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Beyond California

The storm did not stop at the state line.

Both ARkStorm studies look at California only. The storm of 1861–62 hit what are now seven western states. It reached into Mexico and north into British Columbia.

The winter of 1861–62

Where the water went.

The rain and snow began in Oregon in November 1861 and kept coming into January 1862. This is what the record says about each place.

  • California

    The Central Valley became an inland sea about 300 miles long. Read the full story.

  • Oregon

    The towns of Champoeg and Linn City were destroyed. Neither was rebuilt.

  • Nevada

    Dayton was flooded by January 2, 1862. Adobe buildings turned to mud and collapsed.

  • Idaho

    The Boise River flood is believed to have been four times larger than the biggest one measured since. Miners at Florence were cut off and starved.

  • Utah

    Early settlements on the Virgin and Santa Clara rivers were nearly destroyed.

  • Arizona

    Fort Yuma became an island. Water stood 20 feet deep where the Gila River meets the Colorado. More on Arizona.

  • New Mexico

    The Rio Grande flooded and changed its course in the Mesilla Valley.

  • Mexico

    Flooding is recorded in Sonora, across the border from Arizona.

    [JIM: your source for how far into Mexico it reached]

  • Canada

    Flooding is recorded in parts of British Columbia.

    [JIM: your source for the Fraser River Valley]
Source: Great Flood of 1862, Wikipedia, which lists the original newspaper and government records.

The two ARkStorm studies were made for California, with state agencies among the partners. So both stop at the state line.

We have not found a study like ARkStorm for any other state. Outside California, this history is almost ignored. We are starting with Arizona.

State 1

Arizona

In January 1862 the Colorado and Gila rivers flooded together. Almost no one lived in their path. Millions do now.

  1. January 20, 1862

    The Colorado starts to rise.

    The river is already running high.

  2. January 22, 1862

    Six feet in three hours.

    In one afternoon the river rises nearly six more feet and leaves its banks. Fort Yuma becomes an island in the middle of it.

  3. The days after

    Whole towns are gone.

    Colorado City and Jaeger City are washed away. So is most of the mining town of Gila City. Where the Gila meets the Colorado, water stands 20 feet deep. Mud covers the road to Tucson.

Source: Great Flood of 1862, Wikipedia.
6,482
People counted in what is now Arizona at the 1860 census.
U.S. Census, 1860
7.4 million
People living in Arizona today, and counting. Its biggest cities are built on old, dry river paths.
Under 1%
of Arizona homes have federal flood insurance. In California it is under 2%. Across the country it is about 3%.
FEMA policy counts, August 2026
Analysis · Jim Francis, founder

What stands in the water’s path now.

This part is ARkStorm.org’s own analysis. It is kept separate from our summaries of published research.

  1. Phoenix and the Salt River Valley

    Hundreds of thousands of properties

    sit in mapped river floodplains or on active alluvial fans across Maricopa County. Greater Phoenix lies in a desert basin that drains a large share of Arizona’s mountain runoff. Neighborhoods in Scottsdale, Phoenix, Mesa and Glendale are built around dry washes and canals. In an ordinary storm those channels cope. In a storm the size of 1862, the Salt and Verde rivers would overtop them and flood large residential areas.

    [JIM: source for the Maricopa County count]

  2. Yuma and the Gila River

    Over 40,000 homes

    and nearly 100,000 people are in the Yuma valley today. Yuma sits where the Gila River meets the Colorado, the same ground that was 20 feet under water in 1862. The Gila crosses almost the whole width of Arizona. Rain on the state’s mountains drains west toward Yuma. It is the bottleneck for Arizona’s floodwater.

  3. Tucson and Pima County

    Over 30,000 structures

    in Pima County stand inside mapped 100-year floodplains. Tucson depends on rivers that are dry most of the year, such as the Santa Cruz and the Rillito, and on a web of desert washes. The 100-year maps were not drawn for the inland rain a megastorm brings. Count the ground that flooded in 1862 and we expect that number to triple.

    [JIM: source for the 30,000 figure. The Pima County page you sent does not give a number.]

  4. Away from the rivers

    Pavement makes it worse

    Where desert soil is covered with concrete and asphalt, rain cannot soak in. In a megastorm, streets miles from any river flood because the drains cannot carry that much water. Researchers at the University of Georgia found that a large share of flood damage happens outside the mapped flood zone.

Opinion · Jim Francis, founder

Under 1% of Arizona homes have federal flood insurance. In California it is under 2%. This is what our founder thinks that means.

“Most of these people will have no choice but to walk away.”

“When you add up all the people who will default on their mortgages in all seven of these states, you are in the millions of home defaults. I believe this will create a banking crisis.”

“I will bet that if you went to Arizona, fewer than one person in a thousand has even heard of this.”

This is our founder’s opinion. It is not a finding from any study.

More states are coming.

We’re adding them one at a time. Get an email when the next one is published.

[JIM: which state do you want next?]
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Sources