The Pacific Gas & Electric Co. is surrendering the Federal Energy Regulatory Commission, or FERC, license for the Potter Valley Project, or PVP.

PG&E identifies this action as a business decision because of the project’s failure to produce revenues that offset its operating costs, even though PG&E customers pay higher rates for delivered energy than just about everywhere else in the United States.

In our opinion, PG&E has determined to rid itself of the PVP for a different kind of economic consideration, after determining that the Scott Dam represents an economic liability that the company cannot afford.

A key factor in this determination is the increased understanding of the seismic hazards represented by the Bartlett Springs Fault Zone, or BSFZ, which runs through Lake Pillsbury approximately 5000 feet east of Scott Dam.

As part of the PVP relicensing process, FERC held an auction for potential alternative licensees for the PVP. No takers made offers to accept the ownership of and responsibility for this existing hydropower license, for the same reason that PG&E does not want the responsibility for these existing conditions: a recognition of the outstanding risk that the BSFZ represents for the PVP licensee.

The history of the PVP doesn’t need to be repeated here, but the scientific understanding of Earth sciences that has developed in the past century, which is critical in considering the best options for the future of the PVP, is less well-known.

The geological framework represented by plate tectonics is particularly significant in understanding the circumstances presented for the PVP. The dynamics of plate tectonics were not understood in the early 1900s when the Cape Horn and Scott Dams were designed and constructed.

Over this past century, the scientific understanding of plate tectonics (including the Bartlett Springs Fault Zone) has developed continuously, and most of our current understanding of how tectonic dynamics affect northwestern California has developed fully only in the most recent 30 years.

A short summary of western California’s geological history shows that until about 28 million years ago the western continental margin was a “subduction zone” with the Farallon Plate subducting beneath the western margin of the North American Plate. West of the Farallon Plate was another plate (the Pacific Plate), with a surface movement direction toward the northwest. When the margin between the Pacific Plate and the Farallon Plate reached the edge of the North American Plate, the relative dynamics of the plate boundary changed to become a “transform margin”, with the Pacific Plate moving northwest relative to the North American Plate. This margin is known today as the San Andreas Fault Zone, or SAFZ.

The SAFZ is not just a line on a map, but a 50-mile-wide zone of fault activity on a number of collateral major faults in addition to the San Andreas Fault itself. The Bartlett Springs Fault Zone is the easternmost fault in the SAFZ.

The BSFZ extends 50 miles from the Middle Fork of the Eel River southeast to Round Valley, past Lake Pillsbury and Bartlett Springs to just north of Cache Creek. Related faults in the same alignment system to the south include Wilson, Hunting Creek, and Green Valley faults.

Nobody we know can accurately predict when a seismic event might occur. However, based upon the length of the fault zone and other criteria geologists can estimate the potential magnitude of a major seismic event. Recent studies have identified the Bartlett Springs Fault as capable of producing an earthquake of Moment Magnitude between 6.7 and 7.2 (as documented by geological studies published by B.L. Melosh et al. 2024, V.E. Langenheim et al. 2023, and J.C. Lozos et al. 2015).

Earthquakes with magnitudes between 6.7 and 7.2 are major seismic events. Prior events within the memories of individuals living in northern California that fall within this magnitude range include the 1994 Northridge Earthquake (M6.7), the 1992 Cape Mendocino Triple Junction Earthquake (M7.2), the 1989 Loma Prieta Earthquake (M6.9), and the 1980 Eureka (Gorda Plate) Earthquake (M7.3).

The 1992 Mendocino Triple Junction event (M7.2) resulted in damage in Ferndale (in Humboldt County) very similar to the damage that occurred in Ferndale from the 1906 (M7.9) event in San Francisco. The 1980 Gorda Plate earthquake (M7.3) resulted in a collapsed Highway 101 overpass near Humboldt Bay. The 1989 Loma Prieta Earthquake (M6.9) was on an oblique fault very close to the San Andreas Fault in the Santa Cruz area, and may be more directly indicative of effects associated with the Bartlett Springs Fault at Lake Pillsbury. The event was memorialized on TV because it occurred at the start of a World Series game in San Francisco. Long sections of the I-580 freeway in Oakland collapsed during the quake, and a large part of the Marina District in San Francisco was damaged because of liquefaction in the poorly consolidated fill on which it was built.

An earthquake in this magnitude range on the Bartlett Springs Fault in the vicinity of Lake Pillsbury could result in an immediate failure of Scott Dam as a consequence of the seismic shaking per se. Moreover, the existing large landslide at the south end of the dam, on which the south abutment is based, would likely be mobilized (as occurred widely in northwestern California in 1992 with the Triple Junction event), leading to the destruction of the south end of the dam, and the rest of the structure would follow. This location was not then, and is not now, a safe location for a dam.

Dams do fail and while the specific dynamics are different, the St Francis Dam failure in Los Angeles County in 1928 is a relevant example. A common joke among geologists is that a sure way to find a new fault is to look for an older dam, an indication of how significant a risk geologists consider fault movement to be with respect to dam safety, particularly for older structures. Geological science clearly indicates that the BSFZ represents a significant risk of failure for Scott Dam. While we have yet to see the internal studies conducted by PG&E for the Scott Dam, we suspect that those studies say the same. We strongly believe that discussions among members of the public and their elected decision-makers about the future of the Potter Valley Project should include a greater appreciation of these geological realities.

Bob Schneider has a Bachelor of Science degree in Geology from UC Davis. Chad Roberts is a senior ecologist with a Ph.D. in ecology from UC Davis. They live in Davis, California.