3:15 p.m. update: Barton Springs Pool will remained closed until at least Friday out of an abundance of caution, city officials said as they await test results for a sediment that clouded the water Wednesday.

Deep Eddy Pool will open at 6 a.m. on Friday for morning swimmers, officials said.

10:15 a.m. Thursday update: Barton Springs Pool has cleared up since Wednesday afternoon when a mysterious sediment clouded the water, causing city officials to shut down the iconic Austin swimming hole.

Officials are waiting for lab results that are expected to come later Thursday to explain what exactly got into the pool, said Thain Maurer, an environmental compliance specialist with the city's Watershed Protection Department.

Watershed officials also will be searching for the source of what Maurer described as fine sediment. The particles did not settle in the pool Wednesday and had been flushed out by springs flowing into the pool at 100 cubic feet per second by around 3 p.m. or 4 p.m., Maurer said.

"The things that we have been able to analyze more quickly in the field aren't indicating anything particularly harmful," Maurer said of the sediment.

The earliest the pool could reopen Thursday would be by 7 p.m., he said.

Wednesday: Barton Springs Pool was closed Wednesday after sediment began coming out of the springs at a level city officials said they had never seen before.

Thain Maurer, an environmental compliance specialist with the city's Watershed Protection Department, said that at 12:20 p.m. lifeguards noticed cloudy water was coming from the two main springs at the pool. The sediment was coming from the groundwater in the Barton Springs Aquifer itself and not from creeks upstream as it sometimes happens after rain.

City officials said the cloudy water spewed from the springs until about 4 p.m. and then began to clear up. This caused turbidity levels to rise to three times as much as the most recent high rain event in December.

"In terms of this level of sediment, no staff that we are aware of that I spoke to today has ever seen anything like this," Maurer said. "The reason that the pool is closed is because we don't know what the sediment is."

The city's spill response team has collected samples from the water to test it. Officials would not speculate about what the sediment could be.

Turbidity is the measure of the amount of particles in water, including sediments and pollutants, which can contribute to increased cloudiness and might be problematic for lifeguards who may not be able to see struggling swimmers.

The city has closed the pool in the past because of high turbidity, which Maurer said has always been tied to rain events and never before to problems with the groundwater.

Maurer said it is possible for muddy water to seep into the channels that feed into the aquifer. He said at least 14 city staff members drove around South Austin on Wednesday trying to find the source of the contamination with no luck.

"We didn't want people to be in (the water) in case it was dangerous," he said. "We certainly don't want to close Barton Springs for no reason, but we want to be sure everyone is safe."

By Wednesday evening, security guards were standing by barricades blocking both entrances to the pool. It will also be closed Thursday for a regularly scheduled cleaning.

Maurer said the staff will flush the pool, and, since the water coming out of the springs is clear now, that should be all that is needed to make sure it is safe for swimming, pending the lab test results.

So far, he said it appears that the cloudy water has not affected the endangered salamanders that call Barton Springs Pool home.

The earliest the pool is expected to open is at 7 p.m. Thursday, but that's not certain, Maurer said.

Officials have advised morning swimmers to head to Deep Eddy Pool instead, which will open at 6 a.m. Thursday.