This story is part of a special National Geographic News series on global water issues.
As the crest of the Mississippi River flood moves through New Orleans and out to sea this week, peak river levels recorded during the month-long deluge threaten to top even the Great Mississippi Flood of 1927.
The most destructive river flood in U.S. history, the 1927 event moved about 2 million cubic feet (65,000 cubic meters) of water—enough to fill about 26 Olympic-size swimming pools—every second. (See pictures: "Mississippi River at Its Worst.")
"The numbers are still provisional, but [the current flood's peak water discharge] looks to be about the same" as the 1927 flood, said James O'Connor, a hydrologist with the U.S. Geological Survey (USGS).
Still, the 1927 and 2011 Mississippi River floods remain just drops in the bucket compared to other known freshwater "megafloods" around the world, according to O'Connor.
The scientist co-authored a 2004 USGS report that ranked all freshwater floods known to have occurred during the past two million years. The list, which remains largely unchanged since its release, includes only floods that had peak discharges of 3.5 million cubic feet (100,000 cubic meters) a second or more.
As of 2010, the number one flood on the USGS list swept through what are now Oregon and Washington State (map) about 25,000 to 15,000 years ago, during the last ice age.
Scientists think this so-called Missoula flood was caused by a natural ice-dam failure, which occurs when a glacier slides into a river valley—in this case the Clark Fork River—and blocks the flow of water.
Water builds up in a glacial lake behind the blockage, but "because ice is less dense than water, once the water reaches a certain height, the ice dam actually floats up," O'Connor explained.
Once this point was reached in the Missoula flood, water rushed out of the breached dam at a rate of more than 600 million cubic feet (17 million cubic meters) a second.
"Several decades were probably required to fill the lake, but once the ice dam became unstable, breaking up of the ice dam and emptying the lake probably took only a few days," O'Connor said.
The terrible deluge tore away everything in its path and moved huge boulders several miles, stacking them into heaps hundreds of feet high—one of the signs scientists look for when searching for evidence of ancient floods.
Floods and Climate
The second biggest flood on the list is now the Kuray flood, which ravaged what's now the Altay Republic of Russia (map), according to the USGS.
Scientists think the Kuray flood occurred sometime during the last ice age and was also caused by an ice-dam failure. The torrent released an estimated 350 million cubic feet (10 million cubic meters) of water a second.
Russia's Altay region is thought to be the site of several other megafloods caused by ice-dam failures during the last ice age, each one releasing about 70 million cubic feet (2 million cubic meters) of water a second.
"Some of these Ice Age floods may have tremendously affected global climate for several decades or hundreds of years," O'Connor said. (Related pictures: "2010 a Watershed Year for Floods, Droughts?")
"Two notable cooling events as the world was warming up at the end of the last ice age seem to have been triggered by outbursts of large ice-margin lakes surrounding the present Great Lakes" in Michigan, he said.
These outbursts apparently put so much freshwater into the northern oceans that they disrupted ocean circulation, causing global climate patterns to revert back to ice age-like conditions for significant periods.
Alaska's Volcanic Flood
Sometimes it's fire and not ice that sets off megafloods: Coming in as the eighth biggest flood on the USGS list is the Aniakchak flood in what's now Alaska (map), which was triggered by a volcano eruption about 10,000 years ago.
"You don't really think of a volcano producing floods, but they can in different ways," O'Connor said. In the case of the Aniakchak flood, the volcanic eruption crated a large crater, or caldera, that subsequently filled with rainwater.
"Once overtopped, the water eroded the [caldera] rim and all the water came pouring out pretty quickly," O'Connor said. (Also see "Iceland Volcano Erupts Under Glacier, Triggers Floods.")
Scientists think the Aniakchak flood had a peak discharge of about 35 million cubic feet (1 million cubic meters) of water a second.
In addition to ice-dam failures and volcanism, other causes of historic megafloods include landslides and "ice jams," which occur when dislodged river ice accumulates at river bends and other constricted areas.
The largest known meteorological flood—one caused by rainfall, as in the current Mississippi River flood—happened in 1953, when the Amazon River overflowed.
The 13th largest flood on the USGS list, that Amazon deluge pumped water at a rate of about 13 million cubic feet (370,000 cubic meters) a second.
"These large meteorological floods on very large rivers can cause lots of damage, because of the concentrated infrastructure along river corridors," O'Connor said.
Not Listed: Noah's Flood
Perhaps the most famous of all floods, the historic deluge that inspired the story of Noah in the Bible didn't make the USGS list. That's because, if it happened, it was likely an ocean-based flood and not a freshwater event, O'Connor said.
Some scientists think the story of Noah's flood may have been based on a known flood that happened 8,000 years ago, when sea level rise caused waters from the Mediterranean Sea to breach the Bosporus Strait and overflow the Black Sea.
Some previous estimates claimed the Black Sea could have risen by more than 195 feet (60 meters) during this event, but that estimate was recently revised to no more than 30 feet (10 meters). (See "'Noah's Flood' Not Rooted in Reality, After All?")
Even though a real flood may have inspired the story, O'Connor thinks there's a simple reason it couldn't have been a days-long meteorological event like the one suggested by the Bible.
"There's just not that much water in the atmosphere," he said.