Woodland Valley Large Woody Debris Project
Introduction:
Large woody debris is essential to the ecological health of streams and plays an important role in maintaining the long-term stability of the system. It can change the shape of the stream channel, increase erosion or sediment deposition, or change the speed and direction of the flow of water.
Hypothesis:
The persistence, or stability, of large woody debris are subject to three dimensions:
The size of the tree--the bigger the tree the less likely the stream can move it
The size of the river--the smaller the width the greater influence a piece of large woody debris can have on a stream
The presence or absence of a rootwad--contributes to the overall size of the tree, but also contributes to logs “locking” together and forming an accumulation.
Method:
Woodland Creek was selected because its assessment in 2008 was the first the Ashokan watershed to include the locations of large woody debris sites.
At each of the 68 sites, the length and diameter of trees were measured. By collecting this data over several years, we could potentially find which factors are more likely to lead to the stability of large woody debris and in what situations large woody debris becomes problematic.
Map of Large Woody Debris Locations in Woodland Creek
Red = Gone before relocation in Aug 2010.
Yellow = Still there in August 2010, but gone after flood in October 2010.
Green = Still persistent even after October flood!
To see the full Ashokan Watershed Stream Management Program Large Woody Debris Map click here and you will be redirected to GoogleMaps
Results
Observed factors in Woodland Valley:
In 2008, 91 large woody debris sites were recorded. Before the flood in 2010, 68 sites still had some form of large woody debris. After the flood on October 1, 2010, 29 of the 68 sites still had large woody debris present. Those 29 sites can be placed into four categories that can be used to help identify persistent large woody debris sites in the future.
Rootwad
As expected, all of the persistent large woody debris after the flood had a rootwad attached, but the presence of a rootwad alone was not enough to guarantee persistence. Another factor had to be present such as deposition occurring behind the rootwad or roots from the rootwad still partially attached to the bank holding it in place.
Size and Orientation
As expected, pieces of woody debris that were large relative to the stream size correlated with persistence. The following orientations in the stream channel were also associated with stability:
~ Parallel to stream flow
~ Spanning the channel
~ Partially above bankfull
Number of pieces
Accumulations of two or more pieces of large woody debris lead to persistence, but when the accumulation was behind standing live trees or behind a boulder it remained even through flood conditions.
Bank Failure
Bank failures lead to persistent large woody debris locations. The same piece may not remain but additional large woody debris pieces are recruited as the bank continues to fail.
