Changing Pond Densities

        Aerial photography was first applied systematically in Chester County in 1937, making it possible for the first time to evaluate changes in land use, and to identify the changing role of ponds in the local landscape. Students at West Chester University identified ponds in 7 townships within the Pennsylvania portion of the Brandywine watershed from aerial photos taken at roughly 10-year intervals during the period 1937 – 2005. Surface area, “landscape position” (Section II.B), and surrounding land use were included among variables recorded for each pond.

Fig. 18a–b. Changes in (a) pond densities and (b) population densities in 7 townships from 1937 – 2005.

        As shown in Figure 18a, pond densities have increased steadily over the 68-year period, at an average rate of 0.2 ponds/km2/yr; densities in 2005 were more than 18-fold greater than the average in 1937. Rates of increase varied considerably, with more rural townships (e.g., East Fallowfield, Honeybrook, Newlin) showing the lowest rates of pond proliferation. Population densities increased in the townships during the same period (Fig. 18b), but the ratio of “people to ponds” has changed over time.
        In 1937 and 1946, few ponds were present in the watershed, and the ratio of people to ponds was consequently high. Since 1946 the people-to-ponds ratio has generally declined in all but West Whiteland

Fig. 19. Trends in “people to ponds” among 7 townships over time.

Township; East Fallowfield and Newlin townships have continued to experience more rapid pond increases compared to population density since 1958. By contrast, recent population growth has outstripped pond construction in Birmingham Township. Not surprisingly, Caln and West Whiteland townships, with the highest population densities, have relatively more “people to ponds”.
        The ratio of “people to ponds ponds” assumes greater importance when considered as an index of the ability of ponds to perform “ecosystem services” in retaining sediments and removing or transforming nutrients generated by human activity. In effect, as the ratio of “people to ponds” increases,

Fig. 20. Age distribution of ponds present in 7 townships in 2005.

the ability of ponds to perform such services is reduced. These ideas are considered further in Section III.
        Much as one might evaluate the ages of residents in the Brandywine watershed, it is possible to determine the age distribution of the 550 ponds that were present in the 7 townships in 2005. As shown in Figure 20, there are typically fewer and fewer ponds as age increases. A notable exception to the general trend, however, is the large number of ponds in age category 48-59 years, presumably largely representing farm ponds built during 1946-1957.