Spatial variations in trace metal concentration are controlled by a complex set of parameters including reservoir sedimentation rates, dilution with pre-mining sediments, bed material mixing by shrink-swell processes, and factors that influence the sorption potential of the bed materials. Six years of below normal precipitation left much of the reservoir dry during the summer, 1992, and allowed for the precise delineation and mapping of the stratigraphic units that underlie Lahontan Reservoir.

The units were defined on the basis of spatial and topographic position, grain-size, composition, and the presence or absence of weathering zones. Once delineated, the units were mapped on 1: 12,000 scale aerial photographs. Subsequently, the photographic data were transferred to a stable base map for interpretation and analysis. Twelve stratigraphic units have been defined, nine of which post-date dam construction in 1915. One post-dam unit was deposited by eolian processes, while the remaining eight represent sedimentation in three lacustrine depositional environments: coastal, deep-water, and deltaic.