Sacramento Bee: As a young reporter, Mark Twain visited Lake Tahoe. Its clear, cobalt blue water so entranced him that he wrote of it later, in "The Innocents Abroad." "I have fished for trout, in Tahoe," Twain wrote, "and at a measured depth of eighty-four feet I have seen them put their noses to the bait and I could see their gills open and shut." That was then. One hundred fifty years or so later, a new "state-of-the-lake" report issued last week by researchers at UC Davis has found the lake increasingly opaque...