Thesis advisor: Noah P. Snyder / Centuries-long intensive land use change in the northeastern U.S. provides the opportunity to study the response timescale of geomorphic processes to anthropogenic perturbations. In this region, deforestation and the construction of dams following European settlement drastically altered the landscape, leading to the impoundment of sediment in mill ponds. This legacy sediment continues to be released into transport decades after a dam has been removed or breached. Geochemical tracers can help distinguish sediment sources and understand how sediment moves through a watershed. The South River in western MA is located in a formerly glaciated watershed, and these surficial deposits compose 98% of the area. It experienced two generations of damming, beginning with smaller mill dams in the 18th-19th centuries, followed by the construction of the Conway Electric Dam (CED), a 17 m tall hydroelectric dam in the early 20th century. Legacy sediment deposits from sediment stored behind mill dams cover 1.5% of the watershed area. The CED is located near the outlet of the river, providing a century-long depositional record for the watershed, during reforestation. I hypothesize that sediment mobilized from human activity will contain a different geochemical signature than glacial material, that recent erosion in the watershed is primarily from anthropogenic legacy deposits rather than from glacial age landforms, and channel widening is occurring in reaches of the channel composed of legacy sediment, rather than in glacially confined reaches. These hypotheses were tested through a two part investigation, consisting of a sediment tracing study using Hg, and a Geographic Information Systems (GIS) analysis of channel changes using aerial photographs from 1940 and 2014. Samples were collected from river bank exposures of 11 glacial deposits and four mill pond legacy sites. Two vibracores measuring 476 and 500 cm were collected in reservoir sediment stored behind the CED in 2013 and 2017, respectively. Hg concentrations range from 1-4 ppb in glacial sediment, 3-380 ppb in legacy sediment, and 2-18 ppb and 7-50 ppb in the two CED cores. I used Hg as a tracer to estimate percent contributions to the CED reservoir from each watershed source during the 20th century. Results from a sediment mixing model suggest glacial sources contributed 32 ± 15%, and legacy sediment deposits contributed 68 ± 15% during the 20th century. Based on 137Cs dates on the cores, high amounts of legacy sediment filled in behind the CED prior to 1953 (74 ± 35 %), and background erosion from glacial deposits dominated from 1953 until the reservoir was filled in the 1980s (63 ± 14%). GIS analyses using aerial photographs from 1940 and 2014 indicate that the channel did not significantly widen along any section of the river, however, increases in sinuosity (up to 12%) occurred in the legacy sediment dominated reaches of the channel, and minor increases (1-2%) occurred in the glacial reaches. Overall, these analyses show an increase in the amount of sediment released in the channel as a result of mill dams breaching through the mid-19th to early 20th centuries, and suggest a short recovery timescale response from this land-use change. / Thesis (MS) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thesis advisor: Noah P. Snyder / 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. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thesis advisor: Noah Snyder / While it has been shown that extensive sedimentation in historic millponds has greatly affected streams in the Mid-Atlantic Piedmont region (Walter and Merritts, 2008), much less is known about the phenomenon in the heavily dammed areas of post-glacial New England. Some research has found similar deposits behind breached historic dams in the Sheepscot River watershed in mid-coast Maine, but at a smaller scale than those seen in the Mid-Atlantic region (Strouse, 2013; Hopkins, 2014). I attempt to further explore millpond sedimentation in New England by quantifying the volume of millpond sediment, also called legacy sediment, in the 171.3 km² upper Charles River watershed in eastern Massachusetts. Twenty three milldams were located in the watershed on 1850s maps, giving a damming density of 0.177 dams/km². Each historic dam that had since breached, 14 in total, was visited in the field to identify possible legacy sediment deposits. Legacy sediments were identified by their meter or higher terraces made of fine sands and silt and verified by comparison to sedimentary patterns found in other legacy sediment deposits and radiocarbon dating of material both within the legacy sediment and in the underlying layer. Legacy sediment terraces with an area of 1.68*10⁴ m² and a total volume of 1.29 - 2.57*10⁴ m³ were found upstream of two adjacent breached historic dam sites on the Charles River in Medway, MA. Radiocarbon dates from a coarse sand and gravel lower at 1.8 m depth returned pre-settlement dates of 1281-1391 cal AD (two σ). These dams were immediately downstream of a large glacial feature with steep banks along the river. The lack of legacy sediment at other dam sites and the lack of sedimentation behind intact dams suggest that a low sediment supply to millponds prevented legacy sediment deposits from forming in most of the watershed. / Thesis (BS) — Boston College, 2016. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Earth and Environmental Sciences.

In the last few centuries, many streams in the eastern United States have been severely disturbed by land use change and are now disconnected from their original floodplain due to the aggradation of legacy sediment. Currently, stream-floodplain reconnection is advocated as a stream restoration practice to take advantage of ecosystem services. The objective of this study is to compare two current stream restoration approaches for their nutrient flushing ability: 1) a conventional approach leaves legacy sediment on the floodplain; and 2) an ecological approach that involves removing the accumulated legacy sediment in order to restore the original floodplain surface wetland, revealing a buried A soil horizon. Soil cores were taken from the surficial legacy sediment layer and the buried A soil horizon in the floodplain of a 550-meter reach of Stroubles Creek in the Valley and Ridge province near Blacksburg, VA, to evaluate potential for flushable DOC, TDN, NO3-, NH4+, and SRP content. In addition, an inundation model was developed to evaluate the extent of flooding under the two restoration scenarios. The inundation model results and nutrient flushability levels were then used to simulate the release of nutrients as a function of stream restoration approach. Results indicate that the buried A horizon contained less flushable nutrients, but the ecological restoration would have a higher frequency of inundation that allows for more flushable nutrient release at the annual scale. Understanding the nutrient release potential from the floodplain will provide the ability to estimate net nutrient retention in different stream-floodplain reconnection strategies. / Master of Science

Understanding the effects of land use on riparian systems is dependent upon the development of methodologies to recognize changes in sedimentation related to shifts in land use. Land use trends in southern New England consist of shifts from forested precolonial conditions, to colonial and agrarian land uses, and toward modern industrial-urban landscapes. The goals of this study were to develop a set of stratigraphic indices that reflect these land use periods and to illustrate their applications. Twenty-four riparian sites from first-and second-order watersheds were chosen for study. Soil morphological features, such as buried surface horizons (layers), were useful to identify periods of watershed instability. The presence of human artifacts and increases in heavy metal concentration above background levels, were also effective indicators of industrial-urban land use periods. Increases and peak abundance of non-arboreal weed pollen (Ambrosia) were identified as stratigraphic markers indicative of agricultural land uses. Twelve 14C dates from riparian soils indicated that the rise in non-arboreal pollen corresponds to the start of regional deforestation (AD 1749 ± 56 cal yr; mean ± 2 SD) and peak non-arboreal pollen concentration corresponds to maximum agricultural land use (AD 1820 ± 51 cal yr). These indices were applied to elucidate the impact of land use on riparian sedimentation and soil carbon (C) dynamics. This analysis indicated that the majority of sediment and soil organic carbon (SOC) stored in regional riparian soils is of postcolonial origins. Mean net sedimentation rates increased ∼100-fold during postcolonial time periods, and net SOC sequestration rates showed an approximate 200-fold increase since precolonial times. These results suggest that headwater riparian zones have acted as an effective sink for alluvial sediment and SOC associated with postcolonial land use.

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