It cannot be changed. The expression comes from the idea that water that has fallen over a dam cannot be brought back again.

VOA: special.2009.05.10

Nietzel says the most common place the clues about "where warm waters halt" first lead people is to Eagle Nest Lake, about 100 miles north of Santa Fe, because it has a dam that holds back warm water and is known for its brown trout.

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Two days later, the SEALs flew back to Dam Neck, their base in Virginia.

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Egypt, for instance, seems to be stuck with the catastrophic Aswan dam, which is holding back the Nile's silt.

ECONOMIST: Water