The Delaware River flows in a southwest direction along the Pennsylvania-New Jersey border until it turns to flow in a south direction under the Benjamin Franklin Bridge, which links the Philadelphia City Center with the Camden City Center. The abrupt change in the Delaware River direction of flow just north of the Benjamin Franklin Bridge is referred to here as the Benjamin Franklin Bridge Elbow of Capture. Urban development has complicated interpretations of the Philadelphia and Camden urban area elbows of capture, although based on what evidence remains and also on evidence from north of the Delaware River a history of this elbow of capture can be suggested.
Figure 1: Delaware River Elbow of Capture just north of the Benjamin Franklin Bridge. The Philadelphia City Center is located at the map west center edge. United States Geological Survey map digitally presented using National Geographic TOPO software.
The Delaware River valley eroded headward into a pre-Delaware River landscape surface that no longer exists. What can be determined is at the time Delaware River valley erosion began sea level was much lower than it is today and massive and prolonged southwest oriented floods flowed across the Philadelphia region and continued to flow across the region as the Delaware River valley and its south oriented tributary valleys (e.g. Tacony, Pennypack, and Neshaminy Creeks) eroded headward. Floodwaters were probably flowing in anastomosing complexes of shallow diverging and converging channels. Urban development in the City of Philadelphia and the City of Camden has obscured much of the evidence for these southwest oriented flood flow channels, although aligned drainages found crossing south oriented Delaware River tributary valleys and the south oriented Delaware River valley segment found in the Trenton area and upstream are relics of the flood formed southwest oriented diverging and converging channels.
The deep Delaware River valley eroded headward into the pre-Delaware River valley landscape to capture the massive southwest oriented flood flow, which in turn enabled the Delaware River valley to erode headward so as to capture more of the southwest oriented flood flow. South oriented Delaware River valley segments were formed as the Delaware River valley eroded headward across southwest oriented flood flow channels while southwest oriented Delaware River valley segments were formed as the Delaware River valley eroded headward along a captured southwest oriented flood flow channel.
With this background in place the Benjamin Franklin Bridge Elbow of Capture can be explained. Headward erosion of the downstream south oriented Delaware River valley segment (from the Franklin Bridge to just south of the Whitman Bridge) was across southwest oriented flood flow and captured a southwest oriented flow channel just north of the present day Benjamin Franklin Bridge location. That southwest oriented flood flow channel probably was moving large volumes of water as the deep Delaware River valley then eroded headward along that southwest oriented flood flow channel to Bristol and possibly further. South oriented and deep Delaware River tributary valleys (e.g. Tacony, Pennypack, and Neshaminy Creeks) eroded headward in sequence (from west to east) from the actively eroding southwest-oriented Delaware River valley to capture southwest oriented flood flow further to the north and to divert the captured floodwaters to the newly eroded and deep southwest and south oriented Delaware River valley.