A tanker spilled more than half a million gallons of crude oil into the lower Mississippi River on Tuesday evening, one of the largest oil spills in U.S. waters since the Exxon Valdez dumped more than 10 million gallons in Alaska in 1989. The spill near Port Sulphur, La., is about 70 miles inland from the Gulf of Mexico and 25 miles upriver from the Delta National Wildlife Refuge and a state wildlife management area where about 200,000 migratory waterfowl make their winter homes. Cleanup crews have so far kept oil out of the sensitive wetlands areas by setting up floating booms, but some birds have already been affected. The single-hulled Westchester, a Liberian-owned tanker registered in the Bahamas, ran aground after an explosion in its engine caused a power failure.