The Auburn Dam could well be the most talked about water storage and flood control facility in the country that simply doesn’t exist — no matter how much it’s been argued about, advocated for and against, legislatively proposed and architecturally rendered. Dreamed of for decades, and having prompted scores of editorials — as well as a new (and excellent) documentary, “The Dam That Never Was,” by writer-director Steve Hubbard — it still doesn’t exist.  At first, the Auburn Dam sounded like a good idea when it was suggested almost 70 years ago; preliminary construction began a decade later. It would be a concrete weir almost 700 feet high on the American River’s north fork, bordered by two counties: Sacramento and El Dorado.