The unexpectedly heavy floods have also overwhelmed state governments and emergency management agencies, the Nigerian newspaper The Guardian reported.
Delta State Gov. Chibuike Amaechi visited some of the sites of the worst flooding during the weekend and afterward termed the flooding a monumental national disaster.
A resident of Delta State, Chima Abali, who lived near the banks of what he called the usually "small, slow-moving" Omuku River, said the use of traditional measures against the river's flood water, such as sandbags, had failed.
A woman who lived half a mile from the river said her home was half-submerged in the high water.
More than 50 communities in the state were reported under water. More than 4,000 people were living in nine camps set up by the government.
Amos Utuama, the state's deputy governor, said many people were trapped in their homes and on trees. He forecast the situation would get worse because water was still flowing into the area from upstream, and still had to flow into the Atlantic Ocean.
Utuama said national emergency management officials had issued warnings but that the state "did not prepare for this level of disaster."
A spokesman for the Rivers State government in coastal Port Harcourt said more than 100 communities in that state had been affected by the flood.