I had just left this spot after having taken some shots of the water and sunset and had driven along the back roads that surrounded the water which took several minutes. Decided on heading back the way I'd come as the light was fading and clouds coming in. Suddenly this rainbow was just there. In actuality it was a complete solid arc but there was no where to go to get it all in as the reservoir was at my feet. It faded in less than five minutes!