Hydrogeology of the Malad–Lower Bear River Basin, North-Central Utah and South-Central Idaho
By: Hugh Hurlow

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This report characterizes the lithology of the younger (i.e., Quaternary) basin fill of the Malad–Lower Bear River basin in north-central Utah to provide a geologic framework for evaluation of groundwater flow and the potential effects of additional future groundwater pumping. Analysis of well-drillers’ logs yielded a coherent lithologic stratigraphy below the valley floor, that consists of alternating predominantly fine-grained and predominantly coarse-grained layers above the Tertiary Salt Lake Formation. These units grade abruptly across a facies transition to heterogeneous, predominantly coarse-grained deposits below the mountain fronts. The valley-floor lithologic succession is consistent with findings from previous studies in Cache Valley and basins along the Wasatch front, and with established Quaternary lake cycles. Three fine-grained layers separated by two gravel layers compose a heterogeneous, composite confining unit below the youngest surficial deposits. Heterogeneous, predominantly coarse-grained deposits between the confining unit and the Salt Lake Formation make up the deep sand and gravel aquifer. Groundwater pumping from the shallow sand and gravel aquifer would cause rapid and direct depletion of stream flow in the Bear River and Malad River, which traverse the basin. Depletion of stream flow and spring flow due to pumping from the deep sand and gravel aquifer would be delayed and dispersed, but not negated, by the composite confining unit.