One person has died in South Dakota and hundreds of homes have been destroyed in Iowa during flooding in the upper US Midwest.
Some rivers in the two states reached record-high levels, as up to 18in (45cm) of rain fell in some areas over the weekend.
The risk of storms and floods will persist on Monday, the National Weather Service (NWS) says.
Elsewhere, tens of millions of Americans are still living under alerts due to a scorching heatwave.
This is expected to shift to the south-east, mid-south, and central and southern Plains this week, the NWS says.
Scientists say extreme weather events are becoming more frequent and intense as a result of human-caused climate change, fuelled by activities like burning fossil fuels and cutting down forests.
Heatwaves have become more frequent and more intense globally since 1950, says the UN’s climate body, the Intergovernmental Panel on Climate Change (IPCC).
In her update on the flooding on Sunday, Iowa Governor Kim Reynolds called the situation “catastrophic” and declared a state of disaster in 21 counties.
About 250 water rescues have been conducted so far, she told reporters. “I can tell you, the devastation is severe and it’s widespread,” she added.
Drone footage posted by regional officials showed homes and buildings almost completely submerged, with only rooftops visible.
Hundreds of homes have been damaged in the town of Spencer, where the water-level gauge was completely submerged by water and a sewage plant was deluged.
South Dakota’s governor, Kristi Noem, declared a state of emergency herself – warning that the worst of the flooding was expected on Monday and Tuesday.
Around 4,000 residents in Rock Valley, Iowa – about 50 miles (80 km) south-east of Sioux Falls – were forced to evacuate after the Rock River rose to an all-time high.
Residents in the region were left without clean running water as floodwater contaminated the wells, officials said.
The flooding stranded some people and animals in the city early on Saturday, prompting helicopter rescue operations.
The heavy storms come as other parts of the US continue to deal with a week-long heat wave that has surpassed daily temperature records in some cities.
More than 50 million people are under heat advisory alerts as of Monday.
Hot summer temperatures are forecast to hit about 100F (37 C) over the central Plains, though other areas will get relief due to the movement of a weather system south and east.
The heat wave has been unusually early for this time of year, and the NWS warned that it could be the longest experienced in decades for some locations.
Wildfires that ignited in New Mexico during the extreme weather led to two deaths and the destruction of hundreds of homes in the village of Rudioso. The FBI is offering a reward up to $10,000 (£7,900) for information leading to the arrest and conviction of the person or persons responsible.
There has also been destruction in Wisconsin, where a tornado on Saturday evening flattened the 130-year-old Apple Grove Lutheran Church in the village of Argyle, the local parish said.
