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A river is an open curve of water.

What is a name for a closed curve (e.g. a circle) of water?

For example, what is the name of the digging around castle walls, that is filled with water to prevent enemies from getting near the wall?

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up vote 7 down vote accepted

If a body of water is static rather than flowing then its generic name would be a lake, regardless of its shape. If it is small, it may be called a pond.

Man-made features may have names specific to their purpose, such as the moat already referred to, or a canal or a reservoir. A canal could be circular, such as a transport 'ring-road' around a city, eg in Amsterdam.

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If a curve of water is almost a circle of water but is not completely closed then it is an "oxbow lake". – QuentinUK Mar 31 '14 at 15:18

That is called a moat:

A deep, wide ditch surrounding a castle, fort, or town, typically filled with water and intended as a defence against attack.
Oxford Online Dictionaries, s.v. moat

In other contexts it has no particular name.

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