Large, deep-seated landslides typically occur in hillside settings without any natural buttressing, and many of these landslides remain relatively unstable and prone to reactivation. However, where large, deep-seated landslides have moved into incised valleys that subsequently experienced alluvial aggradation, a natural buttress of alluvium may cover the toes of these landslides, increasing stability. This study presents three examples of large, deep-seated landslides that are buttressed by Quaternary alluvium. The McCracken Hill Landslide in southern California and the Potrero Canyon Landslide Complex in central California are proximal to the Pacific coast. The Knights Valley Landslide Complex in northern California is much farther inland than the other examples. We analyzed the stability of one of the example landslides to demonstrate that a buttress of alluvium increases stability. In most settings, base level primarily controls alluvial aggradation. Base-level rise may result from either climatically driven late Quaternary eustatic sea-level rise or local factors such as damming of streams or downstream tectonic uplift. Late Quaternary eustatic sea-level rise caused alluvial aggradation at the McCracken Hill and Potrero Canyon sites. Downstream tectonic uplift likely caused local base-level rise and alluvial aggradation at Knights Valley.

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