NARRATOR：

Listen to a part of a lecture from a geology class.

FEMALE PROFESSOR：

So, when we look at lakes, they seem to be permanent, we assume they'll be around forever.

But in fact, lakes aren't permanent.

They can actually disappear.

Sometimes they disappear through natural processes and sometimes because of human activities.

First, let's look at one way lakes can disappear naturally.

And that is by gradually getting filled in with organic sediment.

This often happens with lakes that have lots of plants growing in them.

When the plants die, they break down into a muddy substance, which falls to the bottom of the lake.

They're then replaced by new plants, which eventually also die and fall to the bottom.

And over the years, all this dead plant material builds up on the bottom of the lake.

And, as it builds up,

it starts to fill up the lake,

and there's less and less room left for water ...

And eventually, the lake gets completely filled in.

And disappears.

OK. And lakes can also disappear¡ª

pretty rapidly sometimes¡ª

as a result of human activities.

For example, we know that farmers need water to irrigate their crops.

And sometimes, to get that water, they pump the water out of a nearby lake.

They install pipes that run from the lake to their farms, and they pump the water out of the lake and into their fields.

That's OK if the lake is continually being refilled with rainwater,

or with water from streams that run into the lake.

But if there isn't enough rainwater or stream water to replace the water the farmers take out of the lake,

the lake will eventually dry up.