A deep freeze followed by a quick thaw has pushed the Red River, in the US Midwest, to its highest levels in 112 years.
One of the levees in the North Dakota town of Fargo burst earlier, and officials have been nervously watching as water levels continued to rise during Friday and early Saturday.
The US tradition of self-help was in evidence in Moorhead - the town across the river from Fargo - as residents scrambled to ensure their homes were protected.
The government too was not taking any chances, supplying thousands of sandbags to worried residents.
Many houses are now bolstered by a secondary wall of levees, as the Red River tests the area's flood defences to the limit.
Thousands of people in Fargo and Moorhead were advised to evacuate their homes - some needed help from the National Guard to flee to safety.
Experts say the river peaked on Saturday, but as it continues to flow at extremely high levels no-one is resting easy in Fargo or Moorhead.