Thesis advisor: Noah P. Snyder / The Souhegan River is a tributary of the Merrimack River that drains a 443 km<super>2</super> watershed in southern New Hampshire. The lowermost barrier on the Souhegan River, the Merrimack Village Dam (MVD), was demolished and removed in August and September 2008. The modern MVD impoundment contained at least 62,000 m<super>3</super> of sediment, mostly sand. Analysis of topographic and historical maps, and photographs suggests that approximately twice the area of what is now the modern impoundment has been affected by over 200 years of damming at the site. I use repeat surveys of cross sections and the river longitudinal profile, and sediment samples, to document the response of the Souhegan River to the MVD removal. A base level drop of 3.9 m caused immediate incision of the sand-sized sediment and channel widening. The impoundment later segmented into a non-alluvial, bedrock and boulder controlled reach; and a quasi-alluvial sand and gravel reach with erosion and deposition modulated by the presence of vegetation on the channel banks. One year after the removal, the Souhegan River has excavated 38,100 m<super>3</super> (65%) of the sediment in the modern impoundment. The response of the Souhegan River was rapid and the channel and floodplain continue to evolve toward a quasi-equilibrium configuration. Continued response will be substantially influenced by the establishment of vegetation within the former impoundment and the magnitude and frequency of high discharge events. / Thesis (MS) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

Thesis advisor: Noah P. Snyder / Dam removal is typically used to restore fish passage, natural flow regimes, and sediment transport in streams. However, dam removal also impacts the riparian vegetation, a change that can have wider effects throughout the ecosystem. Quantifying vegetation change requires a multi-year record to document pre-removal communities and both the immediate and delayed responses. In this study, vegetation change was assessed at the Merrimack Village Dam on the Souhegan River in Merrimack, NH, which was removed in August 2008. The removal caused a ~3 meter drop in water level and rapid erosion of impounded sediment, with ~50% removed in the first three months. The vegetation was sampled using plots at specific intervals along 7 monumented transects that were perpendicular to the channel or adjacent wetland. Tree, shrub, and herbaceous communities were assessed using species percent areal coverage techniques in July 2007, 2009, 2014 and 2015. Change over time was quantified using Analysis of Similarity (ANOSIM) on the Bray-Curtis dissimilarity matrix. As expected, vegetation communities in control plots upstream of the impoundment did not show significant change during the study period. Tree and shrub communities adjacent to the impoundment also did not show significant change. All herbaceous communities adjacent to the impoundment changed significantly (p < 0.05). The herbaceous plots closest to the channel changed to bare sand in 2009 due to erosion in the former impoundment, but by 2014 the riparian fringe community seen in 2007 had re-established and expanded in this area, but at a lower elevation. Between 2007 and 2014, the wetland herbaceous community changed from aquatic species to a stable terrestrial community that persisted without significant change in 2015. From 2007 to 2014, the vegetation community on a mid-channel island of impoundment sand changed from a community with ~50% invasive reed canary grass to a ~98% community of invasive black swallowwort, a species not recorded at the site pre-removal. The vegetation response was greatest in areas with largest geomorphic and hydrologic change, such as along the channel margin where erosion and bank slumping created an unstable scarp or on the mid-channel island and off-channel wetland strongly impacted by the lowered water table. However, large unvegetated areas never persisted nor did the areal coverage of invasive species expand: two common concerns of dam removals. / Thesis (BS) — Boston College, 2016. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Scholar of the College. / Discipline: Earth and Environmental Sciences.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

Field and modelling investigations of eight failed or removed dams have been undertaken to examine the upstream effects of low head dam decommissioning on channel morphology. Failed or decommissioned sites were selected such that no upstream interventions or channel mitigation had been applied since the time of decommissioning resulting in a physically-based analog consistent with the passive dam removal restoration approach. Field surveys of the sites, which failed between 2 years and 70 years ago, included longitudinal profiles, cross-sections and bed material pavement sampling on each riffle, run, and headcut. Findings demonstrate that vertical disturbances typically in the form of headcuts frequently extend well beyond the backwater limits of most reservoirs. Although in most cases, critical velocity and shear stress thresholds were exceeded, the localized increases in friction slope where headcuts occurred demonstrated that the velocities associated with larger flows exceeded critical thresholds more often than critical shear stress thresholds. Findings show that if the grain size distributions of the underlying alluvial geologic units are close to that of critical velocity thresholds, when headcuts are initiated (with their resulting increase in friction slope), they can result in continued channel degradation upstream of impoundment regions.

