Why California rivers saw fewer harmful algal blooms this year

Outbreaks of harmful algal blooms have wreaked havoc on California river ecosystems for years. The toxic algae — a neon green layer of muck that floats atop water — thrives in warm, stagnant conditions brought on by drought. 

Presence of this algae can make life difficult for other plants and fish in the river, and even cause concerns for humans that accidentally ingest or possibly breathe the area around it.

But this year was different. Faster, colder river waters led to fewer outbreaks of the harmful algae throughout the state. 

“We had this increased amount of snowpack, which is what caused a lot of flow down during the summer months,” said Spencer Fern, the Delta science program manager for the environmental nonprofit Restore the Delta. 

California’s snowpack is double the average for May, survey finds

Fern runs a program with Restore the Delta which monitors areas along the San Joaquin River by Stockton for signs of microcystis, a common type of harmful algal bloom. Despite the algae’s consistent presence in recent years, Fern said he saw no signs of it in his coverage area this summer. 

But the harmful algae did not disappear entirely. Officials with the Shingle Springs Band of Miwok Indians reported a low-level presence of the algae in July by the unincorporated town of Verona, just north of Sacramento. Verona lies right next to the confluence of the Sacramento and Feather rivers and is also the site of Wóllok, a historic village for the tribe that would have been located in this vicinity. 

James Sarmento, the executive director for the tribe’s Cultural Resources Division, said algae’s presence caused the tribe to delay a Nisenan and Miwok language camp they’d planned at the site that month.

“We were going to have lots of kids, adults and elders at this event,” he said. “I know for sure there was a concern about it.” 

Zach Gigone, an environmental scientist who runs the tribe’s water quality program, was hired in March as part of the tribe’s expanding efforts to address these blooms. Before he was hired, Gigone said there wasn’t any consistent monitoring of sites in the area. Now, he conducts monthly testing on sites like the river waters by Verona. 

“Having all this information will allow us to have a better picture of what exactly is happening and what factors are most strongly driving these algal blooms,” he said. 

Gigone said heavy flows brought on by last winter’s record snowpack led to fewer outbreaks of the harmful algae this year. However, he said they’ll likely return. He hopes his work will help the tribe not only identify hotspots but also offer safety guidance to tribal members. 

With warmer temperatures brought on by climate change, Fern said these blooms will generally become more of an issue in the future. He said it’s important for California to adapt to these conditions in order to improve the health of state rivers.

“Being able to increase … cold, fresh water flow down the Sacramento and San Joaquin Rivers is what will hopefully prevent this harmful algal bloom problem from happening next year, which I'm crossing my fingers, but I have a feeling it'll be a drought year,” Fern said.