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Using points and examples from the lecture, describe two ways that lakes can disappear.
請用課上的觀點和實例，描述湖泊消失的兩種方式。
Listen to part of a lecture from a geology class.
請聽地理課上的部分內容。
So, when we look at lakes, they seem to be permanent, we assume they'll be around forever.
我們在看待湖泊時，總是覺得它們似乎是永恒的，總是以為它們會永遠存在。
But in fact, lakes aren't permanent; they can actually disappear.
但是事實上，湖泊并不是永恒存在的，它們實際上也會消失。
Sometimes they disappear through natural processes, and sometimes because of human activities.
有時候它們會隨著自然過程而消失，有時會因為人類活動而消失。
First let's look at one way that lakes can disappear naturally, and that is, by gradually getting filled in with organic sediment.
首先讓我們來看看湖泊自然消失的方式，那就是漸漸被有機沉淀物填滿的方式。
This often happens with lakes that have lots of plants growing in them.
這種情況通常發生在有許多植物生長在其中的湖泊上。
When the plants die, they break down into a muddy substance, which falls to the bottom of the lake,
當植物枯萎時，它們就會分解成泥狀物質，沉到湖底。
they're then replaced by new plants, which eventually also die and fall to the bottom.
然后就會被新的植物所代替，而這些新的植物最終也會枯萎并沉入湖底。
And over the years, all this dead plant material builds up on the bottom of the lake,
許多年后，所有這些枯萎的植物便會在湖底不斷堆積，
and as builds up, it starts to fill up the lake, and there's less and less room for water,
在堆積過程中它們便會開始充填湖泊，而湖水所占空間也會越來越少，
and eventually the lake gets completely filled in, and disappears.
當湖泊最終完全被填滿時，它就消失了。
OK, lakes can also disappear, pretty rapidly sometimes, as a result of human activities.
好了，有時候湖泊也會由于人類的活動迅速消失。
For example, we know that farmers need water to irrigate their crops.
比如，我們都知道農民需要水來灌溉莊稼。
And sometimes to get that water, they pump that water out of a nearby lake.
有時候為了獲取水源，他們會從附近的湖泊中抽水出來。
They install pipes that run from the lake to their farms, and they pump the water out of the lake and into their fields.
他們在湖泊和農田之間拉起水管，然后把湖里的水抽上來，湖水就會順著水管流入他們的田里。
Now that's ok if the lake is continually being refilled with rainwater or with water from streams that run into the lake,
如果這片湖泊能夠持續被雨水或者匯入的溪水補充的話，也并無大礙，
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.
但是如果沒有足夠的雨水或溪水來替代被農民抽出的湖水的話，這片湖泊將會漸漸枯竭。
