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Suppose I use a tool to create a circular wave in the river. If there are two fish swimming 1m from the source (2m from one another), they will both feel the wave at the same time.

What will happen if the river flows from one fish to the other? How will it affect the waves?

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Hi Bob, I can't understand your question..! – Waffle's Crazy Peanut Sep 16 '12 at 4:00

Assuming the river is deep enough or equivalently the waves are small enough compared with the depth.

Relatively to the bank the wave will remain its circular form, however as a whole it will be moving with the river. So, assuming the fishes swim against the current to keep stationary relatively to the bank, the upper fish will feel the wave later than the other one.

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