Rates of Pond Disappearance

        Ponds may also “disappear” from the landscape for several reasons. Either they are deliberately filled in and replaced with other land uses, or they succumb to the inexorable accumulation of sediment to the point that they lose permanent standing water. Of the 37 ponds identified in the 7 townships from Breou Farm Atlas maps prepared in 1883 (see above), 27 had disappeared by 1937, a 69% loss rate during the intervening 54-year period.
        These results are similar to those of a larger study of ponds in parts of Kansas and Ohio, summarized by Renwick and colleagues (2006) in the Proceedings of the 8th Federal Interagency Sedimentation Conference Based on aerial photographs taken at regular intervals as in this study, 57% of 520 Kansas ponds present in the 1930s had disappeared by 2002-2003, and 93% of 867 Ohio ponds disappeared during approximately the same time period.
        More recent trends within the Brandywine watershed tell a different story. For example, of 79 ponds present for the first time in 1958 in the same 7 townships, only 2 (2.5%) had disappeared by 2005 (and one of these reappeared later during the 47-year time interval); in effect, ponds currently in the Brandywine watershed have had relatively long life spans compared to ponds in Kansas and Ohio.
        The greater recent pond longevity of recent ponds in the Brandywine drainage may be due partly to much lower sediment loading rates than to the more typically in-line ponds of the colonial era, and to impoundments elsewhere. Sediment accumulation rates (measured as carbon burial) in 40 agricultural impoundments in Iowa studied by Downing et al. (2008) were much greater than in previous studies involving lakes receiving less stream inflow, and the relatively rapid filling in of agricultural impoundments may be the principal cause of the high rates of disappearance observed by Renwick et al (2006) and in the pre-20th century history of the Brandywine drainage. The influences of land use, catchment basin size and topographic slope on sedimentation rates are further discussed by Renwick et al. (2006b).
        In summary, the net proliferation of ponds presently in the Brandywine watershed is largely determined by the construction of new ponds. Greater pond longevity, probably associated with reduced sediment inputs, is gradually increasing average pond age. How long these trends will continue within the watershed is difficult to determine. Pond longevity can be greatly increased by dredging, and dredging may be a more economically viable option for extending pond life when it is required less frequently. It is not known, however, to what extent dredging is currently practiced by pond owners in the watershed.