The summer floodwaters have receded from the swollen Mississippi River, and life in the Midwest is slowly returning to normal. But where did all that water go?
Over the last two months, scientists have discovered that it turned into a well-defined river of less-salty water at the ocean's surface that flowed across the Gulf of Mexico, rounded the tip of Florida and headed north on a course that may already have brought it to somewhere off the New England coast.
A month ago, scientists on a ship off the south coast of Florida detected a strange region of water in the Gulf Stream that was less salty and much cloudier than normal seawater. They soon realized that they were seeing the floodwaters that the mighty Mississippi had dumped into the Gulf of Mexico.
"It's pretty obvious," said David Forcucci, a marine biologist at the Florida Institute of Oceanography who is field manager for a system of automated monitoring stations that first detected the reduced salinity in late August. "You can go out in a boat and look at the water and see the difference."
Off Miami, the stream was 14 miles wide and more than 60 feet deep. Rod Zika, an oceanographer at the University of Miami, told the Miami Herald that "we basically have the Mississippi River off our coastline, or at least a large part of it." (Mr. Zika says he now regrets saying that, since some people took it too literally. The water, of course, has been diluted by seawater, and is only relatively fresh -- about 15 percent less salty than normal.)
Two weeks later Larry Atkinson, an oceanographer at Old Dominion University in Virginia, found the Mississippi water just off Cape Lookout, N.C., on the edge of the Gulf Stream and heading north. It was more diluted but still very distinct. Apparently nobody has looked for it yet off the New England coast, but Mr. Atkinson thinks it has probably reached there by now.
Raymond Schmitt, an oceanographer at Woods Hole Oceanographic Institution who studies the effects of freshwater in the ocean, said that he didn't know of anyone who had been monitoring in the Gulf Stream near New England recently, but he thought the Mississippi water should still be detectable.
Off Miami, the lower-than-normal salt content of the water is about 31 parts per thousand, compared to the usual 36, and it differs from the surrounding seawater in several other ways:
* It has a distinctly different color, a cloudy green instead of the usual deep blue. This is because it still contains a significant amount of silt, scientists think.
* It contains different species of plankton, fish larvae and other creatures, some normally only found in river estuaries.
* It contains chemicals normally found only in fresh water, such as hydrogen peroxide formed from decomposing dead plants.
* It may be laden with herbicides from flooded farmlands, although nobody has yet tested offshore waters for these chemicals. Sampling along the Mississippi by Raj Rajagopal of the University of Iowa showed that in just over a month the river carried a quantity of herbicides greater than the normal flow from entire year.
Although Mississippi waters are not normally detectable so far from its mouth, the water from other large rivers can routinely be traced far out at sea. The Amazon, for example, can be traced hundreds of miles out into the South Atlantic, clearly visible in satellite photographs because of the load of silt it bears. But except for one report in 1975, after a lesser flooding of the Mississippi, no one had seen evidence of the Mississippi so far at sea before.
The presence of Mississippi water off Florida and up the Eastern seaboard is more than a scientific curiosity. Scientists say it may have an impact on sea life, especially sensitive forms such as corals, and might affect fish breeding offshore.
In addition, Mr. Schmitt said, freshwater in the ocean can affect climate, because it acts as an insulating layer to prevent heat from escaping from the warmer water below. While he said this was just "wild speculation," it is possible the flow could result in slightly lower temperatures in Europe next year by reducing the warming effects of the Gulf Stream.
The discovery of the Mississippi water off Florida was a fortuitous accident; if a research ship hadn't been in the right place at the right time, it might have gone unnoticed.
"We just happened to be out there on a survey cruise at the time," said John Milne, an oceanographer at the University of Miami who was onboard the RV Columbus Iselin at the time. The ship's course was quickly altered, he said, to follow the stream.
Mr. Milne said "it shows you how far water masses can spread. If you had an oil spill at the mouth of the Mississippi, it could affect areas quite far away."
In this case, movement of the fresh water out into the Gulf Stream was aided by two factors: a loop current in the gulf, which feeds into the powerful Gulf Stream, was much farther north than usual, helping to funnel the floodwater toward the stream. And strong westerly winds may have helped propel the water toward that loop.
Normally, said Tom Lee, an oceanographer at the University of Miami, the Mississippi outflow moves westward toward the Texas and Mexico coasts.
Has all this happened before? This year's flooding has been described as a one-in-100-years phenomenon, or even rarer, but over the eons such events must have happened many times. There may be ways to find out just how often, scientists say.
One group of researchers was out on the coral reefs off the Florida Keys last week, studying the coral to see if it had been affected by the floodwater. James Porter, a marine biologist at the University of Georgia, said by telephone from a ship off Molasses Reef that the coral initially showed signs of distress, but now it seems to be back to normal.
Mr. Porter said corals show annual growth rings, just as trees do, and since they live for a thousand years or more, careful analysis of the rings may provide a history of Mississippi floodwaters reaching that area.