GALVESTON, Texas (AP) -- Cannon booms reverberate across the Houston Ship Channel, a scare tactic to keep birds away from oil-slicked beaches. On a mainland shore near a line of refineries, crews scour the sand for quarter-sized tar balls that have washed ashore.
Far on the horizon a few ships floated outside the channel, among the dozens of vessels waiting for the U.S. Coast Guard to reopen one of the nation's busiest seaports after a barge collision dumped as many as 170,000 gallons of heavy oil into the water.
Three days after the collision, the cleanup effort is still going on in earnest. But authorities hope the channel's closure could end sometime Tuesday, allowing more than 80 stranded ships to resume activity.
Officials believe most of the oil that spilled Saturday is drifting out of the channel into the Gulf of Mexico, which should limit the impact on bird habitats around Galveston Bay as well as beaches and fisheries important to tourists.
"This spill -- I think if we keep our fingers crossed -- is not going to have the negative impact that it could have had," said Jerry Patterson, commissioner of the Texas General Land Office, the lead state agency on the response to the spill.
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 could 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.
The Coast Guard said earlier Monday that it hoped to have the channel open to barge traffic as quickly as possible but that more tests were needed to confirm the water and the vessels traveling through the channel were free of oil.
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, along with a dark line of a sticky, oily substance, were 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 up 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 one of those things with it being so new, 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.