LONG BEACH >> When it comes to measuring pollution along the Los Angeles River, there’s a saying beachgoers use that describes it best: The first is the worst.

The season’s inaugural storm loosens the river’s dry, unwanted ingredients, mixing rainwater with bacteria, dog feces, oils, pesticides, lawn fertilizer and metals. This year, after the Oct. 9 rainstorm, the toxic flume ride washed into Long Beach Harbor.

All nine of Long Beach’s oceanfront beaches — from Shoreline Aquatic Park stretching to the border with Orange County — received F or D grades in September and October, meaning the water exceeded safe levels for three strains of bacteria, according to the latest numbers from Heal the Bay’s Beach Report Card. Heal the Bay monitors 15 beaches in the city; six are in within the bay or lagoons.

“Right now, this area has been hot for more than three weeks. I don’t have any working theories, but they are near the (outfall) of the L.A. River,” said Mike Grimmer, who manages the group’s Beach Report Card.

In addition, the city’s Colorado Lagoon went from an A grade to a C grade, as did Alamitos Bay near Bayshore Park on the bay side of the Peninsula.

Long Beach received the most failing grades, far more than any other single area of Los Angeles and Orange counties. The environmental group that measures water quality at Southern California beaches failed only two other beaches in all of Los Angeles County: Dockweiler State Beach at Ballona Creek and Mother’s Beach at Marina del Rey.

In Orange County, Dana Point Harbor received an F as did a small area of beach at the mouth of the Santa Ana River between Huntington State Beach and Newport Beach. All the other beaches in Southern California received A’s or passing grades for September and October. A wide majority of beaches from Ventura to San Clemente received A grades, continuing a trend of cleaner beaches in Southern California, Grimmer said.

The rash of red dots along nine beaches in Long Beach on Heal the Bay’s interactive map may be attributed to two factors.

First, these beaches are near where the Los Angeles River’s toxic stew of pollutants mix with the ocean waters off the Long Beach Peninsula.

Second, the bad water quality can’t recirculate because natural ocean tides are blocked by the Long Beach Breakwater, according to water experts interviewed for this article.

As evidence, some point to the beaches in Seal Beach. These get A grades from Heal the Bay even though they are located at the mouth of the San Gabriel River, a major carrier of urban runoff. Also known as nonpoint pollution, urban runoff comes from multiple sources that together infect ocean waters with bacteria and other toxins.

For both rivers, upstream communities are just beginning to work on strategies to reduce the amount of toxins in urban storm runoff or, in some cases, to treat the effluent.

Hence, the difference between the two beaches is not the amount of pollution from upstream rivers but what happens once the pollution arrives at the ocean. In Seal Beach, ocean tides circulate and dilute the runoff stream, says Robert Palmer, chairman of the Surfriders Association, Long Beach Chapter.

“Why? They have waves coming into Seal Beach, and the circulation of those waves bring with it what is diluting that runoff,” Palmer said.

The city is working on a study to determine the effects of removing the breakwater. Many homeowners say the line of rocks out in the ocean calms the sea, prevents erosion and protects beachfront homes.

Tom Modica, a spokesman for the city of Long Beach, said experts don’t know whether the recent bad grades in Long Beach are an anomaly or a trend. Heal the Bay gave those nine Long Beach beaches an F grade from mid-September during what’s called “dry weather flows.” So the rainstorm didn’t cause the bad grades, but it may have added to the problem.

During dry weather, river flows can contain higher concentrations of bacteria than during rain storms. That’s because dry flows contain some wet runoff from overwatered lawns, which in turn drag dog feces and other animal waste, fertilizers and pesticides into storm drains that empty into the ocean.

Last year at this time, two of the nine beaches received flunking grades during dry flows, but most passed. The latest flunking grades are a mystery, said Emiko Innes, an environmental scientist with the county who has examined the data for the group Surfriders Foundation.

“It is weird. This pattern is kind of puzzling,” she said. She described the five to six weeks of bad grades, mostly before a rainstorm, as “kind of alarming.”

Innes suspects that some kind of overwatering of lawns is spiking the bacteria counts by sending polluted water down the L.A. River and into these beaches. She says sampling shows a spike in bacteria on one day, but a leveling off later.

Indeed, in August, the city bragged about its beaches being clean during the summer when most residents visit the shore.

Heal the Bay reported seven consecutive summers of excellent water quality on Sept. 3. Monitoring from Memorial Day to Aug. 21 found 96 percent of 450 beaches along California’s coast earned grades of A or B.

In Los Angeles County, nearly 90 percent of the beaches earned A or B grades, a 3 percent improvement from 2012, the group reported. Catalina, traditionally one of the dirtiest beaches, came up clean. Long Beach beaches were also highlighted in the summer report as showing tremendous improvement.

The improvements to Long Beach beaches began in 2007, when only 12 percent of local beaches received an A or B. In the summer of 2012, the number reached 77 percent, Modica said.

Long Beach currently diverts some dry flows at Alamitos Bay, Belmont, Appian Way and Second Street and the Colorado Lagoon into the Carson wastewater treatment plant operated by the Los Angeles County Sanitation Districts. The city, along with 15 other Gateway Cities, installed special grates on 12,000 storm drains that capture trash before it reaches the river. Long Beach installed a centrifuge-like device called a vortex system that spins waste like a washing machine and allows cleaner water to pass through.

In July, the city received a $4.9 million grant from the State Water Resources Control Board to put in three more stormwater diversion machines and two more vortex systems along the L.A. River. The design phase begins in early 2014, and the projects will take about a year to complete, said Tony Arevalo, the city’s environmental compliance officer.

The city also spent $8.5 million to clean the Colorado Lagoon, a coastal salt marsh lagoon that was once a popular swimming spot for locals but was abandoned for cleaner beaches because it was so polluted.

Until now, it had been receiving A grades since that project was completed in August 2012. The city removed 63,000 cubic yards of polluted sediment from the lagoon and unclogged a culvert to allow tidal waters to recirculate into the lagoon.

Eric Zahn, a biologist with Tidal Influence, a group working on restoring the Los Cerritos Wetlands, which includes the lagoon, says people are returning to Colorado Lagoon to swim. Even triathletes are using the lagoon to train, he said.

“Now it is one of the cleanest beaches in the area,” Zahn said.

Modica believes the city and other upstream cities must work harder on reducing toxins that get into storm drains.

Aracely Lasso, a civil engineer with the county Department of Public Works, said adding upstream stormwater treatment plants is estimated to cost between $17 billion to $120 billion over the next 20 years. The range depends on whether cities implement less-expensive education projects or build more costly stormwater treatment plants.

“Essentially, it is working. But there is more that needs to be done,” Modica said. “All the upstream cities need to do more.”