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Megaflood: the ARkStorm

A scenario by the US Geological Survey based on a real event: the 1861-62 storm sequence that flooded California's Central Valley. A chain of atmospheric rivers would drop weeks of rain, flooding cities and farmland and triggering landslides. It is a good model of how a weather extreme becomes a long-lasting infrastructure disaster.

The tier rates the quality of the evidence, not how likely the event is.

How it could unfold

1. 1A sequence of strong atmospheric rivers lasting weeks● strong evidence
2. 2River and levee failures flood the Central Valley; landslides cut roads● strong evidence
3. 3Power, water and transport failures; evacuations of large populations◐ moderate evidence

Historical anchors

• California's 1861-62 floods and geological evidence of repeated megafloods in the region.

What experts and models say

Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.

What you can do

• Know your flood risk and evacuation route; keep documents in a waterproof bag.
• Keep a stock of drinking water; floods contaminate supplies.

Open the preparedness checklist for this scenario →

Related scenarios

• Mega-drought and breadbasket failure

References

1. USGS Open-File Report 2010-1312: Overview of the ARkStorm scenario. link ↗