Waves may also wash over booms strung out just off shorelines to stop the oil, said Tom McKenzie, a spokesman for U.S. Fish and Wildlife Service, which is hoping booms will keep oil off the Chandeleur Islands, part of a national wildlife refuge.
"The challenge is, are they going to hold up in any kind of serious weather," McKenzie said. "And if there's oil, will the oil overcome the barriers even though they're ... executed well?"
A top adviser to President Barack Obama said Friday that no new oil drilling would be allowed until authorities learn what caused the explosion of the rig Deepwater Horizon. David Axelrod told ABC's "Good Morning America" that "no additional drilling has been authorized and none will until we find out what has happened here." Obama recently lifted a drilling moratorium for many offshore areas, including the Atlantic and Gulf areas.
Two Air Force C-130s were sent to Mississippi and awaited orders to start dumping chemicals on the oil spill. The Navy also sent equipment for the cleanup and Pentagon officials were talking with the Department of Homeland Security to figure out what other help the military could give.
The leak from a blown-out well a mile underwater is five times bigger than first believed. Faint fingers of oily sheen began reaching the Mississippi River delta late Thursday, lapping the Louisiana shoreline in long, thin lines. Thicker oil was farther offshore. Officials have said they would do everything to keep the Mississippi River open to traffic.
The Coast Guard defended the federal response so far. Asked on all three network television morning shows Friday whether the government has done enough to push oil company BP PLC to plug the underwater leak and protect the coast, Brice-O'Hara said the response led by the Coast Guard has been rapid, sustained and has adapted as the threat grew since a drill rig exploded and sank last week, causing the seafloor spill.
The oil slick could become the nation's worst environmental disaster in decades, threatening to eclipse even the Exxon Valdez in scope. It imperils hundreds of species of fish, birds and other wildlife along the Gulf Coast, one of the world's richest seafood grounds, teeming with shrimp, oysters and other marine life.
"It is of grave concern," David Kennedy of the National Oceanic and Atmospheric Administration, told The Associated Press about the spill. "I am frightened. This is a very, very big thing. And the efforts that are going to be required to do anything about it, especially if it continues on, are just mind-boggling."
Oil clumps seabirds' feathers, leaving them without insulation -- and when they preen, they swallow it. Prolonged contact with the skin can cause burns, said Nils Warnock, a spill recovery supervisor with the California Oiled Wildlife Care Network at the University of California-Davis. Oil swallowed by animals can cause anemia, hemorrhaging and other problems, said Jay Holcomb, executive director of the International Bird Rescue Research Center in California.
The spewing oil -- about 210,000 gallons a day -- comes from a well drilled by the rig Deepwater Horizon, which exploded in flames April 20 and sank two days later. BP was operating the rig that was owned by Transocean Ltd. The Coast Guard is working with BP to deploy floating booms, skimmers and chemical dispersants, and set controlled fires to burn the oil off the water's surface.
The leak from the ocean floor proved to be far bigger than initially reported, contributing to a growing sense among some in Louisiana that the government failed them again, just as it did during Hurricane Katrina in 2005. President Obama dispatched Cabinet officials to deal with the crisis.
Cade Thomas, a fishing guide in Venice, worried that his livelihood will be destroyed. He said he did not know whether to blame the Coast Guard, the government or BP.