Map shows epic amounts of water gushing through California rivers

Water rages down the Merced River near Midpines (Mariposa County) on May 11. The historic California snowmelt means an epic season for river rafting but also calls for extra safety measures due to the size of the rapids and the speed of the river.

This year's historic snowpack has meant epic amounts of water flowing through California's rivers, streams and creeks.

"Everything's high right now," said Travis Hiett, head of the United States Geological Survey's Sacramento field office. "The San Joaquin's super high. It's been 10,000 (cubic feet per second) for, seems like months now — and that's very rare."

That's more than the capacity of four standard 40-foot shipping containers rushing by each second.

Around 40% of the roughly 500 stream gauges across the state are running above normal, provisional data from the U.S. Geological Survey shows. A few dozen are registering record highs for this time of year, especially along the central and southern Sierra. With peak melt season expected in the coming weeks, this means plentiful amounts of water running into reservoirs, but also dangerously fast flows and the risk for potential flooding.

"During record-breaking wet years such as we’ve seen in 2023, the high flows and erosional power of rivers can pose serious risk to infrastructure and even human lives," said Emily Burt, a postdoctoral researcher who studies hydrology at Chapman University, by email.

The map above shows sites across the state where streamflow over the past week has been running above normal, based on data collected for the same time and place, over decades. These values, averaged over seven days, represent recent trends better than measurements for a single day.

Scientists can calculate the volume of water moving down a river per second — also known as discharge — through measurements relating river levels to flow rate. The size of each circle in the map reflects how much water is flowing, while the color indicates how streamflows compare to average values.

Spots on some rivers and streams, labeled in purple, are seeing their highest flows ever for this time of year. One example is Pohono Bridge in Yosemite National Park, where the Merced River averaged 7,859 cubic feet — almost 60,000 gallons — of water per second. That's over three times the average amount for this location.

Red "danger" tape is stretched across the Valley View area on May 23, preventing visitors from getting too close to the swift-flowing Merced River in Yosemite National Park.

The high water has caused flooding, forcing the closure of several campgrounds.

The Kern River, in the southern Sierra, is flowing at over eight times the average amount for this time of year. The discharge was about 2,860 cubic feet per second on May 25. "This is equivalent to filling one Olympic swimming pool in just under 30 seconds," Burt said.

All this water means dangerous conditions in rivers across the state: "Snowmelt has area waterways running very COLD and FAST!" tweeted the National Weather Service Sacramento office.

While streamflow is currently very high, it's not unprecedented, Hiett said. At Pohono Bridge, the Merced River peaked at 24,600 cubic feet per second in January 1997.

On the whole, the Sierra snowpack has melted gradually this spring, in part because of temperatures that have generally run cooler than normal for months. A heat wave about two weeks ago bucked that trend.

"That really got the snowmelt going," said Brad Moore, a hydrologist with the National Weather Service California Nevada River Forecast Center.

A more drawn-out version of such high temperatures would be a catastrophic combination with the record-breaking snowpack, experts have said. That could produce far more water than reservoirs and levees could handle, especially in the San Joaquin Valley.

Fortunately, the next week looks to be relatively mild, with even some cloud cover that blocks out snow-obliterating sunlight. "These scattered showers we’re having in the mountains nowadays, that actually does a lot to slow down the snowmelt," Moore said.

Peak snowmelt is expected from the end of May into early June, according to models by the California Nevada River Forecast Center.

Reach Jack Lee: [email protected]