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When someone says that a river drains a city, for example:

The river Thames drains the whole of Greater London.

What does it mean?

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Basically, this refers to water drainage.

So, all of the rainwater, wastewater (from baths, showers, and other processes that produce wastewater). Also included are any rivers and streams. In London everything leads to the Thames - nothing is drained away to other outlets away from the Thames.

A useful read on this is here, and you might also want to research Joseph Bazalgette, who created London's sewer system.

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It means that river is the center of water catchment in the city. Particularly for the city it means that all the city's sewage goes to the river.
In phrases like "a river drains a valley" for instance it means that all the valley's water (streams, rivulets, etc) flows to the river and flows away.
You may find additional information here:
Meaning of drain in English
Definition of drain

  • Hi Alex, when you say "ribbons" do you mean "streams" or "rivulets" or are you thinking of the term "ribbons of water"? "Ribbons of water" does exist but is really a common picturesque metaphor for streams or channels, usually through marshes, river deltas and so on. – BoldBen Feb 12 at 10:57
  • @BoldBen Thank you so much for your correction. Yes, I meant "streams" or "rivulets", the word "ribbons" is inappropriate here. It's already edited. Thank you. – Alex Kuchin Feb 12 at 13:06

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