Rivers swollen to record levels by days of heavy rain continued creeping upwards, keeping people from returning to ruined homes.
Crude oil spilling from a refinery into one flooding river contributed to the mess.
The Kansas National Guard was sent to help with a mandatory evacuation of Osawatomie, a small town in eastern Kansas and one of the hardest-hit communities in the region. The town evacuated 40 percent of its 4,600 residents after two rivers Pottawatomie Creek on the town's south flank and the Marais des Cygnes on the north rose out of their banks.
"It's going to be a few days before we get some of the higher rivers to come down," said Maren Stoflet, a National Weather Service meteorologist in Pleasant Hill, Missouri.
Kansas Gov. Kathleen Sebelius planned to survey the damage Monday.
Rain had mostly stopped falling Monday in Kansas, but the National Weather Service extended a flash flood watch for six counties in the state's southeast corner because major flooding continued on area rivers.
Levees and dikes held after volunteers reinforced them with sandbags, but water pooling in low-lying areas overwhelmed pumps and flooded neighborhoods.
Problems created by the flooding were compounded by a refinery spill of more than 42,000 gallons of crude oil into the Verdigris River, said Sharon Watson, spokeswoman for the Kansas Emergency Management Agency.
The Coffeyville Resources refinery had been shutting down in response to the flooding when the oil spilled Sunday, she said.
The federal Environmental Protection Agency had teams on the scene of the spill, and about a third of the homes in Coffeyville and a quarter of homes in nearby Independence had been evacuated, local officials said, and water intakes had been shut down.
Coffeyville Mayor Virgin Horn said his own house was submerged. "We're very concerned," Horn said. "It's chemicals mixed with water."
The oil was expected to flow down the swollen Verdigris River into Oklahoma and Lake Oologah, said Maj. Gen. Tod Bunting, the Kansas state adjutant. The lake, about 30 miles (48 kilometers) northeast of Tulsa, provides flood control in the Verdigris and Arkansas river basins and is used for boating and fishing.
"The water's moving too fast for us to do anything with it right now," Bunting said Monday.
While Kansas was getting a break, more rain was scattered over Texas and eastern Oklahoma on Sunday and Monday, the latest in nearly two weeks of storms that have inundated parts of Kansas, Texas and Oklahoma. Eleven deaths have been blamed on the storms and flooding in Texas, where two men are missing.
The overnight rainfall flooded a few roads in South Texas but there were no reports of stranded motorists or evacuations, authorities said. At least 200 people were still displaced.
Oklahoma also has suffered flooding, with some of the worst on Sunday near Bartlesville, where the Caney River was more than 3 feet (almost a meter) above flood stage.
Passenger rail service between Oklahoma City and Fort Worth was halted Sunday because of flooding in north Texas, said Terry Angier, a spokeswoman for the Oklahoma Department of Transportation. Passengers were placed on buses.