MEASUREMENTS OF SEDIMENT THICKNESS, SEDIMENT CHEMISTRY, BATHYMETRY, AND HISTORICAL CONDITIONS AT THE WATERTOWN DAM IMPOUNDMENT IN THE CHARLES RIVER, MASSACHUSETTS

The Watertown Dam was first constructed on the Charles River in 1632, and has supplied energy for a variety of mills, while also serving as a fishing weir and a flood control measure. Today, the dam is being considered for removal, as it serves no productive purpose. The dam restricts the range of local fish populations and disrupts natural sediment flow, altering the entire riparian corridor. The goal of this project is to study historical evolution of the area and the sediment accumulated upstream of the dam because bathymetric changes and erosion are expected if the dam is removed in the future. Results will provide decision makers with data relevant to dam removal scenarios. One aspect of the project includes mapping the thickness of the impounded sediment beneath the riverbed and floodplain using ground penetrating radar (GPR). We use these surveys, coupled with depth to refusal measurements using a rebar pole to generate a model of the sediment stratigraphy upstream of the dam. Sediment analysis includes sampling the riverbed and riverbanks to test for grain size, bulk density, and organic content. We use a high-precision global positioning system (GPS) instrument to record coordinates and water-level elevation data at sediment sampling locations. We map the impoundment bathymetry using a depth finder attached to a canoe, and a hand-held GPS device. These data will be used to make a bathymetric map in ArcMap, which will then be merged with LIDAR elevation data of surrounding topography. Preliminary results show water depth up to 1.67 m in the impoundment. Sediment samples reveal a wide range of bed sediment grain size; the left bank has more silt and clay, whereas multiple samples along the right bank have been so coarse that we are only able to sample cobbles. Aside from field work, historical maps and aerial photographs dating from 1855 to the present will be compiled, georeferenced, and compared using ArcMap to gain insight into evolution of the landscape surrounding the dam. Preliminary results show artificial landfilling, channel narrowing and possibly increased riparian vegetation upstream due to impounded sediment.