The Brahmaputra and other rivers in the Assam and Bihar states burst their banks after persistent rain showers.
Floodwaters are rising across eastern India, where hundreds of thousands of people have been trapped on the roofs of their homes or fled to higher ground as more rain has fallen.
Continuous rain showers for more than a week caused the Brahmaputra and other rivers to erupt their banks over Assam and Bihar states.
Up to two meters (6.6 feet) of water flooded many villages.
Experts say the annual floods are getting worse due to climate change. Authorities released water at one dam, fearing the walls would collapse.
Tens of thousands of people are trapped in flood-cut villages, and Assam governments say more than 400,000 have been displaced.
Sixteen-year-old Anuwara Khatun said she and her family spent almost a week on the roof of their home in Ghasbari in the state of Morigaon district.
“The water level has been rising for five days now,” she told AFP by telephone from her troubled village on the banks of the Brahmaputra.
‘Many families are stuck on their roofs. There is a shortage of essential supplies, so we only eat once a day. There is no hygiene here. ”
Santosh Mandal moved his family to a sandbank in Bihar’s Supaul district after his town was flooded.
‘There is no clean water to drink, food to eat and the children crying for milk. We are asking for help, because the government still needs help, ”Mandal said.
The Bihar government has sent lifeboats to bring people to safety, but they are concentrated in the districts hardest hit.
The Bihar and Assam governments said more than 12,000 people were in aid camps.
The Bihar government has opened the Valmiki Gandak Dam and warned people in nearby villages to leave, after 160 mm of rain fell within 24 hours.
The floods also threatened the Kaziranga National Park in Assam, a reserve listed on the UNESCO World Heritage Site, which contains the largest concentration of rare rhino horns.
About 70 percent of the 430-square-mile park is underwater, threatening rhinos as well as elephants and wild boars.
Assaman Prime Minister Himanta Biswa Sarma on Monday made an urgent appeal to traffic to avoid a highway through the reserve.
He said animals seeking shelter on the highway are now at risk.