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.

• 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.

• Canada

  The Fraser River froze solid in January 1862, bank to bank from Lulu Island up to Hope. Cattle were driven up the river on the ice, and ships could not get in. Flooding is also recorded in parts of British Columbia. This is about as far north as the storm’s effects were felt.

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.

1. Phoenix and the Salt River Valley

   Built around dry washes

   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.

2. Yuma and the Gila River

   About 100,000 people

   live in Yuma today. The city 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.

   U.S. Census Bureau

3. Tucson and Pima County

   Rivers that are usually dry

   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 flood maps were not drawn for the inland rain a megastorm brings.

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.

Sources

• Great Flood of 1862. Wikipedia. Where the flood reached and what happened in each place.
• Sharlot Hall Museum, Days Past. Arizona’s early censuses, including the 1860 count of Arizona County, New Mexico Territory.
• MoneyGeek (September 2026). Flood insurance coverage by state, from FEMA policy counts as of August 31, 2026. Arizona 0.68%, California 1.15%.
• U.S. Census Bureau. QuickFacts: Yuma, Arizona.
• University of Georgia (June 2022). New flood maps clarify the risk homeowners face.
• U.S. Geological Survey. ARkStorm Scenario, including the California agencies that took part.
• Delta Museum and Archives Society (February 2018). North Delta history: The fierce frozen Fraser. Cloverdale Reporter.