Before there is no trash can, garbage everywhere, people put the rubbish into the bottom of the river.

之前没有垃圾桶，垃圾扔的到处都是，有人把垃圾到清澈见底的小河里。

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As the algae dies, it sinks to the bottom of the river where it breaks down or decomposes. It then takes oxygen from the water.

随着藻类死亡，它会下沉到河底并分解，然后会带走水中的氧气。

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As the algae dies, it sinks to the bottom of the river where it breaks down or decomposes. It then takes oxygen from the water.

随着藻类植物死亡，它便会下沉到河底发生分解，然后带走水中的氧气。

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PCBs are now embedded at the bottom of the river, getting buried under more and more mud as time goes on.

FORBES: Fact and Comment

But by the early 1900s, the bottom of the river had been scraped and flattened to allow for the logging industry to float timber downstream.

WSJ: Real Rapids in Grand Rapids?

Our tour followed a route along the bottom half of the Fleet River, the biggest and most famous of London's former waterways.

BBC: In search of London's lost rivers