LEGACY SEDIMENT STORAGE IN THE UPPER CHARLES WATERSHED, MASSACHUSETTS

As colonial settlements expanded and developed, many rivers in in the northeastern U.S. were dammed for milling, and forested areas were converted to farmland. Working in the Mid-Atlantic Piedmont, previous researchers have shown that this history caused large amounts of sediment to be stored in river valleys region. We hypothesize that our study site, the upper Charles River watershed in eastern Massachusetts, despite having similar rates of damming and land use change, has less historic sedimentation due to the lower sediment supply and alternative storage locations such as natural lakes. In order to test this hypothesis, we identified the location of historic dams and millponds using historic maps from the 1850s and used a lidar digital elevation model to map possible terraces associated with legacy sediment deposits. We then field checked each dam site in order to verify the presence or absence of legacy sediment. The 171 km² watershed includes 14 breached historic dams with 2 that show evidence for legacy sediment storage. Calibrated radiocarbon dates from a channel bank outcrop included a pre-colonial age (cal AD 1288-1391) at a depth of 1.8 m in a layer of gray coarse sand and gravel with organic-rich lenses. A second sample at 0.8 m depth in a unit of mottled tan fine sand and silt interpreted to be millpond sediment yielded a modern age. Most breached dam sites still exert base level control as the dam is either only partially removed or beaver dams have been developed in their place. This prevents the river from incising into any lower-elevation legacy sediment that may be there. Surficial geologic maps suggest that directly upstream of the sites with interpreted legacy sediment there are thick glacial till deposits offering a direct supply of sediment to the former millponds. These preliminary findings support the hypothesis that legacy millpond sediment storage is uncommon and localized in the upper Charles River watershed.