Hundreds of residents of North Dakota and Minnesota have evacuated their homes as the Red River rose to its highest level in over 100 years in the key wheat and sugar beet growing region.
The Red River topped its record level in Fargo yesterday morning. It is expected to crest today.
"In terms of natural disasters that can strike this country, floods are just the worst, at least in my experience," US agriculture secretary Tom Vilsack said in Washington.
"It is a horrible human tragedy to watch this," he said. "I think the folks in North Dakota and Minnesota have done an extraordinary job in trying to protect against this."
The Red River flows north from southeast North Dakota into Canada's Lake Winnipeg, forming the border between North Dakota and Minnesota. Fargo-Moorhead, with a metropolitan population of 175,000, is the largest city in the US side of the valley.
The river is located in an ancient lakebed and because it flows north, it is especially prone to flooding as water freezes as it moves north into Canada. Its banks are not very high and flood waters can spread out huge distances over the flat landscape.
The Red River Valley produces a large percentage of the US spring wheat crop and more than half of the sugar beets grown in the United States. The flooding could reduce spring wheat plantings by up to 200,000 hectares and result in farmers planting more soybeans once flood waters recede. Sugar beet plantings will not be affected, traders said.