Study Number: 

3041

Project Title:

The role of headwater streams in hillslope erosion and sediment transport in the valley and ridge and blue ridge provinces.

Investigator(s):

E.F. Benfield  E-Mail | Tel. 540.231.5802 | Biographical Sketch
Rebecca  Kavage  E-Mail | Tel. 540.231.8829
Jim Spotila  E-mail
Affiliated Institution(s): Virginia Tech
Address: Benfield:
Virginia Polytechnic Institute and State University
Department of Biology
Blacksburg, Virginia 24061  USA

Kavage:
4044 Derring Hall
Virginia Polytechnic Institute and State University
Geological Sciences Department
Blacksburg, Virginia 24061 USA
Study Category: Aquatic
Project Type: Type 1
Study Period: 05/2001 - 05/2002
Notes:

 
Funding Source(s):

Virginia Tech Geological Sciences Department.
Abstract: I. Project Scope
The Valley and Ridge is perhaps the case example of how rock type and structure influence topography, with alternating layers of bedrock with varying resistance to erosion forming long ridges and valleys. Classical treatment of the fluvial systems in the Appalachians considers the ridges and valleys to be in equilibrium. Relief and form are essentially governed by lithology, and most trunk streams are at grade with their given rock type (Hack, 1954). What has been overlooked, however, is that trunk streams are only a portion of the story. The majority of the surface is actually drained by non-perennial headwater streams (generally 1st - 3rd order), which control the erosion of the hill slopes. Alternating stratigraphy with mixed resistance underlies these streams, in contrast to the trunk streams, which flow over bedrock with similarly weak resistance. We will examine headwater channels to determine if bedrock resistance controls the relief and topography of the ridges, as opposed to drops in base level or climate change. The entire Valley and Ridge fluvial system works together as a coupling between headwater and trunk streams to transport sediment out of the system. Trunk streams carry out sediment transport in the valleys, with the majority of sediment transported in effective discharge events that recur on average every 1.5 years (Wolman and Miller, 1960) and shape the channel. However, headwater streams, which are the pathway for sediment transport from the hill slopes, may require more extreme and infrequent precipitation events to generate effective discharge and channel forming flows. We will determine what precipitation events are important for sediment transport in headwater channels, as well as controls on sediment transport and storage. Holocene climate change and recent land use may cause stream channel changes, if the channel crosses a geomorphic threshold as boundary conditions change. Bull (1979) hypothesized that thresholds of stream power for erosion or sedimentation are important for describing ephemeral channel behavior. We will examine the sensitivity of headwater channels to such boundary changes as climate and woody debris removal and attempt to predict how these changes affect headwater processes and hillslope erosion. For example, it has been suggested that thick aggradational sequences in valleys represent a previous climate in which hillslope erosion by ephemeral channels was more effective than trunk streams ability to transport sediment. By studying how channels function today and comparing this to the Holocene record of sedimentation events, we may be able to get a sense of how ephemeral channels respond to changes in climate. The sensitivity of channels to thresholds will also be studied by looking at the effects of recent modifications to the system (i.e. effect of logging).

II. Objectives:
1. Determine controls on headwater channel profiles and geometry and how they vary with the resistance of underlying bedrock.
2. Determine recurrence intervals of effective discharge events in headwater channels and how they vary with drainage area and bedrock.
3. Determine the sensitivity of headwater channels to changes in boundary conditions such as logging (woody debris removal) and climate change.

III. Our Approach
A) Collect a Project that enables comparison of channel geometry (cross section, long profile, etc.) and substrate to bedrock. This is easy in the Valley and Ridge, where channels cross sections of resistant and weak Paleozoic strata at different orientations. Very resistant layers hold up most ridges, but hill slopes between these ridges and the trunk streams may consist of a wide range of lithologies. This creates an opportunity to explore the equilibrium hypotheses. Headwater channels should be graded to all variations in bedrock along their entire reach. If not, then perhaps bedrock is not the most important thing.
B) Develop an understanding of ephemeral channel function. What is effective discharge and what is the recurrence interval of such events? To get at this we have made some real-time observations of large precipitation events in Allen Hollow, and will continue to monitor this site. We will also do simple modeling of rainfall/runoff routing, discharge and velocity, and effect on sediment entrainment, to see what events are really important for erosion and sediment transport in these channels. Some side questions we may address are what controls channel initiation, what the role of groundwater flow may be (qualitatively), what the effect of woody debris is on sediment storage, etc.
C) Investigate the sensitivity of ephemeral channels to threshold changes. To get at this, we will study channel geometry and substrate in areas of different land use (but similar bedrock, relief, aspect, and so on) to see if minor variations in things like the presence of woody debris can actually change the channel significantly. This comparative study is possible because of sites of known 20th century land use and logging history. A second way we can address this is by comparing short term to long term erosion and sedimentation; by excavating aggradational fans at the base of these channels (such as Allen Hollow) and getting rates of sedimentation and frequency of events by radiocarbon dating. This gets at the question of variations during the Holocene due to climate change.
Location(s), Described:  Watersheds 10 and 34 - length of both streams from weir to channel head.
Location(s), Download GPS: ArcView Shape Files (shp.):  UTM, NAD83, Zone 17
Location(s), Online Map(s):
Watershed Map
Methods/Experimental Design: Survey of stream morphology in watersheds 10 and 34, including distance-elevation measurements the length of the stream (longitudinal profile), general classification of channel substrate, survey of log jams, and characterization of bedrock outcrops. At 3-5 points in the channel, channel cross-sections will be taken and pebble counts performed.
Sampling Frequency: One time sampling for 7-14 days.
Data Columns: None submitted by researchers.
Publications:

Geological Sciences Student Research Symposium, 2001, Virginia Tech:
The Morphology Of A Valley And Ridge Ephemeral Stream Channel In Devonian Shale And Sandstone, Brush Mountain, Virginia
Kavage, Rebecca H., Dept. of Geological Sciences, Virginia Tech, Blacksburg, VA 24061.

Adams, Rebecca Hope Kavage. 2002. The Form and Function of Headwater Streams Based on Field and Modeling Investigations in the southern Appalachian Mountains. MSc Thesis. Virginia Polytechnic Institute and State University
Data Restrictions: Users must adhere to the Coweeta LTER Data Policy.
Metadata: EML Format (XML Schema) | Information about EML
Data Downloads: Download thesis (Serves as data for this project.)