AT&G
- Spotila's active tectonics and geomorphology research
Behavior of ephemeral streams in humid, moderate-relief
drainage basins
Scope: Fluvial erosion has certainly shaped the
ancient Appalachian Mountains. This is easy to see in the Valley and
Ridge, where a trellis drainage network has shaped the land into alluviated
valleys with long trunk streams and adjacent hillslopes that are eroded by
headwater channels. This drainage pattern is somewhat unique from the
classic, fractal nature of dendritic streams. Although it is well known
that the underlying bedrock resistance of Paleozoic strata of the Valley and
Ridge is responsible for this drainage network and the fabric of the landscape,
it is not clear how
this landscape evolves. Perennial trunk streams are well understood in
terms of what size and frequency of precipitation event shapes the channel, how
the channel responds to perturbations, and so on. Very little is know
about what shapes ephemeral, headwater (<2 sq. km drainage area) streams,
however. What size flood shapes the channel? How do fine scale
variations in bedrock, inherent to stratigraphic packages, shape the
longitudinal profile and cross section of these channels? And ultimately,
are the headwater channels and trunk streams in a dynamic equilibrium, such
that the Valley and Ridge relief is quasi-stable? How does climate change
affect such an equilibrium, if it exists? To address these unknowns, we
have investigated the behavior of headwater streams in the Appalachians from
several perspectives (see Adams and Spotila, 2005).
First, to see what effect bedrock variations have on channel
shape, we looked at 9 headwater, mixed bedrock-alluvial catchments from
different lithology (e.g. carbonate, shale, resistant quartz sandstone) and
structure (e.g. dip-slope, strike-slope, gently-dipping). Included in these were several
catchments in the Blue Ridge, where metamorphic lithologies occur. Field observations demonstrate variation with respect to
slope-area channel initiation, basic morphology, slope distribution, hydraulic
geometry, substrate grain size, and role of woody debris. These channels display only some of the
typical downstream trends expected of larger, lowland rivers. Variations are controlled mainly by
differences in bedrock resistance, from the formation level down to
short-wavelength, outcrop-scale variations. Channels
are dominated by very localized resistant bedrock, which is either quartz-rich
or massive (homogenous, unfractured). These form localized knickpoints
that are responsible for shaping the channel above and below, and essentially
"hold-up" the erosion of the entire catchment.
Second, we explored what effect variable land use may have
played on these catchments. It is simply not possible to avoid
human-induced environmental change in the Appalachians, which have been logged
over nearly their entire extent (more than once!). Sites in North
Carolina, some of which have controlled logging history and one (Joyce Kilmer)
that contains old growth, are serving as a control, to see what effect large
woody debris and other factors related to land use may have on channel
development.
Finally, to understand the function of these streams,
rainfall-runoff modeling of these channels was completed. Hydrologic
modeling on these ungauged channels estimates the recurrence of channel-filling
discharge and its ability to erode the channel bed. Two-year recurrence discharge is generally larger and closer
to bankfull height in the Valley and Ridge, due to low soil infiltration
capacity. Discharge that fills the
channel to its surveyed bankfull form is variable, generally exceeding 2-year
flows at small drainage areas (<0.5 km2) and being exceeded by them at greater drainage
areas. This suggests bankfull is not controlled by the same recurrence storm
throughout a channel or physiographic region. Stream power and relative competence are also variable. These heterogeneities contrast
relations observed in larger streams and illustrate the sensitivity of
headwater channels to local knickpoints of resistant bedrock and armoring of
channels by influx of coarse debris from hillslopes. The general lack of predictable trends or functional relationships
among hydraulic variables and the close coupling of channel form and function
with local boundary conditions indicate that headwater streams pose a
significant challenge to landscape evolution modeling.
Personnel: Rebecca Kavage Adams,
M.S. 2002, collaboration with Gary Kapesser (USFS) and Panos Diplas (VPI).
Funding: USFS Fellowship to Rebecca.
Links: The Coweeta LTER hydrologic laboratory in
North Carolina was one of our field sites. The pristine Joyce Kilmer
forest in North Carolina was another. Many other sites were in the Washington-Jefferson
National Forest of Virginia. The biology department at Virginia Tech also
does related research from an ecological point of view; they are known as the Stream Team.
Below is a headwater channel on the north flank of Brush
Mountain, showing some survey points and painted clasts.
Below is a typical headwater channel in the Valley and Ridge,
near the base of a ridge and confluence with a trunk stream. The channel is incised, which makes
bank full appear large and suggests large, rare bank-full flows.
Bedrock reach along Allen Hollow, produced by a stratigraphic
interval that is slightly more coarse and less erodible than typical shale of
the Braillier Formation.
Below is a headwater stream flowing over gneiss at Indian
Spring, in the Coweeta LTER.
Below is a typical headwater stream in the Blue Ridge of
North Carolina; narrow, dark, and lots of rhododendon.
Below is a log-jam (woody debris) trapping fine shaley
sediment in Allen Hollow, Brush Mountain.
Comments to: spotila@vt.edu