How come bodies of water (lakes, rivers, wetlands, oceans even…) exist above ground instead of the water simply being absorbed by the earth?
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Imagine you have a cup without any holes in it. Now, imagine that cup made out of layers of dense rock.
There’s a limit to how much water the ground can absorb. When the ground is full of water, the extra water sits on top.
If you’re near a body of water, and you dig a hole, you’ll find the underground water pretty quick.
1) They *do* sink into the ground. Eventually it’ll hit an *impermeable* layer of dense bedrock that the water can’t go through. The ground will become saturated and no more water can drain down.
2) The ground slows the water down: imagine a cup with small holes in it. The water will drain out, but as long as water goes *in* faster than it drains, the water level will stay elevated.
3) Some kinds of soil are less permeable to water, like clay. Rock, obviously, doesn’t allow water to sink through very easily.
4) Water that drains through the soil will eventually either sink into an aquifer or get forced back up from pressure and form a spring.
One big misconception about lakes is they’re like a cup of water. They get filled and stay that way. Nothing is further from the truth. All lakes are constantly being fed by springs or other flowing waters. At the same time, all lakes have an outflow down a river, stream or into a swampy area which acts like a broad slow moving river. If the rate of drainage in a low lying area is less than it’s rate of fill then you get water building up. If the rate equals out then you simply have a river or stream. And rivers and streams do lose some water through seepage into the ground but the water is moving too fast for any significant loss of water per volume flowing over an area.
Now a couple geological conditions help lakes reatin their water. Chiefly, the bottoms of a lake is either made up of tight nonporous soil like clays or is part of the bedrock itself. If the bottom of an area with a large inflow of water is not like that, that’s when it becomes a swamp because the soil becomes oversaturated.
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Go to the cabinet. Take out a glass. Go to the sink and fill the glass. Now place the glass on the counter.
The glass represents part of the landscape. It’s an area of rock that can hold water — either there’s no cracks in it, or the cracks are small enough it takes a while for the water to get out – let’s call it a basin. The counter holding the glass off the floor is just whatever dirt and rock did underneath that keep it above sea level.
The faucet represents the source of water: rain (or snow). It either falls directly into the basin, or it falls somewhere uphill of the basin and pours downhill as a stream, or water soaking through the dirt an gathers in the basin.
Water is there because it fills faster than it empties. Maybe it’s difficult for the water to get out, maybe water is being added really quickly. Whichever some water backs up to that spot and stays there long enough to form a body of water (anything from a puddle to a great lake).
Any time you see standing water you are seeing a tabletop: the top of the *water table*. This is exactly where the ground stops being 100% saturated. Imagine you take a bowl full of dirt. Scoop out a handful of dirt and make a little hole. Now pour water in until the depression fills with water. It’s the exact same as a lake. The ground at the level of the water table is saturated.
It’s complicated but basically there’s an insane amount of water beneath our feet. Most fresh water I believe is underground. All surface rivers and streams in the world account for something like .001% of fresh water. But there are permeable and impermeable layers of rock in the earth so water gets trapped in certain layers.
It has, to an extent. I think a lot of people here underestimate just how mind bendingly wet rock actually is.
https://www.bbc.co.uk/news/science-environment-34837461
When drilling oil wells you need to constantly fight the combination of this liquid (and often gas) and the pressure it is under, otherwise you get what is known as a kick where all that wet comes back to surface.
Some commenters have left some excellent simple answers like the cup analogy so I’ll give you the more complicated response.
First lets look at rivers. Rivers are the transport of water that has fallen as rain overland, from natural springs or is sourced from melting glaciers/ice sheets. Rivers can exist above ground, but they can also exist below ground, especially in areas dominated by limestone. All rivers will flow towards what we call “base level” which I won’t get into here, but all you need to know is that the ultimate base level is sea level. There are local base levels as well which are not related to the ocean which you would find in areas far away from the coastline. So due to gravity, a river will always flow either to the ocean, or to a local base level which is some depression that exists in the landscape. These depressions commonly form in areas that were previously covered in large glaciers where the ice scoured out a large “hole” in the ground or in mountainous areas. Once you have a depression like this, and you have inflow of water either from a river or from groundwater you have the potential for a lake to form.
For a lake to exist you need a couple of things, first being a positive hydrological balance where the amount of water entering into the lake exceeds the amount of water leaving the lake (evaporation, drainage from an outflow or seepage through bedrock). The other thing you need is layer under the lake that does not allow water to pass through (this is call an impermeable layer). This could be clay, a particular type of rock or it could even be a layer of built up organic matter. So lakes exist both “above ground” but also below it, as water slowly percolates down into the ground until it can’t anymore. An important thing to note is that no lake is every permanent. Over geological time, all lakes will either infill with sediment turning them into wetlands, or something will happen geologically that causes the lake to drain
Oceans are a bit different but very simply put, operate the same as lakes. Ocean basins are the depressions that have formed between the continental land masses. Ocean water does seep into the underlying bedrock, but it happens very slowly.
In detail it is actually a quite complicated question. But one of the most important parts is how the bodies of water above ground interact with groundwater below the ground. Groundwater is the water that fills spaces between grains in soil or rock or fills fractures or cavities in the rock.
Most rivers and lakes are directly connected to the groundwater. They can gain water from the groundwater or lose water to the groundwater. Or gain water in one place and lose water in another. But overall, most river and lake systems gain water from the groundwater. And where does the groundwater come from (aside from from rivers and lakes)? Precipitation on hills and uplands above the rivers seeps into the ground until it reaches the top of the saturated groundwater (called the water table). Then the groundwater slowly flows to rivers, lakes, or the ocean. This is enough to keep most rivers flowing between rainstorms and keeps lakes full of water.
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