Changes in Landscape Position


        By “landscape position” we mean the location of ponds within larger stream-defined watersheds. Ponds in Chester County were categorized as 1) “Headwater – No Inflow” (with no influent stream, but connected to
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Fig. 21. Landscape position categories: 1) Headwater – No Inflow, 2) Headwater – Inflow, 3) Downstream Impoundment, 4) Isolated.

a stream via the outfall, 2) “Headwater – Inflow” (with both a 1st-order stream inflow and outflow to a 1st-order stream, 3) “Downstream Impoundment” (created by damming a 2nd-order or larger stream), or 4) Isolated (not connected to streams). Ponds were considered “isolated” based on their articulation with stream segments in the GIS data layer for streams of Chester County. These categories are shown diagrammatically in Figure 21. Based on site visits to many ponds, however, the available GIS data layer for streams fails to include many small streams and underground pipes, and thus most ponds identified as “isolated” probably release water to streams at least during high flow conditions.
        Ponds constructed during the colonial era were often associated with grist mills, rolling mills and other activities requiring hydropower. Perhaps not surprisingly, of 37 ponds identified from Breou Farm Atlas (1883) maps of the 7 townships described above, 27 (71.1%) were “Downstream Impoundments”, and 8 (23.7%) were “Headwater – Inflow” ponds; by contrast, only 1 (2.6%) was a “Headwater – No Inflow” pond, and 1 (2.6%) was “Isolated”.
        By 1937, the frequencies of these four landscape position categories had changed radically. Of 28 ponds present in 1937, just 1 (3.6%) was a “Downstream Impoundment”, while 10 (35.7%) were “Headwater Inflow” ponds. By contrast, 3 “Headwater – No Inflow ponds” and 14 “Isolated” ponds were present (10.7% and 50.0% of the total, respectively). In effect, there was a marked shift from “in-line” ponds receiving stream
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Fig. 22. Changes in landscape position over time.

inputs in 1883 to ponds that were either unconnected to streams or served as the sources of headwater streams in 1937.
        During the rapid proliferation of ponds between 1937 and 2005 (see Fig. 1a above), the trend away from in-line ponds toward ponds without stream inflows has continued, but the changes have been more modest (Fig. 22). By 2005, the proportion of “Isolated” ponds had further increased from 50.0% in 1937 to 64.1%, and “Headwater – No Inflow” ponds increased slightly from 10.7% of all ponds in 1937 to 13.2% of the total in 2005. By contrast, “Headwater – Inflow” ponds declined further (from 35.7% of ponds in 1937 to 18.2% in 2005); “Downstream impoundments” have remained rare since 1937. A principal effect of taking ponds “off line” is a change in the degree to which they trap sediments. (See Pond Sediments Section.)