Small Artificial Lakes, Big Ecological Consequences

What state in the United States contains the most lakes? Not Minnesota, if you count artificial lakes as well as natural ones. In fact, Minnesota sits well down the list, behind Texas, Florida, Oklahoma, Kansas, Missouri and Mississippi. The good news for Minnesotans is that the license plate claim of "10,000 lakes" is modest – the actual total is more like 100,000.

A Miami geography professor, William Renwick, together with colleagues in Kansas and Mexico, has completed an inventory of small lakes in the continental United States. Depending on whether the lakes are counted using satellite-based maps or topographic maps, the total number of lakes lies somewhere between three and nine million.

While the counting methods do not distinguish between water bodies of natural origin and those built by humans, it is clear from their distribution that the overwhelming majority of lakes are artificial. In Butler County, Ohio, for example, there are no natural lakes. Studies of historical and recent aerial photographs show that the number of lakes in Butler County has increased from nine in 1938 to 2,000 in 2004.

These features, while individually insignificant, constitute an enormous human modification of the landscape with many potentially profound consequences. Science has ignored them, presumably because of their small size. Because they are so numerous, however, their cumulative impact can be large.

Artificial lakes are most numerous in the Great Plains and the Southeast where they provide water for livestock during the dry summer. In the Great Plains the total amount of water evaporated from lakes is more than 10 percent of total stream flow in an area that is already critically short of water.

Lakes and reservoirs also trap large amounts of sediment – an amount similar to the total amount of soil erosion caused by rainfall. The sediment contains organic matter, making them sinks where carbon can accumulate. Carbon, in the form of carbon dioxide, contributes to global warming.

Artificial lakes can have dramatic effects on the distribution of wildlife such as migratory waterfowl. Unfortunately there is little published evidence of such ecological effects. Dr. Renwick hopes this research will awaken others to the significance of artificial lakes on the landscape.