Vegetation changes in the freshwater tidal marsh of the Delaware estuary

We examined the areal extent and changes in thefreshwater tidal wetlands along a 56.4 km and a80.6 km reach of the Delaware River between Chester,Pa. and Trenton, N.J. Most of the remainingfreshwater tidal wetlands of the Delaware River arefound along tributaries which drain the coastal plainof New Jersey. We identified polygons of marsh, mud,and open water using color infrared aerial photographyobtained at low tide in 1977 and 1978. Marsh polygonswere classified into either high marsh or low marshaccording to the dominant visual signature of thevegetation of each polygon, and placed in a geographicinformation system for subsequent analysis. The totalarea of marsh within the two reaches totaled 1416 ha,of which 71% was high marsh and 29% low marsh. Asite re-examination in 1997 and 1998 of marsh arearepresenting 32% of the total marsh area revealedthat, while the total area of wetland appears to haveremained constant, high marsh vegetation along thelower reaches of the tributaries has been replaced bylow marsh vegetation. The fraction of the sample thatwas low marsh increased from 9% in 1977–78 to 34% in1997/8.