Extreme runoff generation from atmospheric river driven snowmelt during
the 2017 Oroville Dam spillways incident
Abstract
In Feb. 2017, a five-day sequence of atmospheric river storms in
California, USA, resulted in extreme inflows to Lake Oroville, the
state’s second-largest reservoir. Damage to the reservoir’s spillway
infrastructure necessitated evacuation of 188,000 people; subsequent
infrastructure repairs cost $1 billion. We assess the atmospheric
conditions, snowmelt, and runoff against major historical events. The
event generated exceptional runoff volumes (second-largest in a 30 year
record) partially at odds with the event precipitation totals
(ninth-largest). We explain the discrepancy with observed record melt of
deep antecedent snowpack, heavy rainfall extending to unusually high
elevations, and high water vapor transport during the atmospheric river
storms. An analysis of distributed snow water equivalent indicates that
snowmelt increased water available for runoff watershed-wide by 37%
(25-52% at 90% confidence). The results highlight an acute flood risk
to public safety and infrastructure projected to increase in severity in
a warmer and more variable climate.