The Role of Atmospheric River Rain-Snow Levels and Pre-Existing Snowpack in the 2017 Oroville Dam Crisis

The spillway damages at Oroville Dam in February 2017 were a major public safety crisis, resulting in mass evacuations, risk of catastrophic inundation, and damage costs nearing $1B. While the spillways have received significant attention, the storms and runoff events that filled Lake Oroville and enhanced the crisis have not yet been explored. Over February 9-11, Feather River flows to Lake Oroville reached 190,000 cfs, their highest value since the January 1997 flood of record. The high flows resulted from a sequence of strong atmospheric river (AR) events making landfall in Northern California from February 3-10. These ARs maintained intense onshore moisture transport from the Pacific concurrent with high rain-snow levels, resulting in multi-day precipitation totals over the Feather River watershed exceeding 500 mm, falling primarily as rain upon a deep pre-existing snowpack from colder ARs in January. The degree to which the duration of AR conditions and their rain-snow levels were historically anomalous are evaluated here. Using in situ observations of the rain-snow level and precipitation and a distributed snow water equivalent reanalysis dataset, the role of the rain-on-snow ARs in modulating streamflow generation is illustrated. The role of snow is strongly elevation-dependent, with lower-elevation snowmelt contributing to rain-driven runoff, while deeper, higher-elevation snowpack likely entrained a portion of the rain and limited runoff. Given observed trends and projections showing warming and strengthening of wintertime ARs, the February 2017 event may be indicative of enhanced future flood risk in the region.