What's Wrong With This Picture? Part 3: The Mass Wasting of Del Puerto Canyon

It's no secret that I am against the construction of a useless wasteful dam in Del Puerto Canyon. I have had a lot to say about the project in public testimony and in recent blogs, and my opposition is deeply rooted in my appreciation of the canyon as a unique and irreplaceable outdoor laboratory for understanding the geology and natural history of the Diablo Range. It ought to be a national park or monument, not a stagnant pool of mud. But my opposition to this project also follows from my understanding of the hazards of constructing a reservoir in unsuitable and unstable rocks with a proximity to earthquake fault zones of unknown potential activity.

The words "mass wasting" in the title might be interpreted in several ways. It would certainly be a huge waste of money, and a squandering of a treasure of national significance. But to a geologist, mass wasting has a specific meaning: it is the downhill movement of rock and debris under the influence of gravity. Most people call this landsliding, but "land" is a non-specific term, and "sliding" is too specific. Mass wasting can involve sliding debris, but also flowing and falling material.  This post is not about the future potential hazard of mass wasting in Del Puerto Canyon. It is about the active slides that are already there.

Let's see what the Geotechnical Memorandum of the Environment Impact Report has to say about the mass wasting hazard in Del Puerto Canyon:

A significant number of landslides are found within and in the immediate vicinity of the reservoir inundation area, the majority of these landslides are located within units of the Cretaceous Moreno formation, upstream from the proposed main dam. At least seven landslides are mapped within the inundation area of the proposed reservoir – six are in the Moreno formation and one landslide occurs in the Panoche formation (Figure 5). It is expected that additional small landslides and movement of existing landslides would occur as a result of reservoir infilling and operations. These landslides would be expected to experience continuous deformation without some form of stabilization/mitigation. The rate of movement of these landslides would likely be slow. Stability of the reservoir rim, including potential for seismically triggered landslides would be required for design of the Project.

But wait, as they say, there's more...

The proposed reservoir would inundate areas underlain by the Cretaceous Moreno and Panoche Formations. Landslides are found within and in the immediate vicinity of the Project Area, the majority of which are located within units of the Moreno formation, upstream from the proposed main dam. Movement of these landslides is expected as a result of infilling and seasonal operations of the reservoir. It is expected that additional landslides would form as well. Movement of existing and any newly developed landslides resulting from reservoir operation is expected, any deformation of the landslide would be relatively slow and at scale that would not form seiche waves of significant magnitude that would overtop the proposed dam. An assessment of landslide potential and impacts to the Project would be needed for final design of the reservoir and dam.

That's pretty much it. Seven landslides within the reservoir inundation zone, with no specifics about the age of the failures, or the volume and length, and nothing about the current activity. These are precious few words for what could be one of the most hazardous aspects of the proposed reservoir.

Here's figure 5 from the EIR (above). It is a geologic map that shows the different rock formations found at the dam site and the location of the landslides mentioned in the quote above. The proposed reservoir is outlined in blue, and the landslides are the white areas are in white, with black arrows showing the direction of movement. The scale of this map is very roughly 1 inch = 1 mile. These mapped landslides are not small. The smallest mapped slides are about 1/5 of a mile in length, while the largest is about a mile. A mile. These are not minor earth movements.

The three pictures in this post provide a perspective for understanding the size of these unstable masses. The images show the same slide from three different angles, and the trees and farm buildings provide scale. It's huge. It is probably several thousand years old, as it filled the canyon it occupies, pushing the stream all the way to the right against the mountain slope. But it is not done...as Del Puerto Creek has undercut the base (the toe) of the slide, the lower parts have been reactivated, forming the sharp terraces (scarps) above the creek.

Whatever stability exists with this slide lies in the friction between the mass and the underlying rock. But the slide is slated to be at least halfway inundated by the waters of the reservoir, and water is a hugely destabilizing force in mass wasting. It gets between rock surfaces and in essence breaks down the frictional resistance. Geologists other than myself will have to assess the possibilities of inundation, but one of the worst-case scenarios would involve a rapid flow or slide of a huge volume of rock and debris into the reservoir, displacing vast amounts of the water over the top of the dam (the seiching mentioned in the EIR). A less catastrophic outcome, but no less significant, would be the slow flow of debris into the lake. Every cubic yard of debris going into the lake is a cubic yard less of water storage. The slide in these pictures is the smallest of the mapped slope failures. Imagine what happens when all six or seven of the slides are rejuvenated by the waters of the proposed reservoirs.

These are serious concerns, and although the draft EIR addresses some of them, they are buried within something like a thousand pages of the EIR itself, and the background memorandums. This dam is being proposed for construction in an area of unstable and unsuitable rocks and sediments within an area where the earthquake risk is poorly known. I've been learning about the region for thirty years, and I was surprised to find that there was a 6.1 magnitude earthquake in 1881 in the immediate vicinity of the dam site (six miles), but that "the location is highly uncertain given its pre-instrumental age and is based on intensity estimates documented in the public record". That is the kind of uncertainty that should give everyone pause about the magnitude and risks of this dam proposal.

There will be many reasons offered as to why this reservoir is SO necessary, and how there will be SO many benefits. But the questions need to be asked, what is this taking away from all of us, and what is the full magnitude of the threats we will face if it constructed?

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Read the Environmental Impact Report at this link. If Del Puerto Canyon has significance to you, please respond and be active in the opposition! If you have expertise in any of the areas that will affected, you need to be heard from.

There are several important meetings and deadlines coming up very soon:

1/27 5:00pm Public Comments DUE. ahansen@delpuertowd.org OR Anthea Hansen PO Box 1596 Patterson CA 95363 (use the forms outlined in the EIR documents)
1/28 9:00am Board of Supervisors Meeting 1010 10th St Modesto CA - voice concerns, they have final decision