DOCUMENTATION OF LANDSLIDE ACTIVITY ALONG THE COLD RIVER, ACWORTH, NH USING DENDROCHRONOLOGY

Located on the Cold River in Acworth, NH is a 22 meter high, 35°-45° slope experiencing various stages of failure. The slope is composed of 8 meters of compact till overlain by 12 meters of sandy glacial lake sediments. Developed over the glacial sediments is a 10-12 cm soil horizon with rooted hemlock and pine ranging in age from ca. 50-150 years. Slope failure occurs along the contact between the till and glacial lake sediments, and along the interface between the soil and underlying glacial sediments. In 2008 the Town of Acworth clear cut 6.2 acres of land, 44 meters above the slope, and approximately 200 meters east of the active slide in preparation for gravel mining. Increased infiltration and runoff resulting from clear-cutting could increase the flow of water into, out of, and along the slope, further destabilizing it.

This study was initiated to determine if dendrochronology could be used to document and monitor slope failure. Dendrochronology, which uses variations in annual growth ring patterns to record environmental change, has been successfully employed to document slope movement by analyzing the downslope development of tree ring asymmetry. Tilting of a tree upslope or downslope is accompanied by a correlating upslope or downslope thickening in tree rings. Five core samples were taken at a monitored intact slope transect with an increment borer, and two sections were cut from fallen trees at the base of an active landslide.

Tree ring data from toppled trees were compared to regional stream flow data as a proxy for runoff. Several periods of slope activity were recorded, with the most recent occurring in 2005. These events can be related to significant storms and periods of high spring runoff. Cores from trees along the monitored intact transect reflect more stable conditions, as would be expected because the slope has not yet failed, and activity there is currently restricted to basal slump and creep. Dendrochronology will prove useful in documenting accelerated and incremental movement along the monitored section. By setting a base line for slope monitoring, the groundwork has been established for future studies.