When the July Fourth crowds cleared out from Tahoe’s beaches this year, visitors left thousands of pounds of trash behind — Zephyr Shoals alone had 8,500 pounds of rubbish. The next day, volunteers flocked to the beaches, picking up broken coolers and lawn chairs, plastic cups and aluminum cans. But more rubbish, unseen by the volunteers, hid just beneath the sand. Across Tahoe’s beaches, scraps like bottle caps, bits of Styrofoam and cigarette butts remained. … Traditional methods for rounding up litter in the water and on the lakeshores are no longer sufficient, according to the League to Save Lake Tahoe, a nonprofit group dedicated to protecting the Tahoe Basin. Enter the BEBOT and the PixieDrone, zero-emission robots designed specifically to clean sandy beaches and the surfaces of lakes.