Pretty stupid I think, but it's not that big of a deal. This time of year in that area they have more water than they know what to do with. They already drain and clean it twice a year.
What's weird is that, according to what I've read, it's actually some kind of holding tank, that's completely open, for water already treated and ready to be pumped into the public water system (with no further treatment!).
David Shaff, Portland's water bureau administrator, said that the reservoir is not shut down for nature's transgressions. "If we did that, we'd be shutting it off all the time. We fish out animals or things that have blown in all the time." Really?