Delaware Bay: Philadelphia’s Estuary or Drowned River Valley

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DelawareBay

Figure 1: Delaware Bay extends from northeast of Philadelphia to Cape May and is a drowned river valley. United States Geological Survey map digitally presented using National Geographic TOPO software.

Delaware Bay is the flooded lower Delaware River valley or an estuary that extends upstream from south of Cape May, New Jersey almost to Trenton, New Jersey. NOAA nautical charts suggest Delaware Bay water depths today are generally less than 60 feet and rarely exceed 100 feet. Delaware Bay is where the Delaware River and several other smaller streams deposit their transported sediments and present day water depths may not accurately reflect the Delaware River valley depth prior to the sea level rise that flooded Delaware Bay. Some topographic details required to determine the complete erosional history of the drowned valley are no longer visible.

What information can be determined from the figure 1 regional map? The Delaware River flows in a southwest direction from near Bristol, Pennsylvania (along figure 1 north edge) to the Wilmington, Delaware area. While not determinable from figure 1 this southwest drainage route is located along the boundary between crystalline metamorphic rocks to the northwest and much more recent and easily eroded sedimentary rocks to the southeast. Interestingly, the southwest oriented Delaware River segment is on the same alignment as the Elk River estuary and the northern reaches of Chesapeake Bay seen in figure 1. However, instead of continuing in a southwest direction the Delaware River valley turns in a south and south-southeast direction to form what is today Delaware Bay.

Streams flowing into Delaware Bay from the east and northeast tend to be oriented in southwest or south directions while at least some streams flowing into Delaware Bay from the west are oriented in northeast directions, which suggests they entered the pre-sea level rise Delaware River as barbed tributaries. Also streams flowing from the northern Delmarva Peninsula to Chesapeake Bay tend to be oriented in southwest directions and also tend to be much longer than the northeast and east oriented streams flowing to Delaware Bay. These drainage orientations and characteristics suggest the pre-sea level rise Delaware River valley eroded headward across a complex of shallow diverging and converging flood flow channels. If so, Delaware River valley headward erosion would have captured southwest oriented flood flow that had been moving across the northern Delmarva Peninsula and diverted the floodwaters more directly to the Atlantic Ocean.

This interpretation is consistent with the interpretations of landforms eroded by Delaware River tributaries flowing across the crystalline rock material found north and west of the southwest oriented Delaware River segment seen in figure 1 (see other entries on this website). Further this interpretation suggests the Delaware River valley probably eroded headward across the southwest oriented flood flow to reach the Wilmington area and then headward along a captured southwest oriented flood flow channel almost to Trenton, New Jersey at a time when a large North American ice sheet was melting. There is excellent evidence one or more continental ice sheets once covered much of northern North America and that water contained in the ice sheet(s) significantly lowered sea level. Melting of a large North American ice sheet would have created massive and prolonged floods that would have deeply eroded all regions the floodwaters crossed and that would have gradually raised sea level as that erosion occurred. This interpretation is consistent with Philadelphia area landform evidence and with the flooded Delaware River valley, but raises significant questions as to how Coastal Plain and other sediments relate to the massive and prolonged melt water floods as well as to how the large ice sheet(s) and massive melt water floods fit into Cenozoic history.

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