A sheen of oil coated the Mississippi River for nearly 100 miles from the center of this city to the Gulf of Mexico on Friday following the worst oil spill here in nearly a decade. Around noon, after two days with no ship traffic, the Coast Guard opened the river to limited use.

Nationwide, zebra mussels have destroyed ecosystems and damaged water treatment plants. Control efforts and economic losses have cost more than $5 billion a year, according to the U.S. Coast Guard. Colorado is preparing to fine boaters and spend more than $7 million to keep the thumbnail-size creatures from taking over local reservoirs.