Rattlesnake Reservoir at sunset

The Rattlesnake Reservoir is declared to be a “working reservoir” by the signs posted on the fence encircling it. What’s it’s job? I know that there are large amounts of water dumped into it daily in a waterfall (from where? It’s on a hill; is there a pump somewhere? Is it urban runoff? Or drinking water emergency supply?) There are hundreds of birds on it: cormorants, ducks, grebes, coots, egrets and herons, with turns and swallows coursing above. It’s certainly working out for them, but I am forced to watch them from a distance.