Amy Graff, SFGATE
Oct. 5, 2021
Fisheries biologist Pat Higgins said he was shocked when he discovered on Sept. 17 a section of the largest tributary in California’s third-largest watershed was dry.
Higgins explained there was no water flowing above ground in the section of the South Fork Eel River where it meets the main stem in Dyerville below Highway 101 in Humboldt County. The riverbed was exposed and the water just stopped, ending in a still pool. The south fork and the main stem were no longer connected. He believes this is unprecedented based on his observations going back to 1995 and historic data from the U.S. Geological Survey’s gauges measuring river flow.
“When I was standing there, I thought, ‘This is historic,'” said Higgins, managing director of the Eel River Recovery Project. “I hope it’s the worst I ever see.”
Luckily, a week after his observation, Higgins was astounded and relieved when he saw the South Fork was flowing after a cold front delivered a couple inches of rain to the farthest reaches of Northern California. He believes the river was dry for more than a week based on USGS data.
California’s rivers have struggled throughout modern-day history with increasing demands from urban and agriculture water users in the state’s climate marked by periods of natural drought. But the South Fork running dry may be a symbol of a new extreme, signaling escalating environmental neglect, increased agricultural demands, changing weather patterns and droughts that are more extreme than they were in the past. It’s an example of what unfolds when the balance of Mother Nature is disrupted. Scientists are trying to understand the threats to this unique river with a future that’s clouded by climate change.
The 196-mile Eel River flows northward from Mendocino County to Humboldt County, winding over coastal mountains and through towering forests and narrow canyons, before emptying into the Pacific Ocean about 10 miles downstream from Fortuna, a town known as the gateway to California’s redwoods. The South Fork is 105 miles long, stretching from Laytonville to Dyerville, before joining with the main stem. With no major dams, it stands as a remote and stunningly beautiful waterway more untouched than many of the state’s other rivers. There are several other tributaries in the Eel watershed including the Middle Fork, North Fork and Van Duzen River. More than 350 miles of the watershed’s waterways have both state and federal Wild and Scenic River designations.
“It’s more than 3,300 miles of watershed,” said Scott McBain, who has fished the Eel River and worked on the main stem and North Fork. “It’s like one of the largest rivers in California and historically it had one of the largest salmon and steelhead runs. There used to be a million anadromous fish that went up it. It used to be one of the biggest salmon producers in the state, and it still could be. Its ability to recover is high.”
After two consecutive dry winters, the tributaries and the main stem of the river are running at levels below historic averages and in some cases have reached all-time lows. The flow on the South Fork is significantly lower than some of the other tributaries, a non-peer-reviewed report from the Eel River Recovery Project revealed.
A river’s flow is determined by measuring the number of cubic feet that passes by a fixed point per second (cfs). At one point on the South Fork, for example, the flow was recently measured at 6.98 cfs. The previous historic low was 8.86 cfs in 2002, the report said.
Researchers are trying to understand and quantify the factors contributing to the river’s low flow — including climate change, water diversions and the health of the surrounding forests. Climate change undoubtedly impacts the South Fork and all rivers across the state. In California, warming temperatures are reducing the size of the snowpack that feeds rivers in spring and summer, and research also suggests the state’s droughts are becoming more common while rain is more sporadic and intense.
“It’s not entirely surprising the South Fork is so low because it’s one of the driest years we’ve had in 100 years,” said Eli Asarian, a hydrologist and aquatic ecologist with Riverbend Sciences. “It was a very low rainfall year and the flows were very low. It’s not surprising it’s at or below historic lows. As far as what’s causing it, it’s a combination of many different factors.”
Another of those factors is an increase in water diversions. “As the population has increased, as the cannabis cultivation has been increased, there has been more water used over time,” said Asarian, who has researched the Eel River watershed. “There are not great numbers for that but it’s self-evident there has been an increase.”
Asarian explained the impact of diversions can vary depending on the time of year. “In March when it has been raining, the amount of water being diverted can be 1,000 times less than the river flow,” he said. “Then you get to now, and the amount of water being used on the South Fork Eel could be quite a bit greater than the total river flow. The effects are most intense in the late summer to early fall and in drought years — and especially this year in what may be the worst drought ever.”
