At least 2.6 million small, artificial water bodies dot the landscape of the conterminous United States; most are in the eastern half of the country. These features account for approximately 20% of the standing water area across the United States, and their impact on hydrology, sedimentology, geochemistry, and ecology is apparently large in proportion to their area. These features locally elevate evaporation, divert and delay downstream water flow, and modify groundwater interactions. They apparently intercept about as much eroded soil as larger, better-documented reservoirs. Estimated vertical accretion rates are much higher, hence, inferred sedimentary chemical reactions must be different in the small features than in larger ones. Finally, these features substantially alter the characteristics of aquatic habitats across the landscape.