Des Moines Register: Council Bluffs officials are trying to determine how far invasive zebra mussels have spread from one of the basins where the city draws drinking water from. The Daily Nonpareil reports the mussels were found during maintenance this fall, but it’s not clear if they have begun to colonize and clog the pipes leading into the city’s water treatment plant. The mussels do not harm drinking water, and actually can improve it by filtering plankton. But they crowd out other species and clog pipes. They also cluster on docks.