The declaration allows the federal government, specifically the Federal Emergency Management Agency (FEMA) to coordinate relief efforts. It also provides federal funding for disaster relief.
"This action will help alleviate the hardship and suffering that the emergency has inflicted on the local population," the White House said in a statement.
"Specifically, FEMA is authorized to identify, mobilize, and provide at its discretion equipment and resources necessary to alleviate the impacts of the emergency."
As of Monday morning, Harvey was hovering on the Gulf coast of Texas, about 100 miles (160 kilometers) southwest of Houston.
It was expected to start moving toward the northeast on Tuesday, bringing heavy rains through Friday to already inundated parts of east Texas and into neighbouring southwestern Louisiana.
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