THE IMPORTANCE OF GROUNDWATER IN LAKE BONNEVILLE HISTORY

The geology of the Bonneville basin is characterized by extensional fault-block mountains composed of Paleozoic and Mesozoic sedimentary rocks and Mesozoic and younger igneous rocks, and intervening valleys filled with Paleogene, Neogene, and Quaternary sediments. Hydraulic conductivities vary considerably in these rocks and sediments, but good bedrock and unconsolidated-sediment aquifers are common. High mountains supplied river runoff and clastic sediment to Pleistocene lakes in the basins, and were recharge areas for groundwater that flowed to the lakes. Traditionally, groundwater terms in lake water-budget equations have been acknowledged but largely overlooked because quantitative information on groundwater inputs and outputs is difficult to obtain. Groundwater flows, however, were probably very important in the water budget of Lake Bonneville, although they have not yet been quantified. Groundwater outflow from the basin, as Lake Bonneville reached its highest levels and formed the Bonneville shoreline, may have been responsible for a decrease in the rate of transgression in the closed basin, and groundwater sapping weakened the external threshold of Lake Bonneville immediately before the catastrophic collapse of the alluvial-fan dam and the ensuing Bonneville flood. Groundwater discharge from mountain and piedmont aquifers was probably responsible for the prolonged period of overflow of the shallow lake in the Sevier basin during and after the regressive phase of Lake Bonneville. Groundwater discharge during a period of cool but dry climate during the early Holocene may have been responsible for a freshwater cap and the resulting stratification of hypersaline Great Salt Lake. Groundwater inflow rates to the lake would have changed as lake level rose and fell, and discharge of old groundwater to the lake may have led to a temporally and spatially variable radiocarbon reservoir effect.