Abingdon - The crystal -clear waters of the Clinch River, which meanders southwest across the Virginia-Tennessee border, look clean and healthy. But a mysterious die-off of freshwater mussels in the river has scientists scrambling to find the cause.{snip}Since 2016, though, the Clinch River has seen 10 of its 56 species of mussels have gone extinct with another 20 species now considered endangered - like the fluted kidneyshell, snuffbox, birdwing pearly mussel, and the shiny pigtoe. The pheasantshell mussel, once one of the Clinch’s most abundant mussel species, has plummeted by more than 90 percent, from a population estimated to be...