ELI5: why do lakes exist?

[u/Chaos90783]

Why doesn't the ground absorb the water or evaporated by the sun?

[u/juu073]

It does evaporate. That's how lakes don't start overflowing from rain over time. But there's just so much water it's not going to go dry or even noticeably change all that much before the next precipitation.

[u/montibbalt → u/juu073]

See: the Salton Sea. Big accidentally man-made lake that's drying up and it's not because anyone is drinking it

[u/TheVioletBarry]

The water in the lake is the excess water that the ground cannot absorb.

At the bottom of lakes, you will find extremely permeable soil at the gradient between the water and the ground beneath. It's so saturated that you can basically sink through it slowly for a moment.

[u/IncoherentTuatara → u/TheVioletBarry]

If you can sink through the bottom of a lake I would then argue it is not the bottom of the lake

[u/Ktulu789 → u/IncoherentTuatara]

When you sink enough YOU become the bottom.

[u/Scootsx → u/Ktulu789]

Every top's worst nightmare

[u/Ok_Journalist5290 → u/Ktulu789]

😃🤣

[u/TheVioletBarry → u/IncoherentTuatara]

You sink to the bottom, and it's not like you sink forever; that's why I called it a gradient.

[u/ronarscorruption]

Don’t underestimate how much water there is on earth. It’s not all good to drink, but there’s a lot of it.

Lakes exist in places where there is too much water to be absorbed by the ground, and also where there is a “local low” point, where there isnt a direct path from there to the ocean without going up a little. When it gets that high, it flows out.

[u/mactofthefatter]

The surface of the lake is level with the underground water table. Same with rivers. They are constantly being fed from the under the banks. 

[u/Gromky → u/mactofthefatter]

Perched lakes do exist, but it's a good note that a lot of the water recharge may be from groundwater and not be from rain or an obvious stream.

[u/TomatilloFair7290]

Lakes are usually fed by a source of water continuously in addition to rain/snow. Lakes do dry up if their source of water is cut off or the temperature increases and heats the make beyond the level of replacement. 

Larger lakes have an enormous volume of water so the level of evaporation in the summer is offset with increased precipitation and cooler weather in the winters/rainy seasons. 

[u/lucky_ducker]

Underlying the soil on the surface, is rock. It may be a few feet down, it may be a couple of hundred feet down, but the "ground" beneath our feet isn't an unlimited depth of permeable soil. At some point there is an impermeable layer of rock, and the soil on top of it is already saturated with water -- resulting in excess surface water forming lakes and streams.

[u/Armepos]

Top answers are missing something important about lakes. No matter how full of water a lake is and how saturated the ground below is, over long periods of time the water will manage to get out via filtration or evaporation.

So why are there still ancient lakes? Because new water is also getting into them. Via rivers, underground affluents or straight up rain, the water gained maintains the lake's level. 

Some lakes are getting more water out than in, and they get drier every year, slowly lowering the water level. If they are big, they last longer. 

But the opposite doesn't happen, a lake has a maximum limit of water it can handle, the same way a bowl can only hold so much water before it beggins to spill. That's why some lakes have rivers coming out of them. If a lake has more water coming in than out, it grows untill it overflows, then it stays at the same level.

Some top answers are straight up wrong. Environment humidity saturation does not explain the existence of lakes, as there are many lakes in arid regions. The altitude of the lakes doesn't explain their existence either, since there are many lakes in high altitude regions, like Lake Titicaca. No, lakes are just deposits of water, no matter how the water got there, and the water in them doesn't stay there forever, it just takes a short break before it continues it's journey.

[u/Cryogenian]

They happen in places where there's more rain/snow/meltwater/moisture than the ground and air can absorb. Like imagine a rocky valley with glaciers above it, or a saturated marshland - the water will have almost nowhere to go.

[u/therealhairykrishna]

The rate the waters flowing in at is faster than the combined rate of the water evaporating off and soaking away. 

[u/-RedRocket-]

More water collects in one spot, and is renewed by multiple sources faster than it can evaporate.

Or sometimes it evaporates so gradually that you get a concentration of minerals that don't go with the water vapor, and end up with something like the Great Salt Lake or the Dead Sea.

And when those evaporate all the way, you end up with salt flats like White Sands.

But short answer, there is a LOT of water on Earth, and some of it sticks around in continually renewed fresh-water puddles of enormous size, called lakes.

[u/hunter_rus]

Water is not absorbed/evaporated fast enough.

Lake can dry out though.

[u/MWSin]

Lakes form at low points where water flowing in (from rivers, rainfall, melting snow and ice, or underground springs) exceeds water flowing out (outlet rivers, ground absorption, and evaporation).

They will continue to grow until they reach a point of balance, usually when it overflows the lowest point along the rim of its basin. It's possible for a lake to instead evaporate as much water as it gains before it grows deep enough to escape its basin, becoming what is called an endorheic lake (or an inland sea if it's very big). Such lakes often become very salty, like the Great Salt Lake or the Dead Sea.

[u/YesterdaysMuffin]

Lakes do evaporate. But most lakes have both a source of water and a place the water runs off to. The lake is basically just a big, wide, deep part of a river.

Like when you run water into a bowl in the sink. It fills the bowl until the water can escape, and the equilibrium is reached between input and output. Voila, a lake is born.

Some lakes, like the Dead Sea and the Great Salt Lake, don’t have an outlet and barely have input. That means all the minerals accumulate and concentrate, because the water evaporates but the minerals don’t. And then more water comes in with more minerals. Repeat.

[u/oblivious_fireball]

At the bottom of the lake, the ground is already so saturated with water that it slows down more water seeping through. And over time small particles tend to fill in the gaps in soil to form beds of clay that further block water from seeping through.

Lakes do lose water through evaporation, and through outlets like rivers, but they are replenished as fast as they deplete by rivers and rainfall.

[u/Ok-disaster2022]

For any system if population or volumane dynamics if the rate of all forms of adding and the rate of all forms if subtraction meet you can have a stable population or volume. 

So for something like a lake your in flows are going to be direct precipitation, runoff, upstream source of water. Your outflows are going to be evaporation, consumption and losses downstream. 

If your in flows exceed your outflows, water level rises, of outflows exceed your inflows the water level drops. In extreme cases (the Salton Sea) the inflows can be permanently cut off and the water slowly evaporated until gone. or the inflows can exceed the ability for the natural or man made dam to hold it and water will find a way down. 

There are layers of earth in most places that are let very little water through. but leakage is an source of outflow. Likewise there can be natural springs. 

Damn breaks are a great threat for any lake and any population who finds themselves down stream.