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Earth and Environmental Sciences Home > Rappahannock Watershed Information > Research Projects > Geologic Influences on Horsepen Run (Upper Rappahannock Basin)

The Geologic Influence on the Stream Characteristics of Horsepen Run in Stafford County, Virginia

Cynthia Cheatham, (B.S., Geology major, Environmental Science post-baccalaureate major)

Brian Laposay , (Senior, Geology major)

 

Introduction

Study Area

Results

Interpretation and Discussion

References

Introduction (Back to Top)

Geology has an important influence on the characteristics of streams and rivers, as well as the drainage basin. The slope of the basin and drainage patterns are determined by the underlying rocks. Stream patterns and the quantity and types of sediments delivered to the streams are influenced or determined by the geology. A study of the characteristics of a portion of Horsepen Run in Stafford County, Virginia illustrates the importance of geologic influences on the stream. The study resulted in observing how many other factors equally influence the characteristics of Horsepen Run.

Study Area (Back to Top)

The area of study is located near the Atlantic Coast of the United States, in the state of Virginia. Horsepen Run is located approximately 50 miles (80.5 km) southwest of Washington D.C. and 4 miles (6.4 km) northwest of the City of Fredericksburg, Virginia. Fredericksburg, which is located on Interstate Route 95 between Richmond, Va. and Washington, D.C., is on the south bank of the Rappahannock River. Horsepen Run lies upstream of Fredericksburg on the north bank of the Rappahannock River. The mouth of Horsepen Run is located approximately 4.5 river miles (7.2 km) below the confluence of the Rappahannock River and the Rapidan River.

Horsepen Run is shown on the U.S.G.S. 7.5 minute Salem Church topographic map. The local topography consists primarily of moderate to narrow ridges incised by first to fourth order streams. The slopes are commonly very steep with rock outcrops. Floodplains are frequently pinched by steep valleys on one or both sides of the streams. In the upper end of the basin, the topography adjacent to Va. State Route 17 is rolling and incised by streams. The topography reflects the underlying igneous and metamorphic rocks of the Piedmont physiographic province.

The climate of the region is humid temperate. The average annual temperature is around 57 O F(13.9 O C) and the average annual precipitation is about 39.5 inches (100.3 cm). This climate supports an oak-hickory forest with some pines. With the exception of the land adjacent to Route 17, most of the drainage basin is forested. Forest cover has been removed for some agricultural and residential uses. There are a few minor commercial uses adjacent to Route 17.

The area of study is limited to a portion of the Horsepen Run drainage basin (figure 1). The study area begins at the mouth of Horsepen Run and includes the area adjacent to the 1Figure 1: Horsepen Run Drainage Basin, Stafford County, Virginia.

stream for approximately 1.25 miles (2 km). Portions of two tributaries of Horsepen Run were included in the study area. One of these is located near the mouth and the other is located upstream.

The area of study was determined from field observations of the stream characteristics. The lowest portion of the stream is steep and cuts through bedrock. Upstream, Horsepen Run flattens and exhibits the characteristics of an alluvial stream. This modification is observed within the study area.

Results (Back to Top)

Field observations of Horsepen Run began a day after a large discharge. This was one of several large storm events occurring between January and March of 1998. The Rappahannock River was colored brown from the sediment that it carried. The surface water in the mouth of Horsepen Run was marked by two areas of color (figure 2). Brown

2

Figure 2

water from the Rappahannock River is distinct from the green water flowing from Horsepen Run. This tributary carries little suspended sediment, but washes algae, sand and gravel from upstream.

Above the mouth of the stream, large boulders, sand and gravel cover the bed and banks. The boulders contain large crystals of potassium feldspar and quartz. Quartz veins are common in the rocks. The sand deposits contains quartz, biotite, muscovite and some accessory minerals. Steep valley walls with rock outcrops surround both sides of the stream. Trees and underbrush stabilize the slopes and reduce the erosion. Debris from trees washed down the stream is deposited on the rocks and trapped by standing vegetation.

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Figure 3

Further upstream, the slope of Horsepen Run steepens considerably. There is little, if any, floodplain along this portion of the stream. Large boulders line the bed, bank and valley walls. Joint sets are visible and indicate that the stream bed runs parallel to two of these planes. This structure creates a staircase of small falls and pools in the stream (figure 3). Some of the pools appear to be about five feet (1.5 m) deep. Sand and gravel deposits are found between the boulders. Gneiss is exposed in this portion of the stream (figure 4).

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Figure 4

Approximately 1800 feet (~550 m) above the mouth of Horsepen Run, the slope flattens and a floodplain, approximately 100 feet (30.5 m) wide, is present on both sides of the stream. The floodplain appears to have been recently disturbed from overbank flow. The stream is approximately 20 feet (6 m) wide, relatively straight and somewhat shallow in this area (figure 5). The bed of the stream is bedrock, with outcrops of gneiss visible. The banks are eroded, sometimes undercut, and commonly 2 to 3 feet (0.6 to 0.9 m) above the water surface. Sand and gravel deposits are present along the banks.

