Two barges struck a Mississippi River bridge Sunday morning, causing an oil spill that kept part of the waterway shut down to ship traffic and held up at least 21 boats, barges and vessels.One of the two tank barges was carrying more than 80,000 gallons (302,833 liters) of light crude oil, an unknown amount of which spilled into the Mississippi River after the vessel hit the Vicksburg Railroad river bridge. Cleanup crews have not yet determined the extent of the oil spill, but a pollution response team is currently assessing the size of the spill and monitoring tank levels for further leakage, the US Coast Guard relayed in a press release. Each of the two ships contains eight tanks of oil.A section of the lower Mississippi River has been closed to all traffic, backing up at least 21 tugboats, barges and other vessels going both northbound and southbound, Coast Guard spokesman Lt. Ryan Gomez told the Associated Press.The spill occurred at around 1:30 a.m. Sunday, and oil was reported as far as three miles downriver later that day. Cleanup clews, including the US Environmental Services, are frantically trying to prevent the oil from spreading and have laid out containment booms – temporary floating barriers used to contain an oil spill.“They have the boom to contain any cruise oil that’s leaking out of the barge. They have a secondary boom to corral any crude oil that gets past the first boom,” Gomez told AP.“They are continuing to try to remove the product from the damaged tank to one of the non-damaged tanks on the same barge. The ultimate goal is to transfer all of the crude to a different barge,” he added.Gomez said investigators are still trying to determine how the accident happened. Third Coast Towing LLC, the company that owns the two barges, has not made any public statements about the collision and has not responded to reporters’ requests for comment. At the time of the accident, both vessels were being pulled by the tugboat Nature’s Way Endeavor.The Mississippi River has suffered numerous oil spills in the past few years, which carry the risk of further devastating the Gulf Coast. In February 2012, part of the river was shut down for one day after an oil barge collided with a construction barge and spilled about 10,000 gallons of oil. In 2008, a fuel barge collided with a tanker, breaking in half and spilling 283,000 gallons of heavy crude into the waterways and keeping a large part of the river shut down for nearly a week.Large oil spills can leak into the Gulf of Mexico, but Gomez believes the newly spilled oil will not travel the 344 river miles to the gulf. If more oil had been released, then it could have affected residents and businesses in the Gulf Coast states who are still recovering from the 2010 explosion of the Deepwater Horizon drilling rig, which released 200 million gallons of oil into the water and killed 11 people.Still, Environmental Protection Agency spokesman Jason McDonald said it’s too early to predict the environmental impact.“Rivers have a different dynamic than oceans and gulfs,” he told the Clarion-Ledger. “It’s too early to tell.”The Coast Guard does not know how long the afflicted portion of the Mississippi River will remain closed.