The United States is dotted by about 2.6 million man-made ponds that significantly alter natural runoff schemes by trapping a previously unknown amount of sediment.

The number was generated in a new review of satellite images. Most of the ponds are less than 1.5 acres in size, but they add up.

"These ponds capture the runoff from about 20 percent of the area of the U.S.," said Jeremy Bartley of the Kansas Geological Survey. "Most large-scale studies of sedimentation haven't taken these small water bodies into account. Taken together, they have a dramatic impact."

The ponds collect about a quarter of the sedimentation that would otherwise end up in rivers and deltas, Bartley and his colleagues report in the journal Geomorphology. The trapped sediment could fill more than three million railroad boxcars a year.

"Before these ponds were built, much of that sediment was deposited in river valleys," Bartley said. "Now it goes into these small impoundments, changing the nature of sedimentation and drainage in this country."

Many of the ponds were constructed during the 1900s to provide water for livestock or for recreation. But thousands more are built each year, many in the Great Plains or in the southeastern United States in locations where natural lakes are rare, the researchers say.

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