Why does a dam (like the hoover dam) foam/bubble?

Question

So, where I live we have a dam. On a couple occasions I have driven by it, today being one of them, and besides the water being really high because of our record setting snow fall and rain fall this year, the water was all foamy/bubbly. Obviously the dam was open, but why does it foam?

Answer 1

I used to live by a creek that had a small dam and it used to do the same thing.  I called E.P.A. and during the night oil tanks would dump into the creek!!!  This may not be your problem but I agree with the response that it is some kind of impurities!!

Answer 2

Impurities in the water, if there's been flooding it's most likely sewage.

Answer 3

There was a bum who wanted a bubble bath.