February 23, 2022  

The Potomac River Floodplain

This post is going to talk about fluvial processes during the last few millennia, with the Potomac River as an example. A previous post discussed the geology of the Potomac’s fall line, where it drops out of the foothills of the ancestral Appalachian Mountains to the coastal plain before entering Chesapeake Bay. I’m going to keep this simple because, to be honest, fluvial geomorphology is not a straightforward topic. Rivers are constantly changing at time scales from years to millions of years. We won’t be walking back billions of years today, only a few hundred thousand, maybe a couple of million.

Figure 1. Schematic map, showing the location of the study area within northern Virginia (inset) and North America (double inset). Several features are labeled that will be referred to in the text and following figures. All photographs were taken within the black circled area. Note the labeled Holocene levee and Pleistocene terrace because they will be referred to later.

The lower Potomac River is braided, with multiple channels defining wooded islands (e.g. Van Deventer Island in Fig. 1). I won’t be talking about them but instead focus on what I saw, what the rocks (river sediment is unlithified rock to a geologist) tell me. The river flood plain extends to the Pleistocene terrace (yellow line in Fig. 1), which is about 80 feet higher in elevation than the river surface. No permanent structures have been constructed on the flood plain.

Some of the features we will examine are shown schematically in Figure 2. Note however, that the image shows a meandering stream whereas the Potomac is braided, which means that its channel doesn’t take those big loops shown in Fig. 2. That’s because the lower Potomac drops rapidly from Great Falls just upstream of the study area, to Washington D.C. in this area.

Figure 2. Schematic image of common river features.

We started out on the area labeled “Bluffs” in Fig. 2 and traversed the flood plain, following a tributary called Horsepen Run (see Fig. 1 for location). Note that Horsepen Run is a meandering stream, so we’ll see several features that scale downward from Fig. 2 as we cross the Potomac flood plain.

Figure 3. View looking downstream along Horsepen Run, showing fork in channel where debris collects during high flow. Note how shallow the flow is, the presence of large cobbles (some more than a foot in diameter), and erosion along the banks.

Horsepen Run (aka creek) drops quickly from the Pleistocene terrace (Fig. 1) but then crosses the Potomac flood plain and begins to meander. The photo in Fig. 3 is from a location just before this change in stream topography occurred.

Figure 4. View downstream of Fig. 3, showing coarse sediment inside of a meander forming a point bar.

The changes in stream morphology seen between Figs. 3 and 4 occur in larger streams (like the Potomac) but on much longer spatial scales.

Figure 5. Eroded bank of Horsepen Run, showing roots of a tree that cannot be more than a hundred years old. This indicates rapid downcutting and lateral movement of the channel. This kind of incision indicates a lowering of the stream’s base level, either due to uplift of the source or lowering of the receiving basin. For Horsepen Run, base level is where it enters the Potomac River (Fig. 6).
Figure 6. Confluence of Horsepen Run and Potomac River. The stream and river both have cut banks about 6 feet in height. There is no substantial delta at the mouth of Horsepen Run because of the coarseness of its sediment. Note the boulders seen in Figs. 3 and 4. This location is only a few miles from exposed Precambrian rocks.
Figure 7. View looking north, across the south fork of the Potomac River, towards Van Deventer Island which separates Virginia and Maryland. This is a rather large island in a terrain underlain by Precambrian rocks that are resistant to erosion. It is not a gravel bar as depicted in many representations of braided streams. Note the cut bank on the opposite shore, which is more than six feet in height (estimate only).

Figure 1 indicates the presence of natural levees (lower center of Fig. 1) near the main river channel. There is no “Yazoo Tributary” (see Fig. 2) at this location, so Horsepen Run cut across the Potomac’s natural levee. This can be seen beautifully in Fig. 8.

Figure 8. Meander in Horsepen Run cutting across the Potomac River’s natural levee, which is shown by the slope to the right side of the photo. The view is looking parallel to the Potomac’s channel at this location.
Figure 9. It is difficult to see the levee in this photo, which was taken from the top of the approximately 3-foot natural levee. Note the swale (low area) to the left side of the image. The levee isn’t as high as some (e.g., the Mississippi River) but it is quite noticeable, especially from ground level.

It was a beautiful February day to hike to across the Potomac River flood plain. I hadn’t expected to find so much dynamic geology so close to my new home, but there it was. The historic Potomac River transitions from its rocky confluence with the Shenandoah River at Harper’s Ferry, to the tidal river that defines Washington D.C., right here and, like America, it is not in equilibrium. The cut banks of the Potomac and its tributary, Horsepen Run, portend of rapid changes in the relative elevation of the land and the sea.

We are in for a wild ride…

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