About 5 million gallons of untreated sewage has spilled into the Dominguez Channel, which discharges at Cabrillo Beach in San Pedro and the Port of Long Beach, prompting widespread beach closures.

The Los Angeles County Department of Public Health announced the sewage spill Monday.

In its announcement, the health department said Cabrillo Beach would be closed until further notice.

The Long Beach Department of Health and Human Services, later that same afternoon, also announced that all swimming areas in the city — about 7 miles of waterfront — would be closed as a result of the spill.

The 5 million gallon spill, which occurred at about 6 a.m. Monday and was initially estimated to be about 1 million gallons, discharged into the Dominguez Channel and Compton Creek, which lead to the Port of Long Beach and the L.A. River, respectively, according to Long Beach’s announcement.

In addition to the massive spill, Long Beach’s announcement said, another 40,000 gallons of sewage spilled into Commerce on Monday morning.“This additional spill also flows into the (L.A.) River,” the city said, “compounding the environmental impact and posing further concerns for the affected areas.”

The cause of the 5 million gallon spill, according to Los Angeles County Sanitation Districts spokesperson Bryan Langpap, is the recent storm system that has pummeled the region and prompted Gov. Gavin Newsom to declare a state of emergency in L.A. County.

“That amount of rain overwhelms the storm drain system, which causes the streets to flood,” Langpap said in a Monday email. “In turn, that flooded water gets into wastewater sewers and can cause the flow in the sewer to exceed the sewer’s capacity.”

The excess sewage, Langpap said, can then cause an overflow from a sewer cover.

“That appears to be the case in Rancho Dominguez today,” he added, “though our crews were still investigating the cause.”

Long Beach’s announcement said the recent rains were likely the cause of the smaller leak in Commerce as well, though that sewage spill is also still under investigation.

Though the county health department, in its announcement, said about 1 million gallons of sewage had spilled, Langpap said the estimate is now around 5 million gallons.

The waters at Cabrillo Beach will be closed until bacterial levels meet state standards.

Both the health department and the Sanitation Districts will conduct water quality testing. Long Beach’s health department will also conduct water testing along its coastline.

“(Those two departments) will be taking water samples from nearby water bodies to check the water quality for the next several days,” Langpap said, “until there have been at least two straight days where water quality meets requirements.”

The beaches could reopen as early as Wednesday night if lab results taken Monday and today come back good.

A rain advisory warning residents to avoid contact with ocean water because the rain can cause an increase in bacteria that are harmful to human health is also in effect until Wednesday.

For more information about water quality, go to publichealth.lacounty.gov/Beach and tinyurl.com/LB WaterQual.