Field Research in the Mid-Atlantic Piedmont, 2005-2007

This page is a photo journal of my research into soil transporting processes in the forested hillslopes of NW Delaware and NE Maryland. I have been ably assisted by undergraduates in the Environmental Science program and the Ag School at the University of Delaware. In particular Steve Sciscione used digital elevation data to locate knickpoints in the local streams draining the Piedmont near the Fall Line, and John Doroba explored soil disturbance around tree throws. Both of these bright students have since gone on to much bigger and better things. Long may they run…

Here’s a summary of my work with tree throw profiles and soil transport in a poster I presented at the annual AGU meeting in San Francisco, 2006:
AGU Fall 2006 Poster (6.5 mb, pdf): Pits, Mounds, and Soil Transport on Hillslopes in NW Delaware.

The bottom line: I think I have established a proof of concept in getting at soil transport rates on hillslopes using the decay of pits and mounds left by tree throws. This would facilitate estimates of longer term fluxes of sediment from hillslopes, and hence could play a role in establishing a sediment budget for local drainage basins.

Below you will find a photo record of observations and research into soil transport on hillslopes in the Piedmont of northwest Delaware…

Steve Sciscione lends a scale to this section of regolith and soil overlying bedrock in a cutbank of a tributary to White Clay Creek in Delaware. Steve is just shy of 2 m high. Most regolith in this area is <2 m thick. At right a stream cutbank exposes bedrock and soil. This is typical of the soil mantled landscape in the Piedmont.

Gray scale shaded relief map of the Piedmont in NW Delaware. White Clay Creek Coastal Plain/Piedmont boundary

At left is a typical stream profile in the Piedmont. Arrows mark locations where the stream gradient increases, i.e., at the lower end of knick zones or knickpoints. The exponential trend line is plotted as a reference line to assist in knickpoint identification. Steve Sciscione identified dozens of oversteepened reaches in 13 drainages (2^nd to 3^rd order) in White Clay Creek State Park.

At left, earth worms leave trails in a drying mud puddle. Clearly, there’s a lot of them in this area, and they could be the dominant soil transporter in many cases!

Below, earth worm castings dominate this O soil horizon. In many places, the annual thickness of castings can exceed 2 cm. Earth worm activity, however, is not uniform across a forested landscape.

Frost needles: a highly effective mechanism for eroding a stream cutbank in winter. Ice needles lift soil at a right angle to the ground surface. Upon melting, the soil particles fall according to gravity, often into the creek below, which then carries the sediment away…

At left is a moderately recent tree throw (last 10-20 years??). The mound exhibits a nearly vertical face, so one mechanism of transport off the mound is likely to be slope collapse/failure. Mounds often exhibit rock-capped pinnacles—a clear sign of rain splash and surface runoff erosion. The log shows some signs of age—moss is growing, and bark is missing. I have set up a relative age system for describing log decay: Stage 0—Leaves still on; fell this year Stage 1—No leaves; bark is still solid; small rootlets and branches still present Stage 2—Bark is missing; log is still solid Stage 3—Log is >50% solid Stage 4—Log is < 50% solid Stage 5—Log is covered with decayed wood Stage 6—Log is a mound of duff Stage 7—Log is missing Now we need absolute time for this series…

John Doroba, a former undergraduate Environmental Science major at University of Delaware, exhibits style and balance in taking a core from this tree growing on an old mound. To get at soil transport rates or erosion rates, one needs to establish time and age. For tree throws, one might be able to use a decay rate for logs. This has not yet been done for trees in Delaware (an excellent research project!). One could also establish a minimum age if a tree is growing on a mound, such as at left. We counted ~23 rings for this tree. The junction of the exposed roots is ~100 cm above the mound, yielding a minimum erosion rate of ~4 cm/yr!

John Doroba mapped soil profiles up and down slope from tree throw pits and mounds. Here is a typical profile upslope with O, A, and B Horizons.

Soil profile downslope from a tree throw mound; Here, we see O, A, B, then O/A, then B. This is typical of soil transported downslope from the tree throw.

At left are typical profiles of pits-and-mounds, plotted against qualitative estimates of log decay (Stage 1 is solid log with bark on; Stage 6 log is duff; Stage 7 has no log). The dashed lines represent the pre-throw hillslope profile. Clearly, the mounds are evolving, which implies a non-zero sediment transport rate.

Curved tree trunks are usually signs of soil transport around the growing tree. The tree, perhaps rooted into soil below the creeping surface layer, experiences a rotation, and tries to grow up into the light. Curved trunks are not that common in the Piedmont, but occur primarily near streams with high cutbanks. This leads one to ponder the relationship between stream activity and hillslope sediment transport rates. In the Piedmont, one would expect almost no erosional activity in this old landscape. However, on my numerous hikes through the local forests, I have observed many localized areas of higher erosion rates, usually on the outside banks in a stream close to a valley side slope, and these are well-worth investigating further…

Page set up and maintained by Les Hasbargen: lesh@udel.edu
Last modified December 25, 2007

Les currently teaches in the Earth Sciences Dept., SUNY Oneonta

Les is solely responsible for the content on this page.