News

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.

---

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.

On Saturday, January 13, 2024, 29 participants set out with bags, clippers, and loppers to clear significant overgrowth along the little-known Meadow Trail within Eaton Canyon Natural Area. Among them was the We Explore Earth group, a non-profit organization that seeks to “build stewards through nature experiences.” The Meadow Trail is a small nature trail that parallels the main trail. It begins near the last set of trash cans across the wash and ends only 0.16 miles later under an oak tree beside the main trail. The trail traverses an oak woodland known to support many different species of plants, animals, mosses, lichens, and fungi.

The Meadow Trail’s origins can be traced to the 1940s when the California Institute of Technology used it as a dirt road during its weapons manufacturing era of World War II. It is unknown exactly how the road was used, but it is known that the U.S. military tested weapons in Eaton Canyon during this time. More on Eaton Canyon’s involvement during World War II can be read here.

In addition to clearing the trail for human access, the crew also removed enormous amounts of the invasive Castor Bean plant growing in the area.

According to a new United States Department of Agriculture (USDA) plant hardiness zone map released in November 2023, parts of lower Eaton Canyon have been elevated to the 10b zone classification. The USDA 10b zone is characterized by annual average extreme minimum temperatures being within 35°F and 40°F. In other words, the average year is not expected to drop below 35°F even on the coldest winter nights in 10b zones.

This warming is a direct result of the combination of the urban heat island (UHI) of Los Angeles and global climate change.

The USDA plant hardiness zone map is a reference that can be used by gardeners to understand which of the plants they wish to grow can survive the coldest temperatures of the year. Since it is based solely on the year’s minimum temperature, the map has been criticized for its inability to incorporate other aspects of a particular area’s climate. For example, most of Miami-Dade County in southern Florida shares a USDA hardiness zone of 10b with Eaton Canyon. This is because Eaton Canyon and Miami-Dade County both see an average annual extreme minimum temperature of 35°F – 40°F. While Miami-Dade County is a warm tropical rainforest that has never exceeded 100°F, Eaton Canyon has a relatively cooler mediterranean climate that can top 115°F in summer. 

Being located in a dry, mountainous region of the world, Eaton Canyon is subject to dramatic temperature swings between the ridgetops and the canyons at night. On dry winter nights, these temperature differences can be most pronounced. Here, temperatures within the wash can plunge into the 30s while the homes overlooking the canyon remain in the 40s. A given night’s minimum temperature can be as much as 15°F warmer at the Pinecrest Gate compared to the Eaton Canyon Nature Center. At a more microclimate scale than just broader Eaton Canyon, it’s possible that some areas are already in USDA hardiness zone 11a, which does not drop below 40°F on even the coldest nights of winter.

Here is a look at how annual extreme minimum temperatures have warmed locally over the past 116 years: