TIMING OF SLUMPING DETERMINED FROM GROWTH ASYMMETRY IN TSUGA CANADENSIS, MOHAWK RIVER WATERSHED, NY

Slumping and downslope mass movement introduces sediment to rivers and streams, and therefore the occurrence and frequency of these events affects sediment transport and siltation in the watershed. We have used the tilting and subsequent annual ring asymmetry in Tsuga canadensis to determine the timing of ground movement of three slumps developed in glacial till in two small tributaries in the Mohawk River watershed of eastern NY. All three slumps show a clear and significant ring asymmetry with reaction wood growth likely related to tilting and slip between 1972-77, which coincides with an extremely wet interval well documented in local river discharge records. In the Plotterkill Creek, a main slide (PKm) has active and continuous flows over a large fraction of its surface, and a small slump occurs immediately to the south, and is unconnected with the main feature (PKs). The small PKs initiated in 1943. The larger PKm appears to have been initiated in 1955, but experienced significant movement in 1972. Since that time, there has been nearly continuous movement of small surface flows. The Bowman Creek slump (Bw) is a large, slow-moving slide that has three prominent slip planes that have clear surface expression. Slip in July 2008 of the lowest part (c. 10% of the total area) resulted in exposure of a 1-2 m scarp and partial blockage of the creek. This is the latest movement on what appears to be a complex history over over the last 125 yr. The most significant tilting on the hillslope appears to have been between 1941-45, and the onset of slip might have been c. 1886-89, or earlier. The uppermost trees cored on the Bowman slide show pronounced eccentricity in the past few years. This finding, combined with the higher than normal precipitation during this time, suggests we are entering a new period of enhanced slope instability.