英语听力：自然百科 泰晤士河

The River Thames runs over 200 miles through the British countryside and on to the city of London. Each day, thousands of visitors enjoy the pleasures of just being on the river. But as little as 40 years ago, this body of water had become one of Europe's most polluted rivers with a noxious legacy.

In 1858, the British Houses of Parliament were evacuated when the stench from the River Thames had become unbearable. Untreated sewage had been leaking straight into the Thames. Victorian engineer Joseph Bazalgette remedied the situation by designing an extensive network ofsewers. However, a century later, the growing population has stretched it to the limit. The majority of the old system is still in use today. It's simply too expensive to replace it, and it's still causing big problems.

"And that's Western Pumping Station. It's part of the original Victorian sewage system for London. When the system is overloaded by rainfall, we have this problem again with overflowing sewage, and that pumping station can discharge up to about 50 thousand tons an hour with mixture of untreated sewage and urban run-off."

The sewage causes oxygen depletion in the river, killing fish and other marine life. Enter one of these vessels. This is the Vitality, and it can pump up to 30 tons of oxygen into the water in a day. The whole process of injecting river water with pure oxygen takes only a few minutes. The irony is that the only reason a system like this is necessary is because the Thames is now so much cleaner than it was even 50 years ago. Fish have returned. It's one of the most important fish nurseries in Britain with around 120 species.

"The Thames is fantastic asset. It's a flood defense mechanism and drains the whole basin. It supplies, for instance, 2/3 of London's drinking water. I really like it to be a busy, active river which is sort of appreciated and enjoyed by all types of people, yet one which is stillrenowned as being a haven for wildlife."

The regeneration of this river is a wonderful success story. But it will be a constant battle to keep it that way.