QUATERNARY GEOLOGY AND GEOMORPHOLOGY A KEY TO ASSESSING LEVEE FOUNDATION CONDITIONS IN THE SACRAMENTO VALLEY, CALIFORNIA

Flood protection infrastructure in California’s Sacramento Valley typically is founded on unconsolidated Late Quaternary fluvial, basin, and estuarine sediments. The stability of these sediments as foundation materials depends on characteristics such as grain size, sorting, density, permeability, bedding, and cementation. These characteristics are a reflection of the geologic origin and history of the sediments. Original 1:24,000 scale mapping of Quaternary geologic units was conducted to provide a framework for foundation assessments. The mapping builds on previous geologic mapping, and the interpretation of early topographic and soil maps, 1937 aerial photography, and limited drilling data.

The Sacramento Valley is divided into five major environments of deposition: Coast Range alluvial fans, Sierran alluvial fans, Sacramento River flood plain, flood basins, and the delta estuary. Coast Range alluvial fans are derived primarily from clastic sedimentary rocks and are clay-rich, especially in the distal fan reaches. Prominent natural levees form winding low ridges that may extend along the length of the fan. Sierran alluvial fans are derived from granitic and metamorphic rocks and are dominated by micaceous sands and silts. Coarse sediments derived from 1800s-era gold mining spoils form wide-spread deposits in several Sierran streams and alluvial fans, extending downstream to the Sacramento River and Sacramento – San Joaquin Delta. Alluvial fans from glaciated Sierran watersheds exhibit multiple older fan terraces at the mountain front. In the axis of the valley, the Sacramento River winds southward, flanked by silt-rich natural levees. Flood basins occupy the lowlands on either side of these natural levees, and their thick clays and silts interfinger with the coarser fan and levee deposits. Flood basins empty into the delta-- large marsh islands underlain by peat, clay, and silt, and bordered by winding sloughs and their natural levees.

Sacramento Valley sediments are longitudinally and laterally diverse reflecting the interplay of geomorphic process and depositional environment. This understanding provides a regionally continuous framework for estimating foundation conditions and facilitates identification of levee reaches for subsequent engineering characterization.