Simon Norfolk/NB Pictures, for The New York Times
"As the 20th century progressed, many water managers came to believe that the 1950s, which included the most severe drought years since measurement of the river began, were the marker for a worst-case situation. But recent studies of tree rings, in which academics drill core samples from the oldest Ponderosa pines or Douglas firs they can find in order to determine moisture levels hundreds of years ago, indicate that the dry times of the 1950s were mild and brief compared with other historical droughts. The latest research effort, published in the journal Geophysical Research Letters in late May, identified the existence of an epochal Southwestern megadrought that, if it recurred, would prove calamitous."
Read more in this sobering article from NYTimes Magazine: LINK