The 2023 Kaweah River High Flows in Sequoia National Park, Compared With the Past

The Marble Fork of the Kaweah River in the foothills of Sequoia National Park, January 9, 2023. The dark, muddy water is partly debris from slopes burned in the 2021 KNP Complex Fire. NPS / Dave Fox.

Years of severe drought have focused California on water shortages and wildfire. But the state also has a history of megafloods, and climate change is raising the odds of a very large flood within our lifetimes. The high rain and river flows of winter 2022–2023 were far from megaflood scale, but they can be compared with past events. And large fires in 2020 and 2021, which stripped vegetation from slopes above roads and trails, made it more likely that storms would bring rockfalls and debris that damage infrastructure. A longer view helps park managers prepare for these combined water and fire hazards.

Watching the river

Park ecologist Erik Meyer compared peak flows at a river gauge in Three Rivers, California, which has recorded hourly and daily flow since May 2007. He matched every peak hourly reading to weather records from the National Oceanic and Atmospheric Administration and to a written history of local floods. The peaks of January 9, March 10 and March 15, 2023 were the highest hourly and daily flows at the Three Rivers gauge in 14 years, though each was lower than the 1997 peak of 56,595 cubic feet per second at a gauge downstream, where the river enters the Lake Kaweah reservoir.

Cubic feet per second (cfs) measures the volume and speed of flowing water. A cubic foot is about the size of a basketball, so a stream flowing at 100 cfs is like 100 basketballs rolling down the riverbed together. The higher the number, the higher the river.

Peak hourly flows (top) and mean daily flows (bottom) at the Kaweah River gauge in Three Rivers, California, May 23, 2007, to March 15, 2023. The vertical scales differ; daily means average 24 hours, so they vary less than hourly peaks. NPS / Erik Meyer.

The original dates the 2002 event November 8 in its table and November 9 in its text.

What drove the high flows

• Atmospheric rivers drove at least six of the nine events (the 1997, 2002, 2009 and three 2023 events) and probably contributed to the rest. These long ribbons of moisture carry huge amounts of water vapour from the tropics toward the poles; when they move inland and hit mountains, the air rises, cools and drops heavy rain. They bring nearly half of California's precipitation and cause most of its major floods. One carrying tropical moisture is informally called a "pineapple express".
• Rain on snow contributed to seven of the nine. Rain falling on snowpack releases meltwater from large, high drainage basins; in the warmest storms the area contributing water can become several times larger than in a typical winter storm. Depending on the rain's intensity and the snow's depth, meltwater can start flowing out of the snowpack within hours. As winters warm, rain falls at higher elevations, adding still more meltwater downstream.

Fire, then flood

When big storms follow large, severe fires in the mountains, the damage compounds. Burned slopes have lost the plants that hold soil and rock in place, so intense, long storms send soil, rocks and fallen trees onto roads. Culverts built for normal runoff clog with debris, and water then flows over roads or erodes beneath them, lengthening the time needed to assess and repair them and keeping roads closed longer.

Left, erosion undercutting the Generals Highway between Hospital Rock and Giant Forest; right, the outer edge of the road to Mineral King collapsed near Slapjack Creek. NPS.

The river on peak days

These photographs were taken from a bridge over the Main Fork of the Kaweah just outside the park's main entrance station, on days of peak flow, though not necessarily at the peak itself.

March 8, 2023: a more typical flow, for comparison, two days before the March 10 storm. NPS / Tony Caprio.

January 2, 1997, about 5 p.m., when the Lake Kaweah inflow gauge recorded about 23,000 cfs; by 11 p.m. it peaked at 56,595 cfs. An atmospheric river and rain on snow caused flooding and extensive damage across the region. NPS / Tony Caprio.

March 10, 2023: the peak flow was second only to 1997's in 26 years. NPS / Tony Caprio.

January 9, 2023: the dark, muddy water carries sediment washed from slopes burned in the 2021 KNP Complex Fire, where the loss of plants left soil unstable in heavy rain. NPS / Tony Caprio.

February 7, 2017: rain on a large snowpack, more than 150% of average for the time of year. NPS / Tony Caprio.

December 19, 2010: 8.5 inches of rain at low elevations in a few days, December 17–20, sent the river up sharply. Tony Caprio.

The past and the future

Earlier years brought many higher flows and floods, and larger storms and floods are becoming more likely. The 1861–1862 megaflood is widely treated as the benchmark for a "plausible worst case" flood in California today: it flooded an approximately 300-mile stretch of the Central Valley, large parts of the Los Angeles area and nearly every narrow river valley in the state. Models of a similar event in modern California show catastrophic, statewide flooding causing more damage, disruption and economic loss than a large earthquake near a major city.

Research predicts that as temperatures and freezing levels rise, precipitation at low to middle elevations (4,000 to 6,500 feet) will shift from snow to rain, while above 7,000 feet it will stay mostly snow, and snow there may even accumulate much more. More rain means immediate runoff instead of snow that melts slowly, and more rain-on-snow events are expected, raising flood risk further. The parks can use data and experience from recent fires and storms, together with such predictions, to prepare for growing hazards.

Sources

• National Park Service, Sequoia and Kings Canyon National Parks, "Putting It in Perspective: How do the 2023 high river flows compare to recent past events?", summary by Linda Mutch and Erik Meyer, February 2023. https://www.nps.gov/articles/000/seki-river-high-flow-events.htm
• J. T. Austin, Floods and Droughts in the Tulare Lake Basin, 2nd edition, Sequoia Parks Conservancy, 2015.
• R. Kattelman, "Flooding from rain-on-snow events in the Sierra Nevada," 1997. https://data.sacriver.org/assets/5ffd2763a1b5d1a3078c4bd991651244/application/pdf/Flooding_from_rain-on-snow_events_in_the_Sierra_Nevada.pdf
• National Oceanic and Atmospheric Administration, What are atmospheric rivers?
• D. Swain, "ARKStorm 2.0: Climate change is increasing the risk of a California megaflood," Weather West, 2022.