Eaton Canyon Escapes Major Pacific Storm With Minimal Damage

Fourteen, seven, eleven, and ten: This is how many inches of rain fell on Eaton Canyon over a 3-day period during the most cataclysmic storms of the century. In no fewer than 6 instances between 1943 and 2024, Eaton Wash has been completely reset, ordered, and renewed, giving rise to an entirely new park each time.

Between February 4-6, 2024, the San Gabriel Valley experienced 54 hours of continuous rainfall. In that time, a whopping 8.5″ of rain fell at Eaton Dam while over a foot of precipitation fell at Camp Hi-Hill behind Mount Wilson. The storm entered the top-10 list for rainiest 2-day period ever recorded in Pasadena, a list occupied almost exclusively with dates that saw catastrophic flooding in Eaton Canyon. This time, however, Eaton Canyon emerged essentially unscathed. No such flooding of any appreciable magnitude happened. In fact, not even the Mule Fat that lines the creek was messed with. How is such a thing even possible?

Rising 6,000′ over the nearby Pacific Ocean, Eaton Canyon has a long and complicated relationship with flash flooding. For millions of years since it’s formation, Eaton Canyon’s flash flooding has gotten worse as the Sierra Madre fault continues to push it’s upper peaks higher and higher into the sky, enhancing orographic lift. Being at the intersection of a semi-arid desert, mountains, a reverse fault, and an ocean that covers half the planet, this history is not surprising. Hiking through the wash will yield lots of interesting discoveries, including flash flood debris, 12-ton boulders, and Bigcone Douglas Fir logs sitting out in the shrublands.

In general, there are 4 primary ingredients needed for major flash flooding in Eaton Canyon. Listed from most to least important, they are as follows:

1. Torrential rain rates (>2.00″ / hr)
2. Burned landscapes
3. Waterlogged soils
4. High snow levels

During the February 4-6, 2024 atmospheric river, the two most important ingredients for flash flooding in Eaton Canyon were absent. It has been 30 years since the 1993 Kinneloa Fire, and with so much vegetation covering the canyon, rain had a chance to soak in before running off the steep mountain slopes. Additionally, rain rates peaked at just 0.96″ / hr during the storm system, unlike the December 14, 2021 event when rain rates peaked at 2.51″ / hr. While enormous amounts of precipitation fell and snow levels remained >5,000′ during most of the system, major flash flooding in Eaton Canyon did not occur.

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That all being said, here are some notable changes to the canyon from this storm system:

At approximately 2:42 pm on Sunday, February 4, 2024, a Coast Live Oak tree near the east end of the Meadow Trail collapsed in two parts about 1 minute apart. Parts of the tree are still alive and located at 34.181256342951244, -118.09564831451077. The Coast Live Oak was weighed down by the rain having begun about 40 minutes prior, and several large branches collapsed. Nobody was hurt, though several people by the first crossing heard the tree fall.

Yet another rockfall occured along the Mount Wilson Toll Road below the Pinecrest Gate sometime between Sunday night and 11 am Monday. A second rockfall occured just below Henninger Flats.

Photo by Sandy Chang

A new swale built by Eaton Canyon Nature Center staff last summer performed well during the storm. It prevented runoff from the parking lot from eroding other trails or being lost to Eaton Creek.