GALVESTON, Texas -- Traffic backed up on both sides of the Houston Ship Channel that was shut down for a third day Tuesday, after a barge spilled as much as 643,500 litres 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.

Officials were hopeful Monday that most of the oil that spilled Saturday was drifting out of the channel into the Gulf of Mexico, thus limiting 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 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.

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 3,406,770 litres 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 42 million litres of oil into the Prince William Sound, or the Deepwater Horizon spill, in which more than 380 million litres of oil spilled into the Gulf of Mexico four years ago.