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Figure 5

Route 655 crosses Horsepen Run about 2670 feet (814.8 m) above its mouth. Two corrugated metal culverts approximately 7 feet (2.1 m) in diameter discharge the flow from upstream. The water level on the upstream side of the culverts is slightly higher than downstream. The stream flows to the culverts at nearly a 45-degree angle and a portion of the stream bank runs almost parallel to the road. Coarse gravel lines the streambed. Sand and gravel is deposited on the lower banks and point bars. These deposits include quartz, some muscovite, biotite, gneiss and mica schist.

Further upstream, the banks are higher and more eroded. The stream becomes slightly more sinuous with sand and gravel bar deposits along some portions of the bank (figure 6). Some of the eroded banks have a thick layer of gray clay about 1.5 feet (0.5 m) below the bank's surface. Springs flow over this clay surface and discharge into the stream. No outcrops of gneiss were observed along this part of Horsepen Run.

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Figure 6

Outcrops of micaceous schist appear further upstream. The stream pattern of Horsepen Run becomes highly variable in this area. Where schist outcrops appear, the banks frequently are steep. In one part of the stream, the bank is a schist outcrop with a nearly vertical cliff rising above the channel. The other side of the channel has a steep eroded bank that is 4 to 5 feet (1.2 to 1.5 m) above the water surface and wide floodplain adjacent to it. In other stretches, the stream meanders and fining upward sand and gravel point bars form. The floodplain is wide and has standing backwater. Abandoned channels and gravel deposits are present on the floodplain, as well as tree and underbrush debris. A recently active avulsion channel cuts across a portion of the floodplain. Sand deposits in this cutoff channel contain a higher percentage of quartz than the sand downstream.

Horsepen Run commonly has a relatively straight pattern before it begins to meander. In these areas, the bed is covered with gravel. Large gravel bars extend from the bank into the center of the stream (figure 7).

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Figure 7

The field observations stopped at the second road crossing on Horsepen Run. Three corrugated metal culverts, approximately the same diameter as the culverts downstream, discharge the upstream water into the study area. The tributaries that were included in the study generally mimicked the local characteristics exhibited by Horsepen Run. The western tributary, which is on the upstream end of the study area, tends to meander more and carries less bedload than the Horsepen Run. The eastern tributary, which empties near the

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Figure 8

mouth of Horsepen Run, is steeper than Horsepen Run (figure 8). It has several falls and pools where bedrock is exposed. Some banks are high, steep and very eroded. Shallow soils and rock outcrops contribute to the toppling of trees that grow along the tributary.

Geologic studies of the area (Pavlides 1980) (figure 9) indicate that the mouth of Horsepen Run is located on a mapped unit of the Falmouth Intrusive Suite. This unit contains relatively young granitoid dikes that have been folded and crosscut. The unit includes pegmatoid dikes of muscovite-quartz-feldspar.

9Figure 9: Geologic Unit Overlay of Horsepen Run Drainage Basin

Upstream of the mouth, Horsepen Run runs through the Holly Corner Gneiss, which is believed to be Cambrian rock. The gneiss unit extends upstream to approximately 3800 linear feet (1.1 km) above the mouth of Horsepen Run. An upright syncline is mapped striking across the gneiss, approximately perpendicular to the course of Horsepen Run. Above the gneiss, the Quantico Formation is mapped. This unit is often referred to as the Quantico Slate. The metamorphic grade is higher in this area and produces micaceous schists. Garnet, kyanite and sillimanite are also found locally. The Quantico Formation includes lenses of quartzite. (Pavlides 1980)

Geologic maps and the Stafford County soil survey show that upland Coastal Plain sediments are located on the ridges above Horsepen Run. These sediments have been eroded and are transported from the stream valley. Early studies (Wentworth 1930) indicate that these sediments may be the remanents of alluvial fans. Later work (Pavlides 1980) indicates that these Coastal Plain uplands are terrace, alluvium and swamp deposits.

The soil survey descriptions, verified by soil samples taken in the field, indicate that the soils are predominantly fine to medium sandy loams. The major difference between the soils in different locations is the amount of clay present.

Interpretation and Discussion (Back to Top)

Horsepen Run exhibits characteristics that, initially, seem easy to explain. A conclusion can be made that, due to the underlying geology, Horsepen Run changes from a bedrock stream to a alluvial stream. Schists are easier to erode, so the stream can modify itself laterally and vertically as necessary upstream. For the portion of Horsepen Run attempting to incise through the gneiss bedrock, this work is much more difficult. A narrower channel and steep valley walls result from this slow process.

If the underlying geology is the explanation for the stream characteristics, some questions arise. Why are some of the schist banks and valley walls still so steep? Some of these slopes are the steepest in the study area. Why is a portion of Horsepen Run relatively flat with a floodplain, while it is cutting through the gneiss bedrock? Why doesn't it behave the same as the area just downstream?

This study did not result in answers to these questions. More fieldwork than what was attempted for this study may provide the answers. We propose some possible explanations for the characteristics that were observed in the field.

