How do bodies of water exist above ground?

Question

How come bodies of water (lakes, rivers, wetlands, oceans even…) exist above ground instead of the water simply being absorbed by the earth?

In: 855

Answer 1

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.