THE PALEOECOLOGICAL DEVELOPMENT OF A LATE HOLOCENE, TIDAL FRESH-WATER MARSH OF THE UPPER DELAWARE RIVER ESTUARY

Changes in groundwater tables brought about by sea level increases in the Delaware River Basin (near Philadelphia) about 2,500 years B.P., initiated wetland development at the Princeton-jefferson Branch of the Woodbury Creek marshes. Continual increases in sea level pushed groundwater tables further upward, and by approximately 800 years B.P., groundwater tables had risen to the upper limits for woody vegetation at the site. By the time European settlers arrived in the late 1600s nontidal sedge marshes dominated the site. Upon arriving colonists began manipulating the hydrology of the Delaware River Basin by constructing dams and dikes for flood control. Soon many areas were cut off from direct contact with the river. During the next one and one-half centuries sea level continued to rise, and because of channelization of the Delaware River the tidal range doubled. During the early 1900s flood control structures began to fail allowing tidal waters to periodically inundate these protected sites. At that time the site was dominated by a Quercus-Castanea swamp forest with hummocks of Cyperaceae interspersed throughout. In 1940 the dike surrounding the Princeton-Jefferson marsh collapsed and the site was immediately inundated with tidal waters on a regular basis. Within a short period of time tidal freshwater marsh developed and has continued to the present day. It is clear from this investigation that changes in hydrology brought about by cultural modifications have been directly responsible for the ontogeny of this tidal marsh. The influence cultural impacts have had on wetland development at the Princeton-Jefferson marsh suggest that it may be necessary to reevaluate the extent humans have modified the development and structure of the present day upper Delaware River estuary. Although the ability to discern historic vegetation zonation patterns is limited, these marshes can record individual events that have shaped these wetlands through time. Due to differences in the structure of the plant community, rates of decomposition, and processes of accretion, Redfield's model (1972) of tidal salt marsh development does not apply to the Princeton-Jefferson marsh. Along a submerging coast, the development of tidal freshwater marsh in many estuaries may be necessary for the establishment of brackish and salt marshes by creating and maintaining a suitable habitat for the eventual colonization of more salt-tolerant plant species. The roles these wetlands have played in the development of the estuaries has been underestimated in the past.