The 100-mile stretch of goop found floating in the Gulf of Mexico over the weekend was likely caused by river sediment, the U.S. Coast Guard announced yesterday.

The Coast Guard began receiving reports Saturday that a dark substance was spotted south of Grand Isle, La. After testing samples, the Coast Guard found trace amounts of petroleum hydrocarbons, oil and grease at levels within the Louisiana Department of Environmental Quality's regulations.

The Mississippi River likely carried the sediment down to the Gulf, and dredging apparently churned the water and caused the dark substance to appear, the Coast Guard said.

Also yesterday, the Coast Guard was investigating an oily substance on the shore of Louisiana. That material was not likely caused by last April's Deepwater Horizon spill, the Coast Guard said (MSNBC, March 21). -- AP