NEW ORLEANS (AP) - A large fuel spill that has shut down 100 miles of the Mississippi River for four days has had a limited impact on wildlife so far, but officials are worried about fragile wetlands downstream.

Almost 800 cleanup workers used containment booms, vacuum skimmers and other equipment Saturday to continue scrubbing oil-coated riverbanks along the nation's busiest inland waterway.

A tanker and a barge collided early Wednesday, spilling about 419,000 gallons of fuel oil from the barge, closing the river from New Orleans to the Gulf of Mexico and temporarily idling some 200 oil supertankers, grain barges and other ships.

The U.S. Fish and Wildlife Service had reports of almost 60 animals coated with oil, mostly ducks and wading birds, regional spokesman Tom MacKenzie said Saturday.

Officials said they had reports of only limited impact on stretches of the winding river fronted by levees topped with rocky cover, concrete mats and other anti-erosion materials. But MacKenzie cautioned that canvassing such a vast area was a tall challenge and probably only a fraction of the affected wildlife had been spotted.

State and federal officials were waiting to see if any of the oil heading down the fast-moving river would reach the wide marshes teeming with wildlife near the river's mouth.

"When it hits those grassland wetlands, that's when it becomes a problem," Bo Boehringer, spokesman for the state Department of Wildlife and Fisheries, said.

Roland Guidry, a state official coordinating the cleanup, said the oil may never reach wetlands including the federal Delta National Wildlife Refuge and the state's Pass-a-Loutre Wildlife Management Area.

Currents could push it to the banks before it gets that far.

"The river is helping us," Coast Guard Capt. Lincoln Stroh said. "As you know, the Mississippi River is a very winding river. With the high current we have right now, the speed of the current and the way that current moves back and forth and side to side as it takes a bend, it's actually forcing the oil to bank itself."

"It's easy to see those areas from the air," said Carlton Dufrechou, executive director of the Lake Pontchartrain Basin Foundation, an environmental group. "The river looks pretty greasy."

But he added none of the oil appeared to have seeped into wetlands so far.

Although the spill is the second-largest on the river since 2000, it is tiny in comparison to disasters such as the Exxon Valdez spill in Alaska in 1989. That tanker spilled about 26 times the amount of oil held in the barge that split open in Wednesday's collision.

"The challenge with this spill is the complex nature of the terrain, combined with the length of riverbank we have to search," Buddy Goatcher, containments specialist with the Lafayette Ecological Service Office, said in a news release.

Five ships, moving as slowly as safely possible, were allowed to pass through the contaminated area Saturday, Coast Guard Petty Officer Adam Baylor said. Four had been allowed through Friday.

A pair of cleaning stations upstream and downstream were set up this week to use pressurized water and steam cleaners to scrub hulls. Booms at each station contained the washed-off oil so it could be removed.

The cruise ship Carnival Fantasy, with 2,056 passengers, had to dock Saturday at Mobile, Ala., rather than New Orleans. The company bused passengers to visit New Orleans.