Since the 1950s, there has been a 40 percent increase in carbon levels in the river as well as a 9 percent increase in the amount of water flowing in the Mississippi, according to Eugene Turner of the Coastal Ecology Institute at Louisiana State University, one of the researchers.

The phenomenon is another illustration of the dramatic environmental changes caused by human activities.

The scientists reported in the journal Nature that the changes most likely stemmed from farming practices including: irrigation; liming, a soil treatment to neutralize acidity; fertilizer use; and changes in crop types.

"Agricultural practices are causing a greater percentage of rainfall to make it to river water instead of being evaporated back into the atmosphere," Peter Raymond of Yale University in New Haven, Connecticut, one of the researchers, said in a telephone interview.

Precipitation records also showed that the changes were not driven by higher rainfall over the years.

The researchers also used century-old, nearly forgotten river records stashed at two New Orleans water treatment plants. "Musty would be polite," Turner said in a telephone interview. "But they were all in order and they were legible."

Raymond said the amount of additional water flowing in the Mississippi annually is about five times that of the Hudson River, another large North American river that flows past New York City.