Quantifying milldam legacy sediment storage in valley bottoms of two New England watersheds

Abstract

Large-scale human modification of the northeastern U.S. landscape began in the 17th century with forest clearing and milldam construction. In the mid-Atlantic Piedmont region of the U.S., Walter and Merritts (2008) found that millpond deposits persist for centuries after dam breaching, resulting in fill terraces composed of legacy sediment. Stratigraphic observations in the mid-Atlantic indicate that these laminated to massive fine-grained layers typically overly a prominent Holocene hydric soil that overlies a Pleistocene basal gravel. I test whether this set of processes applies to glaciated New England. This study focuses on two New England watersheds: the South River in Massachusetts and the Sheepscot River in Maine. I use stratigraphic analysis and radiocarbon dating to identify legacy deposits, and then use lidar digital elevation models to map planar terrace extents in each watershed. Finally, I use lidar digital elevation models to estimate thickness of legacy sediment found behind breached or removed milldams and estimate volumes of legacy sediment storage in valley bottoms over entire watersheds. The South River watershed has 32 historic dam sites; 18 have been field checked and 14 show evidence for legacy sediment storage. The Sheepscot River watershed has 33 historic dam sites; 13 have been field checked and six show evidence of legacy sediment storage. Stratigraphic analyses of bank exposures in both watersheds show a brown fine sand and silt layer (up to 2.19 m thick in the South River watershed and up to 2.30 m thick in the Sheepscot River watershed) which sometimes is underlain by gravel and/or clay; no buried Holocene hydric soil has been found. Further evidence for legacy milldam sedimentation comes from radiocarbon dating. Three radiocarbon dates from the South River watershed and six from the Sheepscot River watershed are less than 300 years old; no underlying Holocene material has been dated. The maximum volume of legacy sediment estimated using lidar methods for the South River watershed is 2.5 x 106 m3 and for the Sheepscot River watershed the volume is 3.7 x 106 m3. These volumes of legacy sediment can be translated to maximum mean thickness of sediment eroded from each landscape: 37 mm for the South River watershed and 7 mm for the Sheepscot River watershed. The Sheepscot River watershed has most of its legacy sediment terraces in the lower section of the watershed with many lakes and wetlands disturbing sediment transport in the upper section of the watershed. Compared to the Sheepscot River watershed, the South River watershed has more widespread glacial deposits contributing to legacy sediment with few lakes and wetlands.