My first thought about this was “snow on a mountain”.
Then, “spring run-off”.
Some people have mentioned “water towers”.
Could be just about any form of H20: ice, rain, pond, or steaming, erupting geyser.
I was in Iowa City five years ago for the big flood. They told us not to worry that week, while they secretly emptied the art gallery, and we watched the Army Corps of Engineers sandbag the railroad line.
Don’t alarm the (paying) public….