Marble Bluff Dam
The lower course of the Truckee River and Pyramid Lake occupies one of the valleys in the Basin Range Province in northwest Nevada. The Basin and Range Province consists of parallel ranges alternating with basin or troughs. The ranges, bounded by faults, have been uplifted relative to adjacent valleys. Igneous, metamorphic and sedimentary rocks are exposed in the ranges. Unconsolidated continental fluvial, lacustrian and eolian deposits several thousands of feet thick fill the valleys and thins on the lower slopes of adjacent mountain ranges.
Pyramid Lake with a present surface elevation of about 3790 feet, is a remnant of old Lake Lahontah which at one time covered adjacent valleys. The general level of the highest wave cut terrace that marks the upper level of prehistoric lake is about elevation 4300 feet.
Marble Bluff Dam is located on minerals deposited in the delta formed by the Truckee River flowing into Pyramid Lake when the lake level was much higher than the present level. Between 1872 and 1909 the lake level fluctuated between elevation 3890 and 3866 feet depending on precipitation. During this period Pyramid Lake covered the vicinity of Marble Bluff damsite. The development of the Newlands Project in 1909 and other diversions from Truckee River Basin, together with changes in precipitation, have reduced the inflow into the lake. The lake surface in 1964 had been lowered to elevation 3789.5 feet. , A drop of 101 feet in the 92 year period. A general decline of approximately 80 feet in the lake surface occurred between 1910 and 1972.
The lower lake level resulted in a lower in a lower base level at the mouth of the Truckee River The lower base level set for the river caused it to downcut its channel and flood plain into deltaic deposits.
Unconsolidated surficial deposits encountered in the foundation of the dam consisted of stream channel deposits in the flood plain, terrace deposit in both abutments and deltaic underlying both the stream channel and terrace deposits. Fine-grained laminated to thin-bedded lake sediments were encountered below the deltaic deposits in exploration drill holes but were not encountered in the excavation except in the left abutment downstream corner of the spillway where a wet highly plastic silt occurred.