Abstract
Over one hundred large Bigcone Douglas-Fir (Pseudotsuga macrocarpa) logs are scattered among the desert-like fields of sand, cacti, and boulders in the wash portion of Eaton Canyon on the south face of the San Gabriel Mountains. As no specimens of this species grow within a 2-mile radius, these logs are quite conspicuous. In this study, we will attempt to reconstruct historical flash floods in Eaton using aerial imagery. Then, we will back up that method by comparing results with Bigcone Douglas-Fir log locations. The first step, however, is to prove that these logs were indeed delivered to the wash from higher elevations by major flash floods.
This study finds that major debris flows are in fact responsible for placing these logs in the wash portion of the canyon. However, because of differences in magnitude of each flood as well as changes in topography between floods, this study also finds that not all log locations are the result of the same flood. This study has been able to determine which logs have moved as recently as 2021 and which logs have sat stationary for over half a century. In addition, this study has been able to determine how many acres of Eaton Canyons’ wash were destroyed during each of the most recent large floods.
Introduction
Eaton Canyon experiences rapidly changing climatological and ecological changes between its wide elevation range. Elevations between 4,500′ and 6,100′ in the canyon consist of conifer forests full of Bigcone Douglas-Firs, Canyon Oaks, Coulter Pines, and even the occasional California Incense Cedar. Meanwhile, elevations between 900′ and 3,000′ in Eaton Canyon can be very hot and dry, with well draining soil, chaparral ecosystems, and medium sized trees huddled in canyons. Eaton Canyon is also an exceptionally steep mountain face, with elevations rising several thousand feet in a matter of 3 miles. The 6-square mile canyon averages over 100″ of snow each winter at one end and can experience 6 straight months with an average high >90°F at the other end. Owing to their size, shape, and length, the vast majority of logs in the wash should be the Bigcone Douglas Fir, a resilient conifer native to the region that dominates the conifer forest above. At lower elevations, Eaton Canyon’s only primary native trees, the Western Sycamore, the Coast Live Oak, and the White Alder, do not have any of the characteristics that these logs show.
Methodology
To demonstrate that these logs were transported by flash floods, we designed three specific tests that can be conclusively attributed to such events. These tests are as follows:
- Log Distribution: If flash flooding was indeed responsible, there should be a higher concentration of logs in wider sections of the wash, with a noticeable scarcity in narrower sections where deeper and faster-moving waters would carry logs away.
- Log Condition: All logs should be completely devoid of branches, consistent with the forceful removal of limbs during transit via high flow events.
- Species-Specific Evidence: Bigcone Douglas-Fir logs should be completely absent in areas not affected by major flooding in past decades, indicating that their presence in historical flood zones can only be due to the floods themselves.
Over the past 5 years, we’ve carefully mapped out and cataloged 108 individual non-native logs scattered throughout Eaton Canyons’ wash between the Chuck Ballard Memorial Bridge and the New York Bridge. (Figure 1) While some of these logs are out in the open and easy to spot, others are partially buried, deep inside bushes, or in rather inaccessible locations. The largest log on this list has a diameter of 57 inches, while the longest is 31 feet. In order for a log to be considered in this list, it must have a diameter of at least 8″. (Figure 2) Once the mapping of these logs was completed and our maps were analyzed, the first two tests should be answerable.
The next step in this study is to locate the dates and impacted zones of the canyon’s largest debris flow events. Our team noticed that when large floods occurred in the canyon, significant scarring left the landscape with a much higher albedo, indicative of the loss of all vegetation and the exposure of white granite rock, gravel, and sand. Utilizing aerial and satellite imagery courtesy of HistoricAerials.com, several different major flash flood events were made apparent. The most recent significant flood to have struck the canyon occurred in the winter of 2005, with satellite imagery in August 2005 showing much less vegetation in the wash compared to the year prior. (Figure 3) According to the Clear Creek RAWS (CEKC1) station near Mount Wilson, 23.26″ of rain fell during the 96 hours stretch ending at 5:00 am on January 11, 2005. (Figure 4) The nearly two feet of rain in just four days coincides with local recount as to when the major flood occurred. The mapping technique concluded that 31.6 acres of the wash were destroyed by fast moving water and boulders. Once we concluded a major flood last occurred in 2005, the team overlayed a map of the log locations with a map of where the flood is thought to have impacted the canyon. (Figure 5) Of the 108 logs cataloged, only 76 logs were inside the boundaries impacted by the 2005 flood. The other 32 logs, some of which more than 250 feet removed from any location in which the 2005 flood touched, needed a further explanation.
