The Fault Line and the Dams 

Lake Temescal Dam sits directly atop the perilous Hayward Fault. How safe is it, and the nearby Tilden Park Dam at Lake Anza?

click to enlarge The Hayward Fault passes directly beneath the right abutment of Lake Temescal Dam, which is below the large green lawn at center left.

Artem Russakovskii

The Hayward Fault passes directly beneath the right abutment of Lake Temescal Dam, which is below the large green lawn at center left.

The earthquakes that rocked Kern County in early July are a potent reminder of the East Bay's own seismic risk. Last year, the U.S. Geological Survey imagined what could happen during a 7.0 quake on the Hayward Fault, which stretches 74 miles from north San Pablo Bay past San Jose. This so-called "HayWired Scenario" envisioned 800 people dying, 18,000 more being injured, and widespread damage occurring to property and infrastructure. The study also predicted that the surface of the earth would rupture in places "where the fault is currently creeping."

Some fault lines, which mark the edge of plates in the earth's crust, only move during earthquakes. But not the Hayward Fault; it moves slowly between earthquakes, "creep" that can be seen along area roadways and was notoriously visible at UC Berkeley's Memorial Stadium before its 2012 reconstruction. Like the stadium, which was reengineered to withstand such creep, Lake Temescal in Upper Rockridge sits atop the Hayward Fault, which passes underneath the right abutment of the manmade lake's aged dam. Experts agree that creep has been observed near Lake Temescal Dam, but disagree on whether this indicates the area is at risk of suffering major damage during a strong quake.

Scientists say the Hayward Fault experiences a large earthquake with a magnitude exceeding 6.5 approximately every 100 to 220 years. The last major quake on the fault occurred in October 1868, in the middle of the construction of Lake Temescal Dam, which did not start holding back water until 1869. The dam has since survived two major earthquakes, the 1906 earthquake and the 1989 Loma Prieta quake, but both of those occurred not on the Hayward but the San Andreas Fault, which runs to the west of San Francisco.

While many East Bay residents have pondered the impact of a strong quake on infrastructure such as highways, gas lines, and water mains, few may have considered the consequences of such quakes on the region's dams. But the East Bay Regional Park District has studied this eventuality. And according to a document titled "Ch-6_Earthquakes_DRAFT_2-2-2017" downloaded from the district's website, several dams on park property could be at risk of failing during a major earthquake. Lake Temescal Dam is one such structure. Other inner East Bay structures judged to be of similar concern include C. L. Tilden Park Dam and tiny Jewel Lake Dam, both on Wildcat Creek in Tilden Park.

Standing 116 feet tall, Lake Temescal Dam is situated at the intersection of Highway 24 and Highway 13. Built by Anthony Chabot between 1868 and 1869, the structure dams the waters of Temescal Creek, which starts in the Oakland Hills and flows down Thornhill Drive from Montclair to the Temescal Regional Recreation Area, where it is dammed. Below the dam, the creek moves primarily through closed, man-made culverts on its sad journey to San Francisco Bay, following Highway 24 before briefly emerging at the Rockridge Temescal Greenbelt, passing under Telegraph Avenue at 51st Street, veering sharply west, then briefly reappearing again at Temescal Creek Park before finally ending its travels at the Emeryville mudflats near Emeryville's Bay Street shopping center.

C. L. Tilden Park Dam stands 88 feet tall and is located on the north side of Lake Anza, near the parking lot. Built in 1938 to provide a swimming area and the means to water Tilden Golf Course, the dam and the lake sit about mile east of the Hayward Fault. Upon spilling over the dam and exiting the lake, Wildcat Creek flows north in its natural creek bed, above ground through Tilden and Wildcat Canyon Regional Parks. It flows into Jewel Lake and past Jewel Lake Dam, a structure that doesn't hold enough water to warrant monitoring by the state's dam safety agency. About four miles from Tilden Park Dam as the crow flies, the creek starts to flow past the houses of McBryde Avenue in Richmond. It continues its journey downhill into residential areas, passing under I-80, skirting the border of San Pablo and Richmond, then flowing north through San Pablo past the San Pablo Library before veering west at John Herbert Davis Park, into North Richmond and, ultimately, San Pablo Bay.

The California Division of Safety of Dams has classified both Lake Temescal Dam and Tilden Park Dam as having an "extremely high" degree of "downstream hazard risk." According to spokesman Chris Orrock of the Department of Water Resources, the division's parent agency, release of the dams' waters when full would be "expected to cause considerable loss of human life or would result in an inundation area with a population of 1,000 or more." That is not necessarily as scary as it sounds, since it implies nothing about the circumstances under which a dam could suffer an uncontrolled release of water, nor that it is at risk of doing so.

In fact, despite the February 2, 2017 park district document expressing concern about the dams, it remains unclear just what level of risk an earthquake would pose. The California Division of Safety of Dams lists Lake Temescal Dam and Tilden Park Dam as satisfactory, and state dam safety experts say they have never shown any issues related to seismic activity. But another geologist found evidence of creep in the dam's spillway, which the HayWired Scenario says could lead to surface rupture. The concern is that if these dams failed in an earthquake, they could potentially flood the neighborhoods downstream of them, causing property damage, injury, and even death.

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