Robert Haskins walked across a vast expanse of cracked mud, littered with old beer bottles and millions of tiny clam shells, that in most Augusts would be 50 feet underwater. But the San Luis Reservoir, the vast inland sea along Highway 152 that is a key part of Silicon Valley’s water supply, is only 10 percent full, its lowest level in 27 years.”Normally that’s an island,” the Santa Clara Valley Water District maintenance supervisor said, pointing to a towering hill.The nation’s largest off-stream reservoir is high and dry this summer, and it’s not really because of the drought.