Drainage Divide Between the East and West Branches Brandywine Creek Located West of West Chester

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West of the City of West Chester, Pennsylvania the East Branch Brandywine Creek flows in a south-southeast and southwest direction to join southeast, northeast, and southeast oriented West Branch Brandywine Creek and to form southeast oriented Brandywine Creek. Broad Run is a southeast, east, and south-southeast oriented West Branch Brandywine Creek tributary located between the two Brandywine Creek branches. Both Brandywine Creek branches and Broad Run are joined by southwest oriented tributaries from the northeast and by barbed northeast oriented tributaries from the southwest. Several northeast-to-southwest through valleys cross drainage divides between Broad Run and the East and West Branches Brandywine Creek.

EWBrandywine

Figure 1 illustrates the East Branch Brandywine Creek flowing in a south-southeast and southwest direction from location 1 to join the West Branch Brandywine Creek at location 2. The West Branch Brandywine Creek flows in a southeast direction from location 3 to location 4 and then turns in a northeast direction to flow to location 5. From location 5 the West Branch flows in a southeast direction to join the East Branch at location 2 and to form southeast oriented Brandywine Creek. Broad Run is the south-southeast oriented stream at location 6, which joins the northeast oriented West Branch Brandywine Creek segment between locations 4 and 5 as a barbed tributary. Note how a northeast-to-southwest oriented through valley at location 6 links the south-southeast oriented Broad Run valley with the East Branch Brandywine Creek valley and with the West Branch Brandywine Creek valley. Locations 7, 8, and 9 identify other somewhat shallower northeast-to-southwest oriented through valleys crossing the drainage divides.

Questions raised by the East and West Branch Brandywine Creek and Broad Run valley orientations and the orientations of through valleys include, why does the West Branch Brandywine Creek change from flowing in a southeast direction to a flow in a northeast direction and then change again to flow in a southeast direction and how were the northeast-to-southeast oriented through valleys eroded? To answer these questions we need to visualize the region prior to the erosion of the East and West Branch Brandywine Creek valleys. At that time the entire region was at least as high if not higher than the highest drainage divides today and massive and prolonged southwest oriented floods were flowing across that upland surface. Floodwaters were flowing in anastomosing complexes of shallow diverging and converging channels and, because there was no nearby lower base level, floodwaters could not cut deep valleys into the regional bedrock.

Headward erosion of the deep southeast oriented Brandywine Creek valley changed the situation described above. The deep Brandywine Creek valley eroded headward from what was probably a newly eroded deep Delaware River valley to capture southwest oriented flood flow north and west of the Delaware River valley and to divert the captured floodwaters to the deeper Delaware River valley. Headward erosion of the deep Brandywine Creek valley to location 2 captured a southwest oriented flood flow channel on the alignment of the southwest oriented East Branch Brandywine Creek segment upstream from location 2 and its southwest oriented Blackhorse Run tributary. Floodwaters on the northeast end of the beheaded flood flow channel reversed direction and flowed in a northeast direction to the deeper Brandywine Creek valley and created northeast oriented (and barbed) tributaries joining Brandywine Creek near location 2.

The deep southeast-oriented Brandywine Creek valley then continued to erode headward in a northwest direction towards location 5 where it captured the southwest oriented flood flow that was flowing in the dry valley shown at location 8. The Brandywine Creek started to erode headward in a north direction and captured a southwest oriented flood flow channel at location 9. At the same time flood flow on the northeast end of the beheaded flood flow channel at location 5 was reversed to flow in a northeast direction and the reversed flow captured floodwaters from further north, including flow on the alignment of the southwest oriented channel at location 6 and on the alignment of the southwest oriented channel at location 7 (I say on the alignment because the deep valleys at location 6 and 7 had not yet been eroded).

Shortly after headward erosion of the deep south oriented Brandywine Creek valley captured the southwest oriented flood flow channel at location 9 headward erosion of the deep East Branch Brandywine Creek valley captured the same southwest oriented and ended all flood flow across the dry valley shown at location 8 and then captured the southwest oriented flood flow channel at location 9 and ended flood flow to the south oriented West Branch Brandywine Creek tributary valley at location 5. The southwest oriented Taylor Run valley in the figure 1 northeast corner was eroded headward from the newly eroded East Branch Brandywine Creek valley along this captured flood flow channel that had been supplying floodwaters to that south oriented tributary valley.

Headward erosion of the deep East Branch Brandywine Creek valley was beheading in sequence from south to north the flood flow channels supplying floodwaters to the newly eroded West Branch Brandywine Creek valley. However, headward erosion of what is today the deep West Branch Brandywine Creek valley was far enough ahead of the deep East Branch Brandywine Creek valley that it was able to capture new southwest oriented flood flow channels before East Branch valley headward erosion could behead those flood flow channels. The same type of sequence of events occurred as the deep West Branch valley eroded headward from the reversed flow channel at location 4 to capture a major southwest oriented flood flow channel at location 3.

The deep Broad Run valley eroded headward from the reversed flow channel (between locations 4 and 5) to capture the southwest oriented flood flow channels supplying floodwaters to the newly eroded West Branch Brandywine Creek valley, but did not reach location 6 until after the deep West Branch valley had already reached location 3. By angling to the west the actively eroding West Branch Brandywine Creek valley head captured southwest oriented flood flow before the actively eroding Broad Run valley head beheaded those flood flow channels. For a period of time the actively eroding Broad Run valley was also able to capture southwest oriented flood flow channels before the actively eroding East Branch Brandywine Creek valley could behead the flow. However, eventually East Branch Brandywine Creek valley headward erosion did behead all flood flow to the Broad Run valley head and the deep Broad Run valley was not able to erode headward into the present day Chester Valley region (north of figure 1).

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