|Lahontan Dam and Reservoir are situated in the southern Carson Desert, a large intermountain basin in the northwestern portion of the Basin and Range geomorphic province. The dam embankment and spillways are founded almost entirely on Truckee Formation rocks and sediments at a point where the Carson River has incised a 120-foot-deep canyon into an isolated butte capped by 40 to 50 feet of Lahontan Lake sediments. The Late Miocene to Pliocene Truckee formation consists of sandstone, claystone, siltstone, conglomerate, hard sandy clay, tuffaceous sand, tuff and minor volcanic flows with most beds being 1.0 to 4.2 feet thick. Unconformably overlying this formation are the flat-lying, unconsolidated, lacustrine beds of the Pleistocene Lahontan Valley Group. These lacustrine beds typically range in composition from poorly graded and silty sands to silt and lean to fat clays. The only parts of the embankment founded on the Lahontan Valley Group are the far left and right ends of the embankment. On the right abutment the contact between the Lahontan Valley Group and the underlying Truckee Formation is projected beneath the right spillway at about station 12+20 and elevation 4139 feet (dam crest elevation is 4174.0 feet). On the left abutment, this contact is projected to cross the centerline of the dam at about station 4+12 and elevation 4120. An isolated minor occurrence of river alluvium was encountered at a depth of 123.8 feet in Drill Hole DH/P-8.8 drilled in the crest of the embankment at Station 8+80. Any alluvium under the embankment is considered to be discontinuous and limited based on preconstruction, construction, as-built, and later investigation information. The 1915 Final Construction Report stated that: "The riverbed was cleaned to either rock or clay and all sand, gravel and loam excavated and wasted." .|
Typically the Truckee Formation beds at the dam site dip to the northwest, but with dips ranging from 10 to 30 degrees. There are local changes in the dips and strikes which are generally attributed to nearby faults. For instance, outcrops immediately upstream and downstream of the right spillway chute contain beds which strike northeast and dip 27 to 43 degrees northwest. Indirect evidence indicates two faults are present in the foundation of Lahontan Dam. The first fault, termed the Channel Fault, is believed to be present in the right portion of the Carson River channel, thus striking transversely under the dam at its maximum section. The fault strikes about N 35-45oE and dips about 70oSE. The presence of a spring and nearby artesian well along the trace of this fault prior to construction suggest groundwater moves along this fault. The second fault, termed the Right Abutment Fault, shows up as a strong lineament, over 400 feet long, to the right and downstream of the right spillway chute. This apparent fault strikes N 20-37o E, dips 70o southeast and is also a groundwater conduit as evidenced by clear water springs along its farthest downstream trace (about 700 to 800 feet downstream of the dam axis). These faults are not considered to be active (see Section I.G, Earthquake Loading).