After revisiting Shakespeare (specifically King Lear), I'd reluctantly drop my objection to meadow, as the second meaning given in the Oxford dictionary may apply (
http://www.oxforddictionaries.com/definition/english/meadow ). Not the same in US dictionaries, though.
Also interesting:
"Flood-meadows are distinct from
water-meadows in that the latter are artificially created and maintained, with flooding controlled on a seasonal and even daily basis."
https://en.wikipedia.org/wiki/Flood-meadowSo one other answer is most likely out of the race as well.
Although I usually don't like pointing to bilingual dictionaries, this one was at least by an ENS:
https://books.google.de/books?id=6fFWBQAAQBAJ&pg=PA308&lpg=P...There it says:
Au/Aue/Flussaue = floodplain
(also mentions "Niederung" as a synonym, that's the tricky part here)
Would be really interesting to know what you chose in the end, Ingo.
Best wishes