"The Robert T. Stafford Disaster Relief and Emergency Assistance Act (referred to as the Stafford Act-42 U.S.C. 5721 et seq.) authorizes the President to issue 'major disaster' or 'emergency' declarations before or after catastrophes occur. Emergency declarations trigger aid that protects property, public health, and safety and lessens or averts the threat of an incident becoming a catastrophic event. A major disaster declaration, issued after catastrophes occur, constitutes broader authority for federal agencies to provide supplemental assistance to help state and local governments, families and individuals, and certain nonprofit organizations recover from the incident. The end result of a presidential disaster declaration is well known, if not entirely understood. Various forms of assistance are provided, including aid to families and individuals for uninsured needs and assistance to state and local governments and certain non-profits in rebuilding or replacing damaged infrastructure. The amount of assistance provided through Presidential disaster declarations has exceeded $100 billion. Often, in recent years, Congress has enacted supplemental appropriations legislation to cover unanticipated costs. While the amounts spent by the federal government on different programs may be reported, and the progress of the recovery can be observed, much less is known about the process that initiates all of this activity. Yet, it is a process that has resulted in an average of more than one disaster declaration a week over the last decade. The disaster declaration procedure is foremost a process that preserves the discretion of the governor to request assistance and the president to decide to grant, or not to grant, supplemental help."
CRS Report for Congress, RL34146