, it's got motion, chemistry, and a name...) 
is called Hydrology.  It's a really cool field that mixes physics and 
chemistry and geology in an aqueous solution.  Glad you're curious about 
it!  --Sarah