An interdisciplinary collaboration used 600 years of tree rings from the San Joaquin Valley to reconstruct plausible daily records of weather and streamflow scenarios during that period. Modeling based on those scenarios revealed the region has experienced vast variability in climate extremes, with droughts and floods that were more severe and lasted longer than what has been seen in the modern record. This new approach, combining paleo information with synthetic weather generation, may help policymakers and scientists better understand – and plan for – California’s flood and drought risks and how they will be compounded by climate change.