WASHINGTON — The federal government has a large rainy day fund on hand to help mitigate the expanding damage on the Gulf Coast, generated by a tax on oil for use in cases like the Deepwater Horizon spill.

Up to $1 billion of the $1.6 billion reserve could be used to compensate for losses from the accident, as much as half of it for what is sometimes a major category of costs: damage to natural resources like fisheries and other wildlife habitats.

Under the law that established the reserve, called the Oil Spill Liability Trust Fund, the operators of the offshore rig face no more than $75 million in liability for the damages that might be claimed by individuals, companies or the government.

The fund was set up by Congress in 1986 but not financed until after the Exxon Valdez ran aground in Alaska in 1989. In exchange for the limits on liability, the Oil Pollution Act of 1990 imposed a tax on oil companies, currently 8 cents for every barrel they produce in this country or import.