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Using points and examples from the lecture, describe two ways that lakes can disappear.
请用课上的观点和实例，描述湖泊消失的两种方式。
Listen to part of a lecture from a geology class.
请听地理课上的部分内容。
So, when we look at lakes, they seem to be permanent, we assume they'll be around forever.
我们在看待湖泊时，总是觉得它们似乎是永恒的，总是以为它们会永远存在。
But in fact, lakes aren't permanent; they can actually disappear.
但是事实上，湖泊并不是永恒存在的，它们实际上也会消失。
Sometimes they disappear through natural processes, and sometimes because of human activities.
有时候它们会随着自然过程而消失，有时会因为人类活动而消失。
First let's look at one way that lakes can disappear naturally, and that is, by gradually getting filled in with organic sediment.
首先让我们来看看湖泊自然消失的方式，那就是渐渐被有机沉淀物填满的方式。
This often happens with lakes that have lots of plants growing in them.
这种情况通常发生在有许多植物生长在其中的湖泊上。
When the plants die, they break down into a muddy substance, which falls to the bottom of the lake,
当植物枯萎时，它们就会分解成泥状物质，沉到湖底。
they're then replaced by new plants, which eventually also die and fall to the bottom.
然后就会被新的植物所代替，而这些新的植物最终也会枯萎并沉入湖底。
And over the years, all this dead plant material builds up on the bottom of the lake,
许多年后，所有这些枯萎的植物便会在湖底不断堆积，
and as builds up, it starts to fill up the lake, and there's less and less room for water,
在堆积过程中它们便会开始充填湖泊，而湖水所占空间也会越来越少，
and eventually the lake gets completely filled in, and disappears.
当湖泊最终完全被填满时，它就消失了。
OK, lakes can also disappear, pretty rapidly sometimes, as a result of human activities.
好了，有时候湖泊也会由于人类的活动迅速消失。
For example, we know that farmers need water to irrigate their crops.
比如，我们都知道农民需要水来灌溉庄稼。
And sometimes to get that water, they pump that water out of a nearby lake.
有时候为了获取水源，他们会从附近的湖泊中抽水出来。
They install pipes that run from the lake to their farms, and they pump the water out of the lake and into their fields.
他们在湖泊和农田之间拉起水管，然后把湖里的水抽上来，湖水就会顺着水管流入他们的田里。
Now that's ok if the lake is continually being refilled with rainwater or with water from streams that run into the lake,
如果这片湖泊能够持续被雨水或者汇入的溪水补充的话，也并无大碍，
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.
但是如果没有足够的雨水或溪水来替代被农民抽出的湖水的话，这片湖泊将会渐渐枯竭。
