SMALL ARTIFICIAL PONDS IN THE UNITED STATES: IMPACTS ON SEDIMENTATION AND CARBON BUDGET.

The proliferation of small artificial ponds constitutes a major human alteration of the hydrologic landscape. We have estimated the total number of such features across the conterminous United States to be between 2.6 and 9 million, with densities in some areas exceeding 5 per km 2 . The majority of ponds have been built in agricultural settings, and densities are highest in the eastern Great Plains and the Southeast. Ponds intercept and temporarily store about 25% of runoff from the conterminous U.S., and in regions of high pond density that proportion may approach 100%. Ponds are a major sediment and carbon sink. Based on erosion and sedimentation rates typical of the mid-to late- 20th century they capture 0.2 to 1.8 x 10 9 tons of sediment and 4 to 36 x 10 6 tons of carbon annually. Case studies in eastern Kansas and southwestern Ohio indicate that while total numbers of ponds have increased steadily since the early 20th century, ponds are transient features on the landscape, with 30 to 90% of those present in the 1950s disappearing by 2000. Ponds disappear mainly by two processes: infilling with sediment, and replacement with other land uses. These processes are spatially variable with infilling by sediment being the dominant cause of disappearance in most areas. Trends in pond sedimentation rates are likely highly variable depending on local conditions. Erosion rates from lands used for row crop agriculture are declining, and ponds with these types of agricultural land uses in their headwater areas are the first to be affected by a reduction in sediment flux. The locus of pond construction appears to be shifting from agricultural to suburban settings, and the dominant rationales for pond construction are changing from provision of livestock water to aesthetic considerations and urban runoff management.