Ship traffic on the Mississippi River largely returned to normal Tuesday, six days after a big oil spill closed the vital commercial artery between New Orleans and the Gulf of Mexico.

Seventy vessels moved in and out of the Southwest Pass, an entryway from the Gulf into the river, said Capt. Lincoln Stroh, the Coast Guard’s captain of the Port of New Orleans. That represents normal traffic, he said.

Although the commercial transportation crisis was averted, more than 1,500 people were working to clean up the river — a process still expected to take several weeks — and the task remained of salvaging the barge hit July 23 by a tanker and cut in half.

The accident totally stopped river traffic for two days before a limited number of ships were allowed to cruise.

By Tuesday, crews had removed about 85,000 gallons of an oil and water mixture, said Paul Book, an official with American Commercial Lines, the barge’s owner.

Although just more than a quarter of the oil had been picked up, Charlie Henry of the National Oceanic and Atmospheric Administration said about two-thirds of the oil would be taken care of by natural forces. High temperatures in the water, for example, cause the oil to deteriorate, making it less of a hazard.

“You get to a point where you could actually do more harm to the environment by not allowing nature to take care of the rest,” Henry said.

Stroh said about 30 tugboat-barge combinations were still in line for pressure cleaning to remove oil. Most New Orleans-area ferries had returned to service, but tourist cruise vessels on the river were still shut down.

The backlog of vessels getting into and leaving the Southwest Pass had just about cleared Tuesday, the Coast Guard said. Stroh said a safety lane was still in effect, and vessels were traveling closer together than normal at slower speeds to reduce the wake that could hamper cleanup efforts. The Coast Guard said traffic was clearing quickly because so many people were working to scrub oil from ships.

An investigation into the cause of the wreck is ongoing.

A preliminary Coast Guard investigation said the pilot of the tug Mel Oliver, which collided with the tanker, held an apprentice mate’s license — not adequate to be piloting the vessel. No licensed master was on board as is required for an apprentice mate to be the pilot, the Coast Guard said.

A public hearing by the Coast Guard on the accident was ordered for Aug. 12.

The U.S. Army Corps of Engineers said Tuesday that it had suspended dredging operations at the Southwest Pass — a largely continuous process that keeps the channel deep enough to handle ships — after oil contamination was found in dredged material. There was no word Tuesday of when dredging would resume.

Meanwhile, the two pieces of the sunken barge are up against a pillar of the Crescent City Connection, one of the Mississippi River bridges in the New Orleans area. Officials have said the wreck does not pose a threat to the bridge.

Welders and divers have been working on the barge with the intention of first pumping out any remaining oil, then hauling it out of the water.

Although economic losses have not been calculated, the Port of New Orleans earlier said its own study showed a total closure of the river below New Orleans would cost the national economy $275 million a day. The port said that as a result of the closure, six vessels were diverted to other ports and three vessels had their departures delayed.