The Cedar River, which burst its banks after torrential rain, has forced at least 20,000 people from their homes in the US state of Iowa, officials said today.

The monster storm that has hit California with fierce winds and heavy rain dropped a thick blanket of snow on the Sierra Nevada mountains today and kept thousands of people from their homes in mudslide-prone areas.

Thousands of Americans have barricaded their homes against a powerful storm which has led to flights being ground, road closures and power cuts.

More than a foot of rain has washed out highways around the US capital, toppled a 100-year-old elm tree on the White House lawn and caused flooding that closed major government departments and the National Archives, where the Declaration of Independence is kept.

Three people have been killed in the US after a helicopter on a mission to rescue a moose clipped a steel cable and crashed through a frozen reservoir.