Besides the advice (all good) by Richard, you can make a bed behind the
dam, and then you will have a large area to grow watercress while
further minimizing losses during floods. I did build the bed (4 by 8
feet, but it should be much larger if you have space, possibly 10 by
10), the watercress sprouted easily and grew strongly, but then the
stream dried and it all died. Next year some more sprouted on their
own, but when the stream dried, that was it. Now the bed is all grass.
Yes, you need mud and a constant stream of water. Finally, if you make
the bed as far upstream as possible, plants that come loose during a
flood will root downstream and colonize the banks. Even in a few
months, I had several instances of that happening.