# Number of Reservoirs in the World

By building large, engineered dams, humans have added over half-a-million artificial lakes to the world's surface. That total includes all the world's reservoirs amounting to at least one hectare (2.5 acres) in surface area. All combined, the water trapped behind dams on earth covers 258,570 square kilometres (99,834 square miles).

In addition to the large dams, numerous small farm ponds have been constructed to water livestock or crops, raise fish, or trap water sediment. Although small, the artificial ponds add up to cover 76,830 square kilometres (29,664 square miles) of the earth's surface, an area the size of Massachusetts, Vermont and New Hampshire combined.

In the United States alone, farm ponds extend over an area almost the size of New Jersey. The country has 21,600 square kilometres (8,340 square miles) flooded by small ponds. Concentrations of ponds exist in Tennessee and Mississippi, where ponds take up 3 to 4 percent of the agricultural land.

The land area flooded by dams in the United States has grown by about 1% a year since the 1960s. Similarly, the number of small water impoundments on US farmland continues to increase by 1 or 2% a year. Since the 1950s, the volume of water sitting behind dams around the world has increased ten-fold.

Through erecting dams and digging ponds, people have flooded a total of 335,000 square kilometres (129,344 square miles) of the earth's surface. That's an area larger than most countries, including Norway and New Zealand.

## How Many Reservoirs on Earth?

 515,149 Reservoirs one hectare (2.5 acres) or larger in size 24 Reservoirs bigger than 1,000 sq km (386 sq miles) 3 Reservoirs covering 10,000 sq km (3,861 sq miles) or more

These estimates of the number and area of the world's reservoirs were made by an international team of scientists. They analyzed data from fine-resolution geographical information systems and used mathematical equations to calculate the totals.

##### Reference

John A. Downing, Yves T. Prairie, Jonathan J. Cole, Carlos M. Duarte, Lars Tranvik, Robert Striegl, William H. McDowell, Pirkko Kortelainen, Nina Caraco, John M. Melack and Jack Middelburg. 2006. The global abundance and size distribution of lakes, ponds, and impoundments. Limnology and Oceanography. 51(5): 2388-2397.