Why old dams are dangerous: Martis Creek Dam, Polaris fault, and Reno

Martis Creek Dam and reservoir in winter
Photographer Michael Nevins
U.S. Army Corps of Engineers
public domain image

Martis Creek Dam is in Nevada County, California, about 6 kilometers east of Truckee, about 56 km upstream from Reno.  The lake is next to the Truckee-Tahoe Airport (runway in left of photo). The highway in the foreground is California State Route 267.  This winter scene shows the lake in winter when it is fairly full.

This dam is one of 10 dams in the U.S. that has been judged to have "urgent and compelling" safety concerns according to the U.S. Army Corps of Engineers. The dam, which was completed in 1972, has significant leakage and was believed to lie close to two fault zones. If the dam fails, potentially parts of Reno could be flooded.

The dam is an earthen embankment, underlain by glacial outwash. It has a history of excessive seepage during reservoir test fillings, including sand boils through the downstream toe, and seepage along stratigraphic contacts adjacent to the spillway. These concerns, along with the two known faults, have prevented it from being used at its design level. (Note: a post here about sand boils, and an update on that post: the levee did fail and the town of Hamburg is now in danger of flooding.)

The plot has now thickened: this month, Hunter et al. reported in the Bulletin of the Seismological Society of America that Lidar work there revealed a new fault, now named the Polaris fault.  It's 35 km long and has the potential to generate a magnitude 6.4-6.9 earthquake. It exhibits "youthful and laterally continuous tectonic geomorphic features" along its full length. It represents a significant seismic hazard to the greater Truckee-Lake Tahoe-Reno-Carson City area.

Reference: Hunter et al., LiDAR-Assisted Identification of an Active Fault near Truckee, California, Buletin of the Seismological Society of America, vol. 101 (3), p. 1162-1182, June 2011.