GALVESTON, Texas (AP) -- Traffic backed up on both sides of the Houston Ship Channel that was shut down for a fourth day Tuesday, after a barge spilled as much as 170,000 gallons of oil into environmentally sensitive waters along the Gulf of Mexico.
About 100 barges and other ships were waiting to enter or leave through the channel that connects the Gulf with Houston and other parts of Southeast Texas, U.S. Coast Guard Petty Officer Andy Kendrick said.
The Coast Guard hopes to reopen the channel to at least some traffic Tuesday, but Kendrick reiterated earlier statements that officials needed to ensure that the water and the ships moving through them are free of oil.
"We're definitely working hard to ensure that we've gotten clean water so that ships can transit," Kendrick said Tuesday morning. "They're hopeful they can get that done today."
Crews have laid down miles of oil boom and have been picking up black, quarter-sized "tar balls" washing up on shore. Cannons are being blasted on one beach to scare birds from the oil-slicked sand and rocks.
Most of the oil appears to have moved out of the ship channel and Galveston Bay, and into the Gulf of Mexico, said Jerry Patterson, commissioner of the Texas General Land Office, the lead state agency on the response to the spill.
Patterson said he saw "very little" oil sheen on the bay waters during a flyover Tuesday morning.
"The big question is, 'Where is the oil that went into the Gulf of Mexico?'" he said in a phone interview. "That's the next question to be answered."
The best-case scenario is for most of the slick to remain in the Gulf for at least several days and congeal into small tar balls that wash up further south on the Texas coast, where they can be picked up and removed, Patterson said.
However, officials said Monday night that changing currents, winds and weather were pushing the oil not only further into the Gulf, but also southwest along Galveston Island, resulting in expanded oil recovery efforts.
At Galveston Island, a popular tourist destination due to its beaches and parks, crews have laid booms around environmentally sensitive areas.
Some black, tar-like globs, and a dark line of a sticky, oily substance, could be seen along the shoreline of the Texas City dike, a 5-mile jetty that juts into Galveston Bay across from a tip of Galveston Island.
In Texas City, near several refineries, crews picked tar balls out of the sand and set up cannons that boomed every few minutes to scare off birds.
At Galveston's East Beach, workers set up metal posts to hang lines of absorbent material to collect tar balls as they washed up. On the other side of a jetty, crews were scooping oil from the sand and pouring it into plastic bags.
"It's very hard to tell how long we'll be out here," Coast Guard Petty Officer Richard Forte said.
Jim Guidry, executive vice president of Houston-based Kirby Inland Marine Corp., which owned the barge, has said the company -- the nation's largest operator of inland barges -- would pay for the cleanup.
Much of the channel's traffic serves refineries key to American oil production. But Patterson said refineries in Texas City appeared to have enough crude oil on hand to continue operating until the ship channel can re-open.
Environmental groups said the spill occurred at an especially sensitive time and place. The channel in Texas City, about 45 miles southeast of Houston, has shorebird habitat on both sides, and tens of thousands of wintering birds are still in the area.
At least 50 birds of six species have needed treatment due to the oil, said Richard Gibbons, conservation director of the Houston Audubon Society. The species include sanderling, ruddy turnstone and the American white pelican, Gibbons said.
Gibbons agreed that the majority of the oil could wash up as tar balls farther south. If it hits the coast sooner, it could damage the natural habitat of many more birds, he said.
The channel, part of the Port of Houston, typically handles as many as 80 large ships daily, as well as about 300 to 400 tugboats and barges.
The barge was carrying about 900,000 gallons when it collided with a ship. The resulting accident falls far short of major American oil spills such as the 1989 Exxon Valdez disaster in Alaska, which dumped 11 million gallons of oil into the Prince William Sound, or the Deepwater Horizon spill, in which more than 100 million gallons of oil spilled into the Gulf of Mexico four years ago.
Nomaan Merchant reported from Dallas. Associated Press writer Michael Graczyk in Texas City contributed to this report.