A major Texas shipping waterway stayed mostly closed Monday — three days after a weekend barge collision sent thousands of barrels of thick tar-like oil spilling into the ocean, polluting an important migration route for tens of thousands of birds.

The barge, which was being towed from Texas City to Bolivar, was carrying nearly one million gallons of RMG 380, an especially thick oil that officials warn has long-lasting environmental impacts.

One of the barge's tanks was breached, spilling up to 168,000 gallons (4,000 barrels) of oil into the Galveston Bay, according to estimates from the U.S. Coast Guard.

The spill comes 25 years after the Exxon oil spill in Valdez, Alaska and nearly four years after BP's Deepwater Horizon spill in the Gulf of Mexico.

While the size of Saturday's spill is smaller than those disasters, it took place in the direct path of a bird migration path. Clean-up officials and environmental groups report oiled and dead birds.

"I've heard this is the largest spill in 25 years in Galveston Bay," said Rachel Powers, Executive Director for Citizens Environmental Coalition. "It happened right near a huge recreational fishing area that's an important source of local jobs."

“The timing really couldn’t be much worse since we’re approaching the peak shorebird migration season,” said Richard Gibbons, conservation director of the Houston Audubon Society, in an interview with the Associated Press.

The Houston Ship Channel, described by ABC News as "one of the world’s busiest waterways for moving petrochemicals," remained partially closed Monday as the clean-up effort continued.

As of Sunday, oil had been detected 12 miles off-shore in the Gulf of Mexico.

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