Crossword Genius

South to face drought when there's no flow? (5,5)

Ross

I believe the answer is:

slack water

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'flow?' is the definition.
The definition and answer can be both related to natural events as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand how the remainder of the clue works.

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I've seen this clue in The Globe and Mail.
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