Extreme Runoff Generation From Atmospheric River Driven Snowmelt During the 2017 Oroville Dam Spillways Incident

• Published in: Geophysical research letters 2020-07, Vol.47 (14), p.n/a
• Main Authors: Henn, Brian, Musselman, Keith N., Lestak, Leanne, Ralph, F. Martin, Molotch, Noah P.
• Format: Article
• Language: English
• Subjects: Air temperature; Atmospheric conditions; Atmospheric precipitations; atmospheric rivers; Climate change; Cost analysis; Environmental risk; Evacuation; Extreme values; Flood risk; flooding; Floods; Heavy rainfall; Infrastructure; Lakes; Mountains; Oroville Dam; Precipitation; Public safety; Rain; Rainfall; rain‐on‐snow; Reservoirs; Runoff; Snow-water equivalent; Snowmelt; Snowpack; Spillways; Storms; Water vapor; Water vapor transport; Water vapour; Watersheds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL088189

In February 2017, a 5‐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‐yr 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 potential threats to public safety and infrastructure associated with a warmer and more variable climate. Plain Language Summary In February 2017, extreme runoff into California's second‐largest reservoir, Lake Oroville, and cracks in the reservoir's spillways resulted in evacuations of thousands of people and major repair costs. We analyzed to what extent the atmospheric river storms that caused the extreme runoff were unusual in terms of precipitation, snowmelt, temperature, and moisture in the air. We found that the precipitation amounts were less unusual than the runoff amounts, suggesting that other factors were involved. We also found that snowmelt in the Sierra Nevada mountains above the reservoir was the heaviest on record at many locations, driven by unusually warm temperatures and deep preexisting snowpack before the storms began. Thus, the warm temperatures and record melt likely increased the water available for runoff by about a third during the spillways incident. Our findings are consistent with other studies that suggest that unusually warm temperatures during winter atmospheric river storms in the Western United States are associated with flood risk due to substantial rainfall and snowmelt. Other studies show that climate change is expected to increase the type of flood risk in the 2017 incident. Key Points The atmospheric river event causing the 2017 Oroville Dam spillways incident was more exceptional for runoff than precipitation totals High rain‐snow elevations, deep antecedent snowpack, and unprecedented snowm