If you visit Lake Tahoe this summer, the beaches might seem a little smaller than they were a few years ago. It’s not an optical illusion. Large sections of them really are underwater. Dozens of feet of snow that blanked the Sierra Nevada this winter, generated by blizzards from raging atmospheric river storms, have been steadily melting all spring and summer, sending billions of gallons of water rushing downhill and steadily raising the water level at Lake Tahoe.