Pond Densities and Distribution

       Based on inspection of aerial photos taken in 2005, 4531 ponds were identified in Chester County. The “landscape positions” of ponds in the county (describing their connection to streams) are shown in Figure 3. Most (67.5%) are shown as “isolated” (unconnected to streams) based on available GIS data and aerial photographs; in visiting many of these ponds, however, most were found to have outfalls that export small amounts of water either year-round or periodically to neighboring stream systems. “Noinflow” headwater streams, constituting 15.1% of the total, are primarily fed by groundwater but serve as the source of first-order streams. Ponds receiving water from first order streams (streams without tributaries) constituted 12.6% of the total, while downstream impoundments of larger streams (second order or higher) were least common (4.7%). In effect, most ponds currently in the county experience little channelized stream flow, and those that are part of stream systems are likely have their greatest and most direct influence on very small streams.


Fig. 3. Proportion of a) “no inflow - headwater” ponds not receiving stream inflows but typically contributing water to headwater streams, b) headwater-inflow ponds created by impounding “first order” streams, c) “downstream impoundments” of larger streams, and d) “isolated” ponds for which no permanent stream connection to streams could be detected.

       Pond densities ranged from <1 to 2.5 ponds/km2 among the 21 major stream watersheds in the county, with highest densities in the Crum and Ridley Creek watersheds (Fig. 4).

Fig. 4. Map of 21 major stream catchments in Chester County, PA, indicating densities of ponds (number/km2) within the catchment boundaries.

       Land use characteristics within the watersheds of individual ponds are known to strongly influence pond water quality. For example, wooded areas are generally thought to maximally protect pond water quality, as tree canopies and deep root systems retain nutrients that might otherwise enter the pond (leaf fall from trees directly overhanging the water, however, can be a substantial seasonal nutrient source).
       By contrast, agricultural land consisting of row crops (e.g., corn, soybeans) may contribute large quantities of sediments and nutrients to a pond, particularly if they occur close to the pond and/or on moderate to steep slopes. Runoff from erosion-prone land surfaces can carry phosphorus-laden sediments to a pond. Nitrogen is more likely to enter the pond in dissolved form, either in surface runoff or in groundwater inflows.
       Pastureland/hay meadows and residential housing, because they provide nearly complete ground cover (albeit primarily by shallow-rooted turf grasses), are generally thought to export larger quantities of soil and nutrients than forests, but less than row-crop agricultural land. As the density of residential housing increases, higher proportion of impervious surface increase the amount of precipitation that, instead of recharging the groundwater, is exported as overland runoff, impacting ponds by rapid changes in water level, increased sediment and nutrient inputs, and temporarily increased rates of water flow at the outfall. Typical impervious surfaces include roads, sidewalks, roofs and driveways, which increase with increasing housing density.
       The underlying bedrock within the watershed also influences pond water quality. Bedrocks of differing weathering properties also contribute to the formation of hills and valleys, and greater elevation change within the watershed may indicate greater supplies of nutrients and other materials to the pond.
       Much of the northern part of Chester County is underlain by gneisses and quartzites; these are hard, metamorphic rocks that weather slowly and contribute sparingly to overlying soils and surface waters (Fig. 5).

Fig. 5. Major rock types in Chester County, PA.

       Schists, also metamorphic and slowly weathered, comprise much of the bedrock in the southern part of the county. Between these two regions a band of more easily weathered carbonate-rich rock (seen in orange) transects the county along a NE-SW axis, forming the Chester Valley. As a result of this weathering, which causes many rock constituents to become dissolved in water, surface waters in the Chester Valley typically have larger quantities of ions such as calcium (Ca2+), magnesium (Mg2+) and bicarbonate (HCO3-), and often more abundant nutrients (e.g., nitrogen and phosphorus) than in other parts of the county. Not too surprisingly, ponds in the county vary widely in their water chemistry, not only because of human-caused pollution but also because of the influence of the underlying bedrock type.
       The riparian vegetation immediately surrounding the pond helps to intercept nutrients and sediment runoff. Another important function of the riparian vegetation is stabilization of the shoreline, preventing bank erosion and thus reducing sediment load (see Volume 1 Section D). Riparian vegetation surrounding ponds in Chester County is most typically “mowed”, with short grass that provides minimal habitat and minimal interception of pollutants; less frequently mowed meadow is also common (Fig. 6).

Fig. 6. Pond in Birmingham Township, showing mowed grass as the predominant riparian vegetation.

       Woods are less typical as riparian vegetation surrounding ponds in the county (Fig. 7). The impacts of riparian trees are not fully understood, but they likely contribute strongly to pond ecology. For example, trees intercept wind flow and thus reduce mixing of the water column. This leads to “stratification” of the water column during daylight hours in summer, such that water near the surface is warm and oxygen-rich, whereas deeper water (often just a foot or so deeper!) may have cooler temperatures and much less oxygen (see Section H below). Secondly, leaf fall at the end of the growing season and periodic inputs of woody material may provide an important source of chemical compounds that collectively affect pond water chemistry and food availability, snags that enhance fish habitat, and increase organic carbon storage within pond sediments. In effect, trees greatly modify the pond environment, affecting the kinds of algae, invertebrates and fish that are present.


Fig. 7. Proportions of forested land, meadows and mowed grass forming the riparian vegetation immediately adjacent to ponds in Chester County.