MEASURING SEDIMENTATION RATES AND LAND-USE CHANGE IN A DAM-INFLUENCED LAKE DELTA: NARRAGUAGUS RIVER, MAINE

We investigate deposits of the Narraguagus River in coastal Maine to determine sediment transport rates during the past ~150 years, a period of extensive logging in the watershed. Such transport rates will provide a baseline for studies of interactions between substrate mobility and habitat characteristics. Our study area is the inlet of Beddington Lake, which has undergone changes in its surface level due to the construction (circa 1850) and subsequent failure in 1951 of a dam installed at the lake outlet to facilitate log drives on the Narraguagus River. Based on historic (1946-1996) aerial photograph analysis, we identify exposed river deposits from the period of elevated lake level. In August 2007, we sampled these subaerial deposits using soil pits, hand auger cores, bailer-borings, and ground penetrating radar (GPR). In-channel pebble counts were performed at several locations at and upstream of the study area. Overall, sediment samples collected from the subaerial deposit show a fining trend downstream from the Narraguagus River mouth into Beddington Lake. Upstream sediment pits and cores in the deposit show a gravel layer at ~1 m depth. We hypothesize that this layer corresponds to river deposition prior to dam construction. Above this gravel layer, we find a coarsening upward trend in soil pits and cores, from mud to coarse sand, which is representative of a prograding lake delta. This sediment package is ~0.98 m thick, and we believe that its deposition occurred during the ~101 years that the dam was present on Beddington Lake. We calculate an average vertical sedimentation rate of 1.0 cm/year at this proximal position in the lake delta during the time the dam was operational. Above this coarsening upward sequence, soil pits and cores have a topmost layer of fine sand and mud, ~0.22 m thick. Given that Beddington Lake has not been dammed since 1951, we calculate a current average vertical floodplain sedimentation rate of 0.4 cm/year. Organics such as wood chips and fragments of cut logs are found in the subaerial deposit. The concentration of these organics increases with depth, suggesting that logging activities in the watershed were most vigorous in the time soon after dam construction. Ongoing research includes interpretation of GPR data, textural sediment analyses, and correlation of sediment pits and cores.