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The detailed as-excavated foundation maps developed during original construction for both spillways were critical pieces of information used by OER to assess the geologic foundation conditions of the FCO Spillway chute where the initial failure occurred, and were used to estimate the headward erosion that could be expected based on the intersection of discontinuities in the foundation. These maps were also used to identify areas downstream of the Emergency Spillway monoliths that might be more susceptible to erosion under high water flows. Figure 3 - Example of an as-excavated FCO Spillway chute foundation map [6] In addition to the foundation geologic mapping during original construction in the 1960s, DWR geologists created foundation maps of areas-which-were and areas-which-were-not fully cleaned up. These clean-up maps documented the presence of fine-grained excavation debris that could remain in place in the FCO Spillway chute foundation. The design specified that the foundation provide at least 50 percent of the rock foundation to be completely cleaned of excavation debris, while the other 50 percent allowed some amount of clean-up debris to be left in place. These fine-grained materials, referred to on the clean-up maps as “compacted clayey fines”, were left in place and have been the source of some confusion. Recent investigations confirm that these materials were sourced from original construction debris. 2.3. GEOLOGIC INVESTIGATIONS SINCE 1960S CONSTRUCTION Numerous geologic studies have been performed in and around Oroville Dam since original construction in the 1960s. However, only two were located in 107