Background/Question/Methods Constructed ponds are rapidly proliferating in many regions of the United States, yet their abundances are largely undocumented and their increasing ecological impact remains poorly understood.  These ponds have been built for various purposes, including recreation (e.g., fishing, swimming, golfing), water supply (e.g., for agriculture, tree nurseries or livestock), fire protection, water quality improvement (e.g., as wet detention basins), and for aesthetic enhancement.  Ecological function of these ponds is partly related to their location and degree of articulation with stream systems, which have changed over time. Our objectives were to 1) relate the current distribution of ponds to stream network patterns, land use and population density, and 2) trace the recent history of pond construction in southeast Pennsylvania.  Aerial photographs from 2000 and 2005 were evaluated for 73 municipalities in Chester County; aerials from 1971, 1980, and 1990 were also obtained and georeferenced for 7 of the municipalities.  We used ArcGIS 9.3 to create pond polygons from the aerial photographs, including size, landscape position (relationship to stream networks) and surrounding land use as pond attributes. 
Results/Conclusions A total of 2392 ponds > 0.1 ha in area were used for analysis.  In contrast to the distribution of former millponds in the region, most currently extant ponds were near the sources of headwater streams or were isolated from permanent stream flow, with just 5.7% of all ponds impounding 2^nd or higher order streams.  Pond densities increased at an average rate of 0.02 ponds/km²/yr during 1971–2005 (densities in the 7 targeted municipalities approximately doubled during the 35-year period); based on ponds present in 2000 throughout the county, disappearance rates were 0.002 ponds/km²/yr during 2000-2005. Pond construction in the past has often occurred at the expense of wetland habitat loss, and may negatively impact streams, for example by interfering with fish migration and elevating water temperature.  However, many ponds also remove large portions of the sediment load and total nitrogen in stream water, while seasonally influencing phosphorus uptake and export.  The aggregate impacts of ponds have been accentuated by both their increasing density and changing distribution within regional watersheds.