Oxbow Ponds

        “Constructed” ponds of human origin that presently constitute the large majority of water bodies in the Brandywine watershed. It may surprise some that so few ponds in southeast Pennsylvania are natural, in comparison to their rather widespread occurrence farther north. Continental glaciers, that carved out the Pocono landscape and created basins that filled with water after their retreat, simply did not extend this far south. Beavers, known to have a profound effect on pond densities in some areas, are also rare here. The floodplains of the Brandywine and its major tributaries, however, contain large numbers of natural “oxbow” ponds. This section describes their creation and likely history.
        Stream channels are in actuality very restless entities, constantly moving within their floodplains. Water movement, particularly during storm events, is constantly changing the distribution of sediments, eroding the stream bank in some areas while depositing materials downstream. As a consequence, stream channels “move” within their floodplain, often creating distress for riparian homeowners who see portions of their properties carried away by bank erosion.

Fig. 1.(a) Shown is a meandering, low-gradient stream. Erosion is typically more severe along the outer bank of each meander, and reduced in depositional areas on the opposite/inner bank. Erosion eventually cuts through the “neck” (b), forming an isolated “oxbow” lake or pond that then gradually fills in over subsequent time (c and d). http://www.emporia.edu/earthsci

        Comparisons of aerial photos taken on successive dates often show stream meandering, and can further be used to detect the presence of stream segments that have been cut off from the original stream channel. These cut off stream segments are termed “oxbows”, presumably because their elongate shape often resembles that of the U-shaped wooden component of harnesses used for oxen. A typical sequence of ox-bow formation is described in Figure 1.

Fig. 2. A portion of the East Branch Brandywine, East Bradford Township, with outlines of the stream channel and ponds in various stages of oxbow formation shown in yellow.

        Oxbow ponds are a common feature in some portions of the Brandywine floodplain (Fig. 2). Very little is known, however, about their history, distribution and ecology. Visits to individual ponds (e.g., Fig. 3a–b) indicate a wide range of aquatic habitats. Differences in age since separation from the creek, surrounding land cover (e.g., emergent, herbaceous plants vs. tree canopies), and the degree to which they receive floodwaters from the creek during storm events, likely contribute to these habitat differences.
        Oxbow pond density might be expected to vary with “stream order”. Headwater streams with no tributaries of their own are termed first order streams. First order streams are typically high-gradient (flowing down steeper slopes), and convey relatively small volumes of water in channels that show little tendency to meander. Stream segments formed by the confluence of two 1st order streams are termed 2nd order. Third order streams in turn most have at least two 2nd order tributaries, and so on. The main stem of the Brandywine south of Lenape is considered a 6th order stream. As stream order increases, the channel widens, the topographic gradient decreases, greater volumes of water are transported and the degree of meandering (“sinuosity”) increases.


Fig. 3a-b. Oxbow ponds along the Brandywine Creek with different surrounding land cover. Note the elongate channel shape.

        Because stream sinuosity and erosive potential typically increase with increasing stream order, larger numbers of ox-bow ponds should occur next to higher order stream segments.

Fig. 4. Stream network, with some segments labeled by stream order. Sinuosity and floodplain size (in green) increase downstream. Oxbow ponds are shown in Oxbow ponds along the Brandywine Creek with different surrounding land cover. Note the elongate channel shape.

        Based on aerial photographs, 59 oxbow ponds have so far been identified along the main stem of the Brandywine Creek, its East Branch and West Branch. Abundances (numbers per km of stream length) typically increased with increasing stream order (Fig. 5), but declined in the 6th-order main stem.


Fig. 5. Abundance (per km of stream channel) of oxbow ponds along the East Branch, West Branch and Main Stem of the Brandywine Creek within Chester County.

        The abundances of oxbow ponds identified in the Brandywine watershed are almost certainly underestimates of actual oxbow formation, as smaller ponds under tree canopies have likely escaped notice, older ponds have gradually filled in leaving only faint traces of their former existence, and some ponds have been deliberately converted to other land uses (e.g., agriculture). Dam construction (see below) has likely altered rates of pond formation by attenuating the erosional force of storm events.
        Oxbow pond formation is a process that continues today. As population density and impervious land cover within the Brandywine watershed continue to Increase, the increasing erosional force of flooding events may lead to more rapid changes in channel location, accelerating rates of oxbow pond formation. They are important storage areas for water and sediments during stream flooding events, and need to be protected.