All but one lane of the I-5 Freeway in Sun Valley reopened Wednesday night, hours after a mudslide sent dirt, rocks and debris across five lanes, California Highway Patrol officials said.

The mudslide occurred just before the afternoon rush hour, blocking all five lanes of the northbound 5 Freeway below the Sunland Boulevard overpass. On the other side of the freeway, all but a center lane -- not usually intended for traffic -- were closed due to flooding.

Drivers inched single file past a massive puddle that covered four lanes on the southbound 5 Freeway.

By 9:50 p.m., all of the southbound lanes and all but one of the northbound lanes were reopened as cleanup efforts continued late into the night.

CHP received reports of the mudslide shortly after 2 p.m. At least one car was trapped in the mud and had to be towed away, aerial footage showed.

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"With all of the dry weather we've had, the moisture is squeezed out, all along the 5 Freeway," said Fritz Coleman, NBC4 weathercaster. "So that area is a little more vulnerable to mud slides."

Drivers are advised to use Glenoaks Boulevard or San Fernando Road as alternates, according to a SigAlert issued for the area.

There is no estimated time for when the lanes will reopen, CHP said. A CalTrans spokesman said crews will have to haul away the feet-deep mud before drivers are allowed through.

The cause of the mudslide has not been determined, but aerial footage showed workers raking debris from a clogged grate (pictured at right) above the freeway where the slide appears to have originated.

What caused the flooding on the southbound lane is also under investigation.

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