The underlying geology does have a major influence on stream characteristics. Horsepen Run and its tributaries have followed fractures and joints in the rocks. Meanders occur where the stream finds a barrier of resistant rock and attempts to flow around it. The gneiss, granitoid dikes and quartz veins are resistant to physical and chemical weathering. Some of the steep slopes and banks that are located in the mapped schist units may have local areas of quartzite, which is more resistant to weathering. Some of the characteristics upstream may be influenced by the presence of clay layers along the stream. Clay acts as a cohesive medium that is more difficult to erode. Other geologic structure may have influenced the steepened slope near the mouth of the stream. This portion of Horsepen Run runs parallel to a dipping limb of an upright syncline.

Vegetation plays an important role in this area. Vegetation reduces erosion and the quantity of sediment transported to the stream. Additionally, trees growing on shallow bedrock have toppled over and create small dams across the streams. These dams collect sediments being transported downstream, causing the stream to aggrade locally. The response of the stream is to seek a more efficient slope by migrating.

Closely tied to vegetation, climatic change can influence the characteristics of a stream over time. More or less precipitation in the region affects the hydrology. The amount of water delivered to the stream influences the stream's competence. In the past, Horsepen Run may have been able to more efficiently transport the sand and gravel deposits from the upper reaches of the stream.

Human effects may have strongly influenced the efficiency of moving water and sediments downstream. Road crossings can tend to dam the stream. This is evident at Route 655. The culverts are undersized, which limits the discharge through the pipes. This results in eddies, decreasing velocities and the stream dumping its bedload upstream of the culverts. The stream may be aggrading, changing the local base level and affecting all areas upstream.

There are no mapped faults in the study area. There is no evidence of local uplift to explain why the upstream, flat-sloped area of gneiss has not eroded further into the bedrock and formed features similar to downstream. A possible explanation is that the Rappahannock River has been able to erode its bed and its water level is lower than in the past. This would result in lowering the base level of Horsepen Run. Typical of rivers, the adjustment to a change in base level begins downstream and works upstream. Due to the resistant

rock in the lowest reaches, this adjustment will take much time. In the meantime, Horsepen Run will remain an unstable stream. It will continue to alter its stream pattern and characteristics as it attempts to efficiently transport water and sediment to the Rappahannock River.

No one factor can explain the behavior and characteristics of rivers and streams. Their history and future is a combination of many factors that should be recognized. This point will be particularly important for Horsepen Run in the future. Human impacts on the drainage basin will be amplified by the natural effects that are in progress.

References (Back to Top)

Conley, James F., 1978, Geology of the Piedmont of Virginia - interpretations and problems, Contributions to Virginia Geology - III, Virginia Division of Mineral Resources Publication 74, p. 19-32.

Conley, James F., 1987, Relationships of structure to massive sulfide deposits in the Chopawamsic Formation of central Virginia, Contributions to Virginia Geology - V, Virginia Division of Mineral Resources Publication 74, p. 19-32.

Dietrich, William E., James W, Kinchner, Hiroshi Ikeda and Fujiko Iseya, 1989, Sediment supply and the development of the coarse surface layer in gravel bedded rivers, Nature, v. 340, p. 215-217.

Germanoski, Dru and S.A. Schumm, 1993, Changes in braided river morphology resulting from aggradation and degradation, The Journal of Geology, v. 101, p. 451-466.

Lonsdale, John T., 1927, Geology of the gold-pyrite belt of the northern Piedmont Virginia, Virginia Geological Survey Bulletin 30, 110 p.

Mead, Robert H., 1992, Sources, sinks, and storage of river sediment in the Atlantic drainage of the United States, The Journal of Geology, v. 90, p. 235-252.

Milliman, John D. and James P.M. Syvitski, 1992, Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountain rivers, The Journal of Geology, v, 100, p. 525-544.

Miall, Andrew D., 1992, Alluvial deposits, in Walker, Roger G. and Noel P. James, eds.,

Facies Models: Response to Sea Level Change, Geological Association of Canada, Newfoundland, p. 119-142.

Pavlides, Louis, 1980, Revised nomenclature and stratigraphic relationships of the Fredericksburg Complex and Quantico Formation of the Virginia Piedmont, Geological Survey Professional Paper 1146, 29 p.

Ouchi, Shunji, 1985, Response of alluvial rivers to slow active tectonic movement, Geological Society of America Bulletin, v. 96, p. 504-515.

Schumm, S.A., 1993, River response to baselevel change: implications for sequence stratigraphy, The Journal of Geology, v. 101, p. 279-294.

Southwick, D.L., John C. Reed and R.B. Mixon, 1971, The Chopawamsic Formation - a new stratigraphic unit in the Piedmont of northeastern Virginia, Geological Survey Bulletin 1324-D, 11 p.

United States. Department of Agriculture. Soil Conservation Service in cooperation with Virginia Polytechnic Institute and State University, 1974, Soil survey of Stafford and King George Counties, Virginia, Washington: GPO.

Wentworth, Chester K., 1930, Sand and gravel resources of the Coastal Plain of Virginia, Virginia Geological Survey Bulletin 32, Richmond, 146 p.

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