Fire and police officials in Houston have responded to over 5,500 emergency calls since the storm began ravaging the area over the weekend.
The Harris County Flood Control District said it expected the release to start flooding homes around the Addicks and Barker reservoirs. "Over the coming weeks, we will have many opportunities to come together as a community in support of Hurricane Harvey relief, and I hope you will join me in taking part in these efforts".
Regina Costilla, 48, said she and her 16-year-old son had been rescued from their home by a good Samaritan with a boat.
The Red Cross has also set up other shelters throughout the area. A spokesperson for Level 3 Communications told The Register that as of Monday morning, all of its facilities have held up to the storm and despite isolated outages the majority of its network in the region is still able to carry traffic.
More than 1,200 people have been rescued from their homes, many by air, while authorities estimated that more than 30,000 people will be left temporarily homeless by the biggest natural disaster to hit the USA since Hurricane Katrina 12 years ago.
Trump on Monday also approved an emergency declaration due to rain and flooding in Louisiana, where parts of the state could see up to 25 inches of rain fall.
Trump will travel to Texas on Tuesday to survey some of the damage.
"To the people of Texas and Louisiana, we are 100 per cent with you", he said. But rain continues to pound the region as rescuers plucked hundreds of people from rising floodwaters along the ravaged Texas coastline.
The storm is estimated to have affected about a quarter of the Texas population, or 6.8 million people in 18 counties. About 2,000 people were retrieved from flooded homes and cars.
Duke said rivers in south Texas "won't crest until later this week". There was about 40 inches of rain at its peak during that storm, which lasted around 15 days. "You could not draw this forecast up, you could not dream this forecast up", Long said.