Whether the danger comes from a rising tide or a  thunderstorm, flooding is a risk homeowners face across the United States. Yet basic property policies don't insure against flood damage, leaving many people vulnerable to a potential catastrophe.

Homeowners must turn for coverage to the Federal Emergency Management Agency (FEMA) and its National Flood Insurance Program.

"Floods can and do happen at anytime, and they can happen anywhere,'' says Lena Thompson, a lender compliance officer with FEMA in its bureau of federal insurance administration. "We all live in a flood zone; it's just what our degree of risk is within that.''

In fact, the U.S. is divided into low, moderate and high-risk or special hazard locations, Thompson says. In the most serious-risk locations, all home buyers are required to have flood insurance before they can obtain a federally insured mortgage.

But flood insurance isn't just for protecting against water from an overflowing river; it also covers damage from water that builds up during storms. In that way it differs from standard homeowners' policies, which insure against damage from rain that comes through a broken window but not against water from something like a hurricane's tidal surge.

Nationally, flood insurance averages around $300 a year, according to Ruth Gastel of the Insurance Information Institute. It can be purchased through companies such as State Farm, which sells policies through its network of agents even though the coverage is  technically financed by the government's National Flood Insurance Program (NFIP).

To obtain coverage, homeowners must live in one of the approximately 18,800 communities that have an agreement with the federal government , Thompson says. A list of participating communities is available at NFIP's Web site.