Clear Lake is the largest natural lake in California (Lake Tahoe is partly in Nevada, and Salton Sea is not natural). It formed in a basin produced by fault motions, and has had an interesting history in that it once overflowed eastward into the Sacramento Valley, then west into the Russian River drainage, and then back east again. The causes of these changes in outflow have been volcanic eruptions and large-scale landslides that blocked one outlet or another.
Did I mention the day was foggy and gloomy? The picture below actually would have contained the horizon if it had been visible. The Buffleheads provided a splash of color, even if it was just black and white!