Geomorphic Response to Global Warming in the Anthropocene: Levee Breaches in California's Sacramento-San Joaquin Watershed

Geomorphic processes in rivers are likely to be influenced by global warming through alterations of flood, erosion, and sedimentation processes and rates. In California's Sierra Nevada, warming scenarios imply future increases in magnitudes and durations (and changes in timing) of floods as snow packs diminish and rainfall runoff increasingly dominates flow into the Central Valley fluvial system. Geomorphic processes are likely to differ from processes that dominated during the Holocene due to the influence both of projected global warming and land use alterations including levee construction that narrows and separates Sacramento-San Joaquin Rivers and tributaries from floodplains and flow regulation downstream of numerous large dams. Whereas Holocene floods induced overbank flow and avulsion processes that led to vertical floodplain accretion and variability of stages in aggrading multiple-channel systems, modern floods largely transport flow and sediment within incised channels confined by levees. Because the scenarios of warming are developed at coarse scales, only an understanding of the relations between large-scale hydrology and climate on the one hand, and the incidence of levee breaches on the other, will make it possible to project likely geomorphic responses to future warming and flooding. A historical record of catastrophic levee breaks on the Sacramento and San Joaquin Rivers has been developed to allow analyses of these connections. In the current work, we develop statistical relations between historical levee break events and flow discharge, as well as with climatic phenomena such as El Nino and La Nina phases of the ENSO cycle, positive and negative phases of the Pacific Decadal Oscillation, and seasonal propensities towards "pineapple-express" storms. Preliminary results suggest strong relations between levee breaches and discharge, but poor relations to ENSO. Further investigation of these data will provide insight to help inform models and river management policy that addresses rates and magnitudes of erosion and sedimentation.