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Flood debris on the Ohio River is halted by a dam (Photo by Michael Mooney; courtesy of flickr user LouisvilleUSACE)

When the post-hurricane floods drain away, there will be tons of debris left behind. More may be washed away and never seen again. Whole buildings may flow down rivers into the oceans. But what happens then?

Some insight into this phenomenon can be found in Flotsametrics and the Floating World, the 2009 book by oceanographer Curtis Ebbesmeyer and science writer Eric Scigliano:

Today the evening news reports excited on all the houses, cars, and other flotsam washed away in floods. Rarely, however, do we learn what happens afterward to this diluvial debris. Some of the trees washed away in the great 1861-62 flood stranded on nearby shores. Coastal eddies, observable from earth-orbiting satellites, spun others a hundred miles offshore, where the California Current swept them on westward to the Hawaiian Islands. In September 1862, Charles Wolcott Brooks, secretary of the California Academy of Sciences, reported “an enormous Oregon tree about 150 feet in length and fully six feet in diameter about the butt” drifting past Maui. “The roots, which rose ten feet out of water, would span about 25 feet. Two branches rose perpendicularly 20 to 25 feet. Several tons of clayish earth were embedded among the roots”—carrying who knows what biological invaders to vulnerable island habitats.

Any logs that got past Hawaii without being snatched or washed up would, over the next five to ten years, complete a full orbit around the Turtle and/or Aleut gyres.

It might also be possible for flood debris to form a floating island. Not just a fantasy in fiction, floating islands are a fairly common lake phenomena:

The influential early-twentieth-century paleontologist William Diller Matthew estimated that a thousand islands drifted out to sea during the seventeenth, eighteenth, and nineteenth centuries, and 200 million during the Cenozoic era. Such islands, formed when soil collects on dense mats of fallen trees and other debris, were known on the lakes of Europe, the marshes of Mesopotamia, and the log-jammed rivers of the Pacific Northwest….Today engineers and harbor authorities clear out such accumulations [from rivers and inlets] before they block passage and menace shipping. But untended, they would pile up until enormous floods washed them out to sea, there to drift, taunting mariners and bedeviling mapmakers, until they broke apart on the waves or crashed onto new shores.

The most famous floating island on the ocean was spotted in the spring of 1892 off the east coast of Florida:

It was a season of extreme weather: hurricanes, tsunamis, and floods violent enough to uproot whole sections of forest. One such section became the only wooded island ever observed transversing an ocean. Thirty-foot trees enable mariners to see it from seven miles away. The U.S. Hydrographic Office feared it would menace transatlantic steamers, and inscribed it on the monthly pilot charts that marked such threats as icebergs, underwater mines, burning vessels, and floating logs. Many captains stared in disbelief when they received their November 1892 chart for the North Atlantic; it showed an island floating in the stream. But this was no cloud or mirage; it had been sighted six times along a 2,248-nautical-mile course.

(Read more about ocean currents and how they brought lost Japanese sailors to America in this except from Flotsametrics.)