The Delaware River flows in a south and southeast direction to Bulls Island and then turns to flow in an east-northeast direction for approximately one mile to Hendrick Island before turning to again flow in a southeast and south direction. Delaware River tributaries from the northeast are aligned in a southwest direction while tributaries from the southwest are aligned in a northeast direction. Figure 1 illustrates how the Delaware River changes its direction of flow and also the alignment of tributaries on both sides of the river. The short east-northeast oriented Delaware River valley segment has an alignment similar to the tributary valley southwest and northeast alignments.
Figure 1: Delaware River flows in a south and southeast direction to Bulls Island, then turns to flow in an east-northeast direction to Hendrick Island, and finally turns to flow in a southeast and south direction to the map south edge. United States Geologic Survey map digitally presented using National Geographic TOPO software.
Why would a south oriented river turn to flow in an east-northeast direction and then resume its flow in a south direction? The answer is related to the tributary alignments, which in turn is related to underlying geologic structures, but which is also related to the way the Delaware River valley was eroded. Before the Delaware River valley was eroded the entire figure 1 region was at least as high or higher than the highest elevations seen in figure 1 today. Erosion of the deep Delaware River valley occurred during immense and prolonged southwest oriented floods that flowed on that high-level surface. Floodwaters flowed in large anastomosing complexes of shallow diverging and converging channels, which were eroded into the least erosion resistant bedrock units and in this manner the underlying geology helped shape the flood flow direction.
The deep south-oriented Delaware River valley seen in the southeast quadrant of figure 1 eroded headward into this high-level surface as it captured southwest oriented flood flow and more specifically flood flow channels in sequence from south to north. The captured floodwaters provided the water volumes required for the deep Delaware River valley to keep eroding headward across the region. Each time headward erosion of the deep south-oriented Delaware River valley captured a southwest oriented flood flow channel the southwest oriented floodwaters would erode a southwest oriented tributary valley into the newly eroded Delaware River valley wall (on the New Jersey side).
At the same time, capture of that southwest oriented flood flow channel meant headward erosion of the deep Delaware River valley also beheaded the channel and floodwaters on the northeast end of the beheaded flood flow channel reversed direction to flow in a northeast direction to the much deeper and newly eroded Delaware River valley. Because the flood flow channels diverged and converged and because at least some of the floodwaters were not moving in the flood flow channels this reversed flow captured significant floodwaters from yet to be beheaded flood flow routes north of the actively eroding Delaware River valley head and this captured flood flow helped the reverse flow on the beheaded channels in eroding northeast oriented tributary valleys.
When the Delaware River valley reached the Hendrick Island area it captured a southwest oriented flood flow channel and floodwaters on the northeast end of the beheaded channel reversed direction to flow in a northeast direction to the newly eroded and much deeper Delaware River valley. However, the volume of captured water enabled the actively eroding Delaware River valley to erode headward along the reversed flow channel before resuming it headward erosion across southwest oriented flood flow channels further to the north. In summary, the Delaware River elbows of capture between Bulls Island and Hendrick Island were formed when the actively eroding Delaware River valley eroded headward along a beheaded southwest oriented flood flow channel that had captured significant amounts of reversed flow and only after eroding headward along that beheaded and reversed flood flow channel did the actively eroding Delaware River valley turn so as to again erode headward across southwest oriented flood flow channels.