Further analysis through aerial imagery and weather data concluded that the winter of 1980 was also responsible for a major ecosystem-disturbing flood in Eaton Canyon. This time, nearly 20″ of rain fell at the weather station in Pasadena over a period of 8 days. This is more than twice the amount of rain that fell in the four-day period in 2005, though it fell in a longer time span. The Clear Creek RAWS station was not yet operational in 1980 to record this event, though upper portions of Eaton Canyon likely saw 30.00″ – 40.00″ of rain. Albedo mapping of the canyon floor resulted in an estimated 48.5 acres of the wash ecosystem being destroyed by the flood, about 38% more land than the 2005 flood. (Figure 6) After overlaying log location data, 29 of the 32 unexplained logs were located in the areas severely impacted by the 1980 flood. Three additional logs, however, stubbornly remained outside both the reconstructed 1980 and 2005 flood zones.
Using satellite imagery, historical photographs, and stories from locals once again, a third flood identified in 1969 that destroyed 43.0 acres explained the final three logs position within the wash. An interactive view of each flood zone may be found here.
Conclusion
By applying the three previously outlined tests against real-world observations, this study conclusively proves that these Bigcone Douglas-Fir logs were transported by major flood events before being deposited throughout the wash. A mapping of all logs in the canyon showed a much higher density of logs in areas where the wash is wide versus areas where the wash is more narrow. (Figure 7) A physical analysis of all logs also shows absolutely no branches or anomalous outcroppings within each log’s shape, consistent with the forceful and destructive nature of major debris flows. (Figure 8) Finally, aerial mapping of historic flood plains using significant changes in albedo perfectly matches with where logs are located within the lower canyon, with none of the 108 logs cataloged occurring outside known flood zones. (Figure 9)
Results and Discussion
This study has successfully reconstructed past flooding events using both aerial imagery and Bigcone Douglas-Fir log locations. We find that the 1969, 1980, and 2005 floods destroyed 43.0, 48.5, and 31.6 acres of Eaton Canyon’s wash respectively. This shows that although the 1969 flood gets lots of attention due to its destruction of the El Dorado Inn, the 1980 flood was actually larger and more impactful for the local ecosystem. The mapping of these debris flows also shows that just because one particular area was left untouched by a large flood does not mean it is safe from a less severe one. The team found that while the logs are fairly well distributed along the 2-mile long riverbed, there is a downward trend the further one gets away from the canyon mouth. Additionally, these maps show that the lower section of Eaton Wash beside the Eaton Canyon Nature Center has been drifting westward toward the parking lot with each successive flow. Most recently in 2005, the north end of main parking lot was nearly destroyed after fast moving flood water begin quickly eroding the embankment separating the parking lot from the wash, something the 1969 and 1980 floods couldn’t do. Local claims that the upper parking lot was destroying during one of the floods could not be confirmed using satellite imagery. Readers are encouraged to submit evidence should these claims be correct.
Future Work
Field biologists may want to investigate why these logs appear to show little to no signs of decay while sitting out in the desert. It is plausible that the necessary microbiology, including fungi and bacteria, needed to break down the logs does not exist in the hot, dry ecosystem in which these logs have been transported to. As a consequence, even the three logs identified to have not moved since 1969 (55 years since the writing of this study) show little sign of decay. (Figure 10)
An analysis of future flood potential using 3D-imagery, climate data, and historical risk should be conducted, especially in regards to the danger posed to Los Angeles County Parks and Recreation amenities such as the Nature Center by a quickly-changing creek route during extreme flow events.
Keywords: Bigcone Douglas Fir, flash flooding, Eaton Canyon, geomorphology, hydrology, sediment transport, arid environment, log preservation, heavy rainfall, debris flow, compartmentalization, desert, alluvial fan, scarp.
Charts and Figures
Figure 3: A comparison of the canyon wash between the summers of 2004 and 2005 shows a heavy loss of vegetation as well as a change in the creek route. Note that the wash encroached on the upper parking lot, along with the loss of a large oak that once served to reduce erosion.
