RE-VISITING AND REVISING THE QUATERNARY GEOLOGY OF THE LOWER AMERICAN RIVER

Recent stratigraphic and geomorphic mapping along the Lower American River (LAR) downstream of Nimbus Dam in support of levee stability evaluations has refined existing geologic understanding and identified several American River paleochannels of potential engineering significance. The recent mapping synthesizes the results of field mapping and sampling as well as petrographic and pedogenic evaluations with existing and recently collected surface and subsurface data to investigate the origin, continuity, and extent of subsurface deposits constituting levee foundations. Collection of outcrop data and development of 3-D surface contour maps, geologic cross sections, and longitudinal correlations has helped resolve questions regarding the thickness, lateral and longitudinal extents, and contact relationships of key units. The results provide evidence that the Plio-Pleistocene-aged Fair Oaks formation, not the previously interpreted Riverbank- or Modesto-age formations, underlies much of the LAR channel bed. The formation is relatively fine-grained and hard, and appears to be laterally extensive and continuous beneath the levees, which may have implications for channel incision and lateral erosion. A variable thickness of unconsolidated modern and/or upper Modesto Formation-age gravels and cobbles unconformably overlies the Fair Oaks formation. The formation’s top surface lies mostly at elevations too low to be exposed in the modern channel bed downstream of Watt Avenue. Synthesis of subsurface data suggests considerable relief of the eroded top of the formation, which may locally correspond with paleochannels identified via historical maps, as well as geophysical and LiDAR data.