More than 185,000 homes were damaged and 9,000 destroyed as 42,000 people remain in shelters amid overflowing rivers and reservoirs, Texas officials said.
Harvey wrecked water pumping stations in the city of Beaumont, about 100 miles east of Houston, leaving residents and hospitals without clean water.
Many remain trapped by debris and waist-high murk that carries a risk of disease. Amidst all this, death toll has reached 50, Houston Chronicle quoted local officials as saying.
Turner said the US army corps of engineers needed to release water in reservoirs to create capacity lest more rain come and bring fresh disaster. He reiterated that people should not stay in their homes if they had water.
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Texas governor Greg Abbott said the recovery process would be long and arduous.
"This is not going to be a short-term project. This is going to be a multi-year project for Texas to be able to dig out of this catastrophe," he added.
President Donald Trump is due to visit Houston later today, his second visit to the state this week. Yesterday, he tweeted: "Texas is heeling fast".
Displaced residents who had moved to shelters, hotels and other accommodation started making forays home to assess damage and retrieve photos, valuables and other possessions.
With swollen rivers and reservoirs still risking potentially deadly flooding, the Red Cross said the number of people in shelters across the region had increased to 42,000.
Arkema officials acknowledged the fire, adding "we will likely see additional incidents." At least 18 people have been injured since the first fire earlier in the week.
Flooding from Harvey, once a Category 4 hurricane that hit southeast Texas last week, had knocked out the refrigeration system needed to keep the chemicals stable.
Flooding remained severe in eastern Texas and western Louisiana, but the storm is starting to lose its tropical characteristics as it moves toward the Ohio Valley, according to the National Weather Service.
"Flooding is still kind of rampant in the areas where we need to get to to restore the power," said Kay Jones, a spokeswoman.
There were about 136,000 flooded structures just in Harris County, the state's most populous, the county Flood Control District estimated on Thursday night.
About 42,000 people stayed in shelters in Texas on Thursday night, and an additional 3,000 Texans stayed in shelters in Louisiana, Governor Abbott said.