Skip to main content.

First Order Stream Hydrology

Joshua Hurley-Bruno, Junior Environmental Science Major

Ryan Anderson, Junior Environmental Science Major

Marne Dantone , Senior Geography Major

 

Introduction

Study Area

Results

Interpretation

Sources

 

Introduction (Back to Top)

When a precipitation event occurs, water flows on top of or within the soil until it reaches a downhill waterway. Our study along the Rappahannock River focuses on a small drainage basin with lush vegetation and two drastically different areas of terrain. We examined hydrologic processes and the origin of water flow occurring from a precipitation event.

Study Area (Back to Top)

The study area is a roughly 1/4 square mile of densely vegetated forest on Embrey hill. It is located 1/2 mile down River Road along the banks of the Rappahannock River in Fredericksburg, Virginia. Fredericksburg is a quickly expanding city located on the fall line between the inland Piedmont country, and the coastal plain landscape to the east. The landscape is covered in dense brush, with scattered trees throughout. Two locations of first order stream origins exist within this basin. The streams then confluence into a second order stream which joins the Rappahannock. The areas of highland topography consist of rolling hills which then drop sharply down a bedrock cliff and lead to a narrow and flat section of the floodplain.

Results (Back to Top)

Embrey hill is heavily buffered with vegetation. This foliage slows surface runoff, increases infiltration, provides soil stability, and aids in providing wildlife habitats. Both stream origins were densely covered with small shrubs and interspersed large tree growth. This is a representative landscape for a humid deciduous forest with four seasons.

The main hydrological process which is occurring in the area is shallow subsurface stormflow. For this type of phenomena to occur, several conditions need to exist. The top layer of soil must be permeable, unlike asphalt or concrete. Permeability is also aided greatly by vegetation that helps infiltration and stabilization of soils. Under the permeable top layer of soil is a less permeable or impermeable layer such as bedrock or clay. This impermeable layer slows or stops infiltration, forcing water to flow laterally through the soil. Flow continues on top of the barrier until reaching a hillslope where the impermeable layer reaches the surface.

This spot on the hillside is an area where spring sapping will occur. Spring sapping is a method of channel initiation in which water seeps from hillsides and flows over land in a stream. Spring sapping forms a bulbous headcut by causing slope failure in a positive feedback loop with environmental factors, which results in the headward growth of the headcut.

Hillslope Profile #1

The first headcut (Profile 1) lies topographically uphill from the steep bedrock cliffs. The headcut is wider than it is deep, and it resembles the shapes of a light bulb. At the top of the headcut there is an oak tree whose root system is extensive. The roots of the oak tree (Figure 1) have been exposed downslope due to the headward progression of slope

Root System Photo

Figure 1: Root Systems Exposed at Headward Progression

failure. In this instance water after infiltrating through the soil, reaches an impermeable clay layer. The water then flows laterally on top of the clay until reaching a hillslope where spring sapping occurs.

The second area of study (Profile 2) is a small drainage basin occupied by three separate headcuts that conduct water from three small drainage basins. The largest headcut is approximately 4.3 meters across and 8 meters long. All three headcuts have a layer of

Hillslope Profile #2

small grain soils on top of a gravel layer that lies over bedrock. As precipitation falls in this area, it strikes vegetation and is slowed by the litter layer. Water then infiltrates into the soil until striking a semi-permeable clay membrane. A small percentage of water flows on top of the clay layer until it seeps from the hillside forming a headcut and stream source. The remainder of the water continues infiltrating through the clay layer until it joins the local water table on top of bedrock.

Interpretation (Back to Top)

Precipitation in this area is infiltrating into soils and flowing through them. When a precipitation event occurs water takes a longer time to reach major channels, because water flowing within the soil flows relatively slow. In a situation where Hortonian overland flow occurs, water flows directly over land, and quickly reaches channels. The high velocity of flow over unvegetated land erodes surfaces and produces high sediment yields. As water reaches channels more quickly, flash floods have a higher probability of occurring. Flooding is far less drastic if there is sufficient storage in the soils, i.e. vegetation, and undisturbed floodplain.

This study has shown shallow subsurface runoff and channel initiation by spring sapping to be the dominant processes on Embrey Hill. Furthermore, our study revealed the importance of vegetation in basin morphology. Well vegetated and relatively undisturbed landscapes, slow surface runoff to a near minimum and therefore help greatly in reducing flash floods, erosion, and sedimentation. In the past several decades, Fredericksburg has been expanding rapidly, consuming natural vegetation and replacing it with concrete, asphalt, and agricultural land. These land use patterns lead to an increase in Hortanian Overland flow and a decrease in infiltration. If expansion continues without modification of human impacts, then we are in store for a dangerous predicament with rising waters and higher sediment levels.

Sources (Back to Top)

Baker, Victor. Spring sapping and valley network development. Geological Society of America , v. 252, p. 235-265.

Dunne, Thomas. Formation and controls of channel networks. Geophysical Monograph , v.89, p. 27.

Dunne, Thomas. Geomorphic experiments on hillslopes. Zeitschrift fuer Geomorphologie , v. 35, p. 40-49.

Montgomery, David. Channel initiation and problems of landscape scale. Science , v. 255, n. 5046, p. 826-830.

Robb, James. Groundwater discharge and spring sapping on the Eastern United States Continental slope. Geological Society of America , v. 16, n. 6, p. 636.

Robb, James. Spring sapping on the lower continental slope, offshore New Jersey. Geology , v.12, n.5, p. 278-282.

(Back to Top)