Skip to main content.
Earth and Environmental Sciences Home > Rappahannock Watershed Information > Research Projects > Post-Settlement Deposition along Rappahannock River near Falmouth

Post-Settlement Deposition along the Rappahannock River near Falmouth

John Gilreath, Sophomore, Geology & Geography major

Beau Whitney, Sophomore, Geology & Geography major

Methods

Results

References

Deposition on the Falmouth Park floodplain near Fredericksburg, Virginia has increased substantially due to the settlement of the Rappahannock River drainage basin. Agricultural land use and timber felled for fuel of numerous blast furnaces of iron works resulted in widespread clear-cutting of forests near Fredericksburg. Much of the present vegetation in the area of clear-cutting is second growth.

This clear-cutting accelerated the rate of erosion along the side slopes of hills and ridge tops and increased the sediment load of the Rappahannock River. These historic sediments are currently stored within floodplains along the Rappahannock River. Falmouth Park represents a probable historic (i.e. post-settlement) floodplain that is located less than 1 km downstream of the "Fall Line" and immediately east of the Rte. 1 bridge, which crosses the Rappahannock River at Fredericksburg (Figure 1). The gradient of the river decreases

sharply as it crosses the "Fall Line" near Falmouth Park and downstream, the river traverses the Coastal Plain and is tidally influenced. The average elevation of the floodplain surface at Falmouth Park is ~10 feet and the floodplain is bounded to the north by River Road and by the Rappahannock River to the south. Frequent flooding at Falmouth Park occurred during the course of the study in January and February of 1998.

Methods (Back to Top)

Floodplain samples up to 2 m deep were acquired using an auger. In addition, excavations of cut-banks showed cross-bedded and unconsolidated sand in the upper part of the floodplain deposit. A total of 20 sites were sampled; 16 were used to construct two stratigraphic cross-sections (Figures 2 and 3) that show the floodplain stratigraphy. The transect for the first cross section is ~150 yards downstream of the

Figure 1: Falmouth Floodplain Stratigraphic Cross-Section 1.


Figure 2: Falmouth Floodplain Stratigraphic Cross-Section 2.

Route 1 Bridge and a total of 9 sites were sampled with a spacing of 10 feet (Figure 1). The second cross section (Figure 3) was constructed from 7 sites with a spacing of 10 feet at a location ~450 yards downstream of the bridge and ~300 yards downstream of the first cross section. Historic photographs and descriptions of the colonial era were also used to interpret the floodplain deposits near Falmouth Park.

Results (Back to Top)

The upstream cross section consists of 0.2-1.0 m of beige cross-bedded fine sand that overlies 1.2-2.2 m of brown sandy loam and loamy sand and minor orange-red clay (Figure 2). Site 1 contained a piece of glass at a depth of 70 cm and site 2 contained a piece of brown glass at a depth of 54 cm (Figure 2). The downstream cross section shows that the upper cross-bedded fine sand thins to 0.1-0.2 m and that in the underlying deposits, the percentage of orange-brown clay increases and sand content decreases

Figure 2: Falmouth Floodplain Stratigraphic Cross-Section 2.

compared to the deposits in the upstream cross section (Figure 3). For instance, sandy loam and clay are abundant, whereas loamy sand is absent in the downstream cross section. In addition, an organic-rich horizon with many roots is present at a depth of ~0.8 m in the downstream cross section.

Site 3 in the downstream cross section also contained a piece of iron slag at a depth of 25 cm. The slag represents material that was produced since 1721, which is when the Spotswood Iron Furnace, located several miles upstream of Falmouth on the Rappahannock River, went into production. Alternatively, the slag could post-date 1750, which is when the Hunter’s Iron Works in Falmouth went into production.

Several pieces of evidence indicate that colonial-era clear-cutting has directly resulted in the Falmouth floodplain. The presence of historic sediments (i.e. glass and iron slag) show that colonial-era clear-cutting contributed to floodplain sedimentation at Falmouth Park. The stratigraphic context and depth of the historic sediments demonstrates that at least the upper 75 cm and probably the uppermost 1-2 m of the floodplain sediments at Falmouth Park are post-settlement deposits. These historic sediments represent deposits that eroded from hillslopes during colonial-era settlement and clear-cutting.

In addition, a park manager of the Falmouth area visited the study area during the course of this study and mentioned that the blanket of flood-deposited fine sand that covers much of Falmouth Park today, began to accumulate in the 1970’s, following high flow events related to hurricane Agnes. Prior to this time, the park manager stated that sandy flood deposits at Falmouth Park were rare. This observation is confirmed by the scarcity of sand in the lower part of the floodplain deposits (Figures 2 and 3). The full extent to which hurricanes such as Agnes have influenced the movement of sediment in the Rappahannock River remains to be explored.

References (Back to Top)

Kennedy, V.C. "Sediment Transported by Georgia Streams" in U.S. Geological Survey Water Supply Paper, 1964.

Meade, Robert. "Movement and Storage of sediment in rivers of the United States and Canada" in The Geology of North America, Vol. 0-1, Surface Water Hydrology. 1990.

Milliman, J.D. "Sediments of the Continental Margin off the Eastern U.S," in Geological Society of America Bulletin. 1972.

Nordin, C.F. Jr "The Sediment Discharge of Rivers" in International Association of Hydrology. 1965.

Trimble, S.W. "Culturally Accelerated Sedimentation in the Middle Georgia Piedmont" Athens, University of Georgia (reissued in 1971 by U.S. Soil Conservation Service, Fort Worth, Texas).

back to top