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Digging Deeper: What is Dendrogeomorphology?

Note: This is the first part in a series in dendrogeomorphology.

The rate at which streambanks are eroding, and the total annual quantity of sediment being entrained from those eroding streambanks are of a primary concern for ecological, water quality, and sediment studies.  

The U.S. Environmental Protection Agency recently determined that streambank erosion constitutes as high as 90 percent of the annual sediment yield found in rivers.

With strong regulatory pressure to reduce sediment in our nation’s rivers, as well as associated pollutants such as bacteria that may be bound-up in the sediments, the industry is in need of a method to quickly calculate erosion rates at specific locations.

While methods such as bank pins and direct measurement provide accurate tracking of erosion rates over time, they require several years’ worth of data to provide meaningful results, which limits the ability of local agencies, governments and other groups to understand local erosion rates. 

Freese and Nichols ecological engineer Bryan Dick along with colleague Ian Jewell, have developed a method using the exposed roots of trees along riverbanks to provide an accurate and quick method of determining local erosion.

What exactly is dendrogeomorphology? 

By using the tree rings of a root sample, it is possible to identify dates of change in land surface along riverbanks, hillsides, lakeshores etc. Tree anatomy changes in response to environmental factors such as being buried in the ground, or exposed to the elements. 

When a root becomes exposed on a riverbank, the change in anatomy (cells, vessels, etc.) is reflected in the growth rings of the tree. This change can be observed by taking an exposed root sample from a streambank or hillslope and examining a cross-section of the root. Within the cross-section, an anatomical change can be seen (such as the one shown at right) indicating the year in which the root became exposed. From there, by taking the distance away from the riverbank to the exposed root and dividing it by the number of years of exposure (growth rings on the root sample starting at the indicator and continuing outward), the annual erosion rate is determined. With this basic approach, any number of samples of exposed roots can be collected along a stream or river reach and used to determine local erosion rates.

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