Horn Pond is a 227acre water body located on the border of Acton, Maine, and Wakefield, New Hampshire, in the United States. The lake is both spring-fed and fed by the outflows of Great East Lake to the north and Wilson Lake to the east. Horn Pond's outlet is the Salmon Falls River which forms a natural border between Maine and New Hampshire and empties into the Piscataqua River east of Dover, New Hampshire, and ultimately the Gulf of Maine.Horn Pond is classified as "Potentially Non-supporting" under the New Hampshire Department of Environmental Services' revised water quality standards for oligotrophic lakes. The Horn Pond watershed covers that is primarily forested (70%). Only 6% of the watershed is developed, with 34% buildable area. The shoreline of Horn Pond is composed primarily of low density houses and camps (51%) and undeveloped shoreline (44%). 70% of the structures along the shoreline are within of the water's edge.Horn Pond water quality monitoring has been collected since 1982. During this period, 11 years of secchi disk transparency data, 7 years of total phosphorus (TP) data, 2 years of chlorophyll-a (Chl-a) data, and 5 years of dissolved oxygen (DO) data has been collected. From this data, the median TP concentration is 8.0 parts per billion (ppb) and the mean transparency is 6.6m.