By ELDAR EMRIC, Associated Press VISEGRAD, Bosnia-Herzegovina (AP) — Huge islands of garbage floating on some rivers in the Balkans are causing an environmental emergency and threatening a regional hydropower plant. Plastic bottles, rusty barrels and other waste could be seen clogging the Drina River near the eastern Bosnian town of Visegrad on Tuesday. Upstream, the Drina's tributaries in Montenegro,