The pre1939 shapefile displays the boundary between the floodplain and the uplands (a boundary that remains constant), while the 1939-2003 shapefiles (assicated with the geoTIFFs) display both the constant boundary between the floodplain and uplands and the changing boundary between the channel and the floodplain. The pre1939 shapefile is included to represent the floodplain formed before the earliest imagery in 1939. Both the flood plain and active channel of the river were delineated on the 1995 digital orthophoto quadrangles and overlaid on rectified photos. ArcGIS was used to draw the polygons that delineate the flood plain and active channel; the delineation was saved as a SHP file. There are no aerial images associated with the pre1939 shapefile.