DETERMINATION OF STREAM CHANNEL INCISION RATES FROM EXPOSED TREE ROOTS: BLUFF'S PARCEL TRIBUTARY, DE

It is well documented that watershed urbanization often results in stream channel enlargement or incision. This study illustrates how dendrogeomorphic analysis of roots and a single set of field surveys yield a time averaged incision rate for the Bluff’s Parcel Tributary, a small second order stream located in the Coastal Plain of Delaware.

Four trees along the stream channel meet the necessary placement criteria for root sample collection. A combination of macroscopic and microscopic analysis of the cellular structure of the root allows for the determination of the year of exposure. The incision rates vary from 0.019 - 0.022 m/yr. Volumetric estimates of the channel incision indicate that the channel has excavated ~744 cubic meters through incision, contributing an extra 14.88 cubic meters of sediment to Blackbird Creek annually. The incision rates indicate that the channel began incision ~50 years ago. Analysis of land use land cover change through historical aerial photography indicates that urbanization occurred in the uplands of the watershed between 1968 and 1992, which coincides with the onset of incision.

The results of this study illustrate how minor amounts of urbanization in the uplands of a watershed can have significant impacts on small streams. Given the density of similarly sized tributaries undergoing an analogous channel response throughout the Blackbird Creek watershed, the sediment load contribution of these incising tributaries to the main channel is non-trivial. The extra sediment load has the potential to negatively impact ecologically sensitive estuarine tidal environments.

This study also presents a new application of a dendrogeomorphic technique for evaluating time averaged incision rates through a single visit to the field site. The technique thus provides a unique temporal perspective for unmonitored streams, while minimizing field time and expenses.

The ability to unravel the erosional history of a stream channel in a single field day provides a powerful tool for land managers seeking to implement stream restoration plans, as understanding the history of a stream and predicting its future behavior is imperative to the success of environmental or infrastructural restoration efforts.