As worry about San Joaquin Valley’s groundwater supply grows, hydrologists contend there’s another water issue lurking: Sierra runoff contaminated with sediment from last summer’s Rim fire.
“We’re going to see massive amounts of sediment (flowing from the burn area), and much of it will end up in the Don Pedro Reservoir,” warned Jim Branham of the Sierra Nevada Conservancy. “It’s going to have dramatic and significant long-term consequences.”
That’s a concern for Stanislaus County residents because Don Pedro is jointly owned by the Modesto Irrigation District and the Turlock Irrigation District. Water from Don Pedro flows to farms throughout the region, and it is the source of much of Modesto’s drinking water.
Branham recently toured the Rim fire’s 257,000-acre burn zone, which is primarily in Tuolumne County. It was the largest fire in the Sierra Nevada’s recorded history, and forests there were destroyed. Branham said the scorched land is covered with a foot or more of ash.
“It is going to come right off” as soon as rain hits it or snow melts, Branham predicted. He is concerned about what that will do to water quality and to sediment buildup in the reservoir.
MID and TID are taking action to minimize the fire’s impact, and their employees are more optimistic than Branham.
“Runoff water will be black going into the Tuolumne River, but that will be six miles up from Don Pedro,” said Jason Carkeet, a TID utility analyst who specializes in watershed issues like that associated with the Rim fire. “The reservoir is so large that we don’t anticipate water quality impacts for irrigation.”
The reservoir currently holds about 326 trillion gallons of water, which will significantly dilute the ash-laden runoff.
But MID and TID plan to do extensive water quality monitoring to make sure.
“We are in the process of working with the United States Geological Survey and the California Department of Water Resources to establish a streamgage and extensive water quality monitoring at a location downstream from the fireand upstream from Don Pedro Reservoir,” said Melissa Williams, MID’s spokewoman.
“Documenting the quantity and quality of water entering Don Pedro Reservoir and modeling streamflow changes in response to the fire will help give us the tools to understand the cumulative effects of the Rim fire on future water supplies,” Williams said.
MID will pay particular attention to the quality of water going into Modesto’s drinking water system.
“MID closely monitors the quality of the water coming into the Modesto Regional Water Treatment Plant,” Williams said. “We collect daily water samples at Modesto Reservoir. We will continue to carefully monitor and diligently test the water quality to ensure delivery of a safe and reliable water supply to the city of Modesto.”
Sediment from the fire is a separate concern for Don Pedro, the state’s sixth-largest reservoir.
Branham said sediment is an ongoing problem for most of California’s more than 1,300 reservoirs. He said 120 of those reservoirs have lost 75percent of their water-holding capacity because of sediment, and 190 others have lost 50 percent to sediment.
“We are filling up existing water storage capacity with sediment,” Branham told the State Board of Food and Agriculture last week during a gathering focused on California’s dwindling groundwater reserves.
If reservoirs can’t hold enough water to meet agricultural and urban needs, more groundwater is likely to be pumped to quench demand, Branham said.
Stanislaus County irrigation districts are aware of the sediment risks posed by the Rim fire.
“With a potential increase in sediment, logs and woody debris that may wash into the Tuolumne River and Don Pedro Reservoir, MID and TIDhave prepared to deploy some 1,200 feet of boom – temporary floating barrier – to recover floatable debris as it may travel into Don Pedro Reservoir,” Williams said.
Carkeet, the TID utility analyst, said they don’t expect fire debris to block water from flowing into the reservoir, but “we anticipate sediment coming into the upper part of Don Pedro.”
Had the Sierra forests been properly managed over the past century the Rim fire wouldn’t have been so devastating, according to Martha Conklin, director of the University of California, Merced, Natural Reserve System.
Conklin also spoke at the groundwater conference, in part because the health of the Sierra forests affects water flows down to the Valley.
Before dams and reservoirs were built to capture Sierra snowmelt, Conklin said, the San Joaquin Valley “was basically a wetlands because that’s where the runoff occurred.” She said the dams “were designed for flood control.”
But the Sierra forests have changed since many of those dams were built, and Conklin said less water now flows down from those mountains. That, in part, is because too many trees have been allowed to grow.
“The trees in the Stanislaus National Forest were surveyed in 1911 and resurveyed about 100 years later. The average tree density in 1911 was 150 trees per acre and 100 years later 500 trees per acre,” Conklin said.
That’s mostly because fires used to be allowed to burn, which naturally thinned the forests. About a century ago, however, Conklin said fire suppression efforts became government policy and that enabled many more trees to grow.
Although the number of trees in the Stanislaus National Forest more than tripled, “the biomass is only twice as much, suggesting more, smaller trees,” Conklin said, citing research done by a UC Berkeley graduate student.
Those smaller trees burn more easily than larger trees, Conklin said, and they help wildfires spread up to the taller trees.
Fire risk aside, dense forests reduce water runoff, Conklin warned. Not only do trees consume water, but their branches hold snow and allow moisture to evaporate into the atmosphere. Snow that falls to the ground, by contrast, melts into the soil and eventually drains out of the mountains down to the Valley.
“We have what we’d call an overgrown forest,” Conklin said. Although “it is counterintuitive,” she said, the abundance of trees is hurting the Sierras and reducing California’s water supply.
If small trees in the forests are thinned, Conklin said, the fire risk would decline and the water flows to the Valley would increase. Her preliminary research shows water yields could increase 10 percent if half the trees, essentially the small ones, were trimmed.
That still would leave many more trees than existed a century ago, but the Sierras would be healthier, she said.
Conklin said the cost of such thinning would be far less than the cost of fighting massive blazes like the Rim fire.