Continuing my efforts to map landslides in the San Jacinto and Martinez Mountain area, I got research access to go through Deep Canyon, protected by UC's Boyd Deep Canyon Reserve. Deep Canyon is a fascinating and wild canyon up to 1500ft deep, with usually steep slopes and a large catchment draining the Santa Rosa Mountains. The main objective was to confirm and refine the aerial reconnaissance mapping I had previously done, and to look for evidence of older landslide deposits within the canyon system. Technical gear would be necessary to descend the canyon so I recruited two experienced field assistants, Sonny and Calius, to join me.
To make the most of the day we decided to try to enter the canyon from a high unnamed tributary draining the Pinyon Crest area. We found several large drops in this tributary, all of which we were able to downclimb by partner assisting where necessary. This turned out to be a relatively efficient way into the canyon, and we were at the edge of the intermittently flowing creek in no time.
Once in the canyon we could immediately appreciate its pristine nature, with fantastic native succulents and cacti on its slopes, and cottonwoods and palms interspersed along its streambed. The rock hopping along the creek was pleasant enough, and the occasional downclimbs and waterfall bypasses kept the travel entertaining. Interestingly I noticed long stretches of canyon with next to no landslide or scree deposits. This canyon is certainly a very active system when it comes to sediment transport!
About a mile above Hidden Palm Canyon we approached the horizon line of a beautiful fluted rappel with a deep and cold looking pool below, our first rappel. A retrievable anchor was easily found and one by one we braved the frigid pool with slight screams of shock.
I had my iPad at the ready to map the extent of the rockfalls as we continued along at a reasonable pace. All rockfalls appeared to be quite young, with very little residence time in this steep canyon. It was clear that certain rock types (e.g. fractured quartzite) were much more prone to rockfall than others (e.g. orthogneiss).
Below Hidden Palms the canyon continued to deepen and drop with more mandatory rappels and some nice palm oases. As we descended, the water progressively warmed which we were thankful for. The heat of the day quickly dried us out between the brief swims. I even found a few optional jumps and a slide too great to resist.
A particularly beautiful bedrock pothole
After several hours of enjoyable fieldwork the canyon began to open and the water ceased to flow as it sunk into the alluvium. We had to do about a half dozen rappels to get through the canyon, and in only one instance had to leave a small anchor behind to be safe (2ft of webbing). At one of the last bedrock channel stretches a rockfall forced the creek to carve a new channel, representing one of the few older deposits identified (but nothing to date). After a quick stop through the research center we drove out to complete our car shuttle. Here at the bottom was where the surprises were lurking. I immediately recognized the geomorphology of three very large rock avalanche deposits, highly eroded and clearly quite old. Here were the old (likely earthquake triggered) events I was hoping to find. I look forward to another day or two's fieldwork mapping their extents! I continue to be impressed at the number of landslides (particularly large and old ones) that have been lurking unnoticed in the landscape.
Thanks to Sonny and Calius for their field assistance and the staff of the Boyd Research Center for being enthusiastic about the fieldwork.
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