More than 30 vessels were backed up by midday Friday as a stretch of the river remains closed.

The Coast Guard says the spill occurred at roughly 8 PM Wednesday when a boat crash caused a cargo tank on a barge to rupture and spill some of the refinery byproduct it was carrying into the river.

The Coast Guard said in a statement that “no visible signs of oil [have been found] along the riverbank”.

The barge damaged in the collision had a capacity of 1.05 million gal. The National Oceanic and Atmospheric Administration says any such process may be complicated by the Mississippi’s swift current or by oil sticking to sediment and sinking to the river bottom.

On Saturday morning, the Coast Guard said vessel traffic on the affected part of the river was open, with a safety zone in effect from mile marker 938 to 934. “We understand that it is vital”.

Powell adds that the boat operators have been interviewed by the Coast Guard and underwent drug and alcohol testing, the results of which are pending.

Responders are assessing the river and the water column to determine where the oil is and the best tactics to clean it up.

A long gash was evident on the smaller tow boat that was involved in the collision.

Mississippi River in Columbus, Ky.

Slurry oil is heavier than water, so it sinks below the surface, Chief Petty Officer Bobby Nash said.

Keleia McCloud, assistant port director, said Hickman’s port and ferry services are operating normally. That spill forced a popular state park to shut down for two months, and goo from the spill washed up on beaches as far as 100 miles away.