GEOLOGIC ASSESSMENTS OF THE LAKE MEAD AREA FOR THE BOULDER CANYON PROJECT

Hoover Dam was a monumental accomplishment for its era which set new world-wide standards for GEOLOGICAL feasibility studies and post-construction performance evaluations. In 1921-22 the U.S. Bureau of Reclamation (Reclamation) employed F. Leslie Ransome to map the geology along either side of the Colorado River channel at potential dam sites in Boulder and Black Canyons, which influenced all subsequent engineering work thereafter. After the Black Canyon site was selected by an independent Colorado River Board appointed by Congress in 1928, Ransome performed more detailed studies of that site, which he completed in 1931. Ransome’s work included the first use of paleoseismology to assess the age of faulting that pervaded the andesites exposed on the abutments of Hoover Dam. Reclamation then hired Frank Nickell as their first geologist, to map the geology as the project progressed between 1931-36. Despite these pioneering efforts, the foundation grout curtain constructed at Hoover Dam failed miserably, and had to be re-built over the succeeding 14 years, after the dam’s completion. In 1933 Reclamation retained Chester Longwell to map the geology of the Lake Mead area before it was inundated by the rising waters of Lake Mead. Hoover Dam was the first dam to be fitted with three strong motion sensors and three seismographs were placed around the reservoir to measure reservoir-triggered seismicity. These led to the first published studies of reservoir-triggered seismicity, which have continued to the present day. Lake Mead was also the first reservoir to be outfitted with an extensive triangulation survey network to measure crustal deformations, leading to the first published studies on that subject. During the initial filling of Lake Mead between 1935-41 density currents were discovered moving through the entire 116-mile length of the reservoir, as well as the deposition of nutrient-rich sediment which caused heating [problems against the dam’s upstream face. These discoveries led to the establishment of teams to undertake long-term monitoring that had significant influence on the understanding of reservoir dynamics, world-wide.