Edgar McGregor

On Sunday, August 20, 2023, Tropical Storm Hilary made landfall in Los Angeles County, California with sustained winds of 40 MPH, gusting to 50 MPH. The storm is the first tropical cyclone to make landfall on the west coast of the United States since 1939. Hilary brought damaging flash floods to the Coachella Valley, burying some neighborhoods in mud and debris. Rainfall totals of up to 11″ were reported on the eastern slopes of the Peninsular Range, a part of Southern California that is normally very dry and cannot handle such high rain totals. In the Transverse Range, however, the storm was mostly beneficial having interrupted a much warmer than average July-August period with soaking rains.

In Eaton Canyon, light to heavy rainfall fell for about 24 straight hours from 7 AM on the 20th through 7 AM on the 21st. Eaton Dam picked up an incredible 5.75″ of precipitation, bringing the 2022-2023 water year total to 49.20″ of rainfall. This makes summer 2023 the wettest summer in at least 116 years of weather record keeping in Old Town Pasadena. Up at Camp Hi-Hill behind Mount Wilson, the area reported 6.52″ of rainfall for the storm, one of the higher rain totals in the Angeles National Forest. Henninger Flats reported 5.88″ of rainfall, while Inspiration Point reported 5.12″.

Rainfall rates peaked at 1.08″ per hour during the dusk hours on the 20th as the remnant eyewall moved through, and then again at 1.32″ per hour around midnight on the 21st as the back side of the system arrived. These rainfall rates were, while high, nowhere near what is required to get significant flash flooding in the park. On December 14, 2021, a thunderstorm with rain rates of 2.52″ per hour hit the canyon, sending a decent flash flood down the park. That particular thunderstorm’s small size and quick departure prevented it from causing catastrophic flooding in the canyon, though it left nearby Rubio Canyon severely scarred. Park-altering floods in the canyon occured in 1938, 1943, 1969, 1980, and 2005. 

All of this tropical rain will delay the wildfire season in Southern California for several more weeks if not a couple of months. With El Niño taking shape down in the equatorial East Pacific, the 2023-2024 winter is likely, though not guaranteed, to be wetter than average. This is fantastic news for the 23-year long megadrought inflicting the Western United States.

The bad news is that visitation rates will remain elevated in Eaton Canyon as we can now forget about the creek drying up later this autumn. Most of the water that fell with Tropical Storm Hilary is still underground in the upper canyon, and it will slowly drain over the coming weeks and months. Littering rates will remain elevated as autumn waterfall chasers continue to rush the canyon. 

---

Here are some notable changes in the park caused by Tropical Storm Hilary: 

1. An Eaton Canyon Nature Center staff member and a volunteer were there to witness a ~500lb boulder fall 30 feet off a sandstone cliff along the west bank trail the morning after the storm, proving just how dangerous the park can still be even if the flash flood threat is gone. The rockfall occured about one tenth of a mile past the first crossing.
2. A new ~3′ – 4′ waterfall has been built by the small flash flood that did come down the canyon on August 20th. It is located directly underneath the Pinecrest Gate, with tons of sand and silt getting backed up behind it. A waterfall has existed in that exact location in the past, but it was bypassed by the creek during the December 14, 2021 flood when several logs blocked water flow. Now, those logs are gone and the waterfall is once more flowing. We won’t share a picture of it. Go have a look yourself! 
3. A large collection of logs and other flash flood debris that served to catch trash floating downstream from the bridge area is missing, having washed away during the small flash flood. This will allow trash to flow into more inaccessible places for cleanup crews. Bummer.
4. Hundreds of new western sycamore saplings growing alongside the wash in the canyon survived the flash flood, keeping hopes high that a new generation of that species will flourish in the canyon. It has been 18 years since the last generation of western Sycamores successfully took hold in the canyon. Western Sycamores need two wet winters in a row with a cool summer in-between to survive infancy. Climate change has made too many summers in recent years too hot in our area for any saplings to take hold. Throughout the entire park, there are just 3 western sycamore saplings under 6′ tall.  Summer 2023 certainly hasn’t been cool, but thanks to Tropical Storm Hilary, water will not be a problem for these new trees. 

Tropical convection building over the mountains on August 19th

An image of the rock that fell onto the west bank trail 

A wet canyon, near where Moist Canyon joins the main wash

A highly unusual sight in August… to say the least! 

I think the canyon might’ve liked that!