Gabriel Rossi, a coastal rivers ecologist with CalTrout, said these diversions in dry summers such as 2020 and 2021 have the potential to compound the effect of climate change on habitat and make the drought that much worse for native species.
Rossi noted that juvenile salmon migration and rearing in the mainstem South Fork Eel was severely truncated this year. “That’s depressing because we had a stronger than average Coho salmon run in the South Fork Eel this winter,” he wrote in an email. “Juvenile Coho were abundant in the upper South Fork and several tributaries throughout the early summer. While some of these juvenile Coho were able to find cool water refugia upstream, the survival of this year’s cohort would have really benefited from a wetter and cooler spring and summer.”
Poor forest management may have also exacerbated 2021’s paltry water flow on the South Fork, the recovery project’s report said.
A significant portion of the South Fork watershed, the land that drains water into the river, was clear-cut in the 1950s and 1960s before it became part of Humboldt Redwoods State Park. The land was then “replaced with thousands of trees” and has become overgrown, Higgins explained.
The result? “The overstocked trees are sucking up the moisture that should be going into the river,” Higgins said. “The moisture transpires back into the atmosphere … it’s like the trees are breathing.”
The technical term used to describe this process is evapotranspiration, which generally means “the water lost to the atmosphere from the ground surface.”
This hypothesis is compelling, but David Dralle, a research hydrologist with the U.S. Forest Service, said more research is needed. “I’m hesitant to say that flows are low in certain watersheds due to an overgrowth of trees,” Dralle wrote in an email. “This is still a surprisingly unsettled area of hydrological research.”
Dralle pointed to another tributary in the watershed, Elder Creek, that did “a bit better than other watersheds this summer” yet saw less rainfall than other tributaries and has a watershed with “very large, productive old-growth forest that was never logged and has not burned anytime recently.”
Asarian countered that Elder Creek’s higher flow may actually be in part due to the old-growth forest. He said some research has shown the structure and composition of a forest — the size of trees and the species — can impact the amount of water that runs across the landscape and makes it into the river. A healthy old-growth forest with larger trees spaced farther apart may require less rain than a young, overstocked forest.
“When a forest is harvested, you temporarily have no trees there,” he said. “Any rain that falls goes straight into the river or into the ground. There’s an after-the-harvest when generally there’s an increase in flow. There are fewer trees there and less water being used. As the forest grows back, assuming there’s been good regeneration, what you end up with is a lot of smaller trees closer together … the effects of the vegetation on the flow, it will flip.”
Higgins noted that compared to the South Fork, other tributaries of the watershed have seen more significant flow this year. The Middle Fork Eel River flow at Dos Rios, for example, recorded a flow of 7.4 cfs, “which is more than four times the all-time low of 1.64 set in 2014,” the report showed.
Why would there be so much flow in 2021 when the rainfall years were similar or worse?
The recovery project said the Middle Fork may have a decent flow because the watershed around it recently burned in the 2020 August Fire that thinned the forest. A similar situation unfolded on the North Fork Eel River with the surrounding land burned by last year’s fire, according to the project.
“The middle fork and the north fork had the million-acre August fire, and their flow is much better than 2013 and 2014 after similar flow years,” Higgins hypothesized.
This may also need more research. “The flow values on the Middle Fork are high given the amount of precip that fell this year, but they are not so anomalously high (in my opinion) as to definitively point toward the August Fire,” Dalle wrote in an email.
A number of factors are clearly at play, leading to the river going temporarily dry. It’s also clear the river ecology is suffering and this began long before the river bed was exposed.
“The sight of a dry channel in the lower South Fork Eel River is certainly startling – but I think it speaks more to the ecological effects of drought that the river has already experienced this year than the immediate effect of drying in September,” Rossi wrote in an email. “A coastal river doesn’t dry up overnight – and a lot of ecological ‘tipping points’ actually occur in the weeks or even months before the channel dries completely. A dry channel in September means that the spring and summer flow conditions were already critically low. So, by the time this localized section of the river dried up, a lot of the damage was already done.”