This dissertation addresses the relationship between large-scale dam removal and the nearshore ecosystem function for fish. The work is based on almost a decade’s worth of collaborative field work in the nearshore of the largest dam removal in the world recently completed on the Elwha River. The data analyzed span seven years prior to, during, and throughout the first year of each dam removal (January 2008 to November 2015). As of September 2015, approximately 2.6 million m3 of sediment material increased the area of the Elwha delta to over 150 ha. Long term study of fish in the estuary reveals fish community response to dam removal, and indicates likely interactions in the nearshore between hatchery and wild fish, including chum salmon critical to watershed recovery. Continued hatchery releases may therefore further challenge chum salmon recovery, and this interaction should be considered when planning for future watershed recovery. Community analysis revealed that, while species richness and taxonomic diversity do not appear to have a significant response to dam removal, functional diversity in the nearshore does respond significantly to dam removal. Three main shifts occurred in the nearshore: large scale and rapid creation of estuary habitats; delivery of large amounts of sediment to the delta/estuary in a short period of time, and; a shift in original habitats from tidally influenced to non-tidally influenced habitats resulted in changes in estuary function. Changes in functional diversity occur disproportionately in the new sites, which have more unstable, and so less resilient, communities. Functional diversity in the original estuary sites appears to be more resilient than in the newly created sites due to the large-scale environmental disruption that, ironically, created the new sites. However, the functional diversity at the original sites may be defined in part by management activities, including hatcheries that could mute/mask/inhibit other community responses. Further, functional diversity at the newly formed nearshore areas is predicted to stabilize as the habitats are vegetated and mature. Principal components analysis of Elwha fish community over the course of this study reveals that the fish communities of the Elwha are predictably grouped, indicating that while a few new species are observed, dam removal has not resulted in observable disruptions in fish community assemblages. And finally, nearshore habitats are critical for many forage fish species, and an emerging topic for large-scale dam removals. Forage fish spawning response to dam removal appears to be complex and may be related to multiple factors including high interannual variability in physical habitat conditions, geographic factors and complex life histories of forage fish. Habitat suitability for forage fish spawning should increase as restored ecosystem processes and newly created habitats mature and stabilize, indicating that time may be an important factor in nearshore restoration for forage fish spawning. It is therefore important to implement long-term monitoring and incorporate nearshore ecosystem process and function for multiple life history stages of nearshore species, including forage fish, into large-scale dam removal restoration and management planning. / Graduate / 2019-04-12

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Thesis advisor: Noah P. Snyder / Dam removal is a widely used river restoration technique. Historically, dams produced hydropower, controlled flooding, and provided water storage, but currently many dams in the United States, specifically low head dams in New England, are obsolete. This study aims to assess the ability of a simple morphodynamic sediment transport model, Dam Removal Express Assessment Model (DREAM-1), developed by Cui et al. (2006a). I compare simulations to a dam removal monitoring project that quantified the physical response of the Souhegan River to the removal of the Merrimack Village Dam (MVD), Merrimack, NH. Pearson et al. (2011) reported results of field monitoring from August 2007-May 2010 and found that the Souhegan River responded to dam removal in two phases: initial rapid incision of impoundment sediment induced by immediate base level drop of 3.9 m (~50% of impounded sediment eroded in ~2 months), followed by an event-driven phase in which impoundment sediment eroded primarily during floods. The reach downstream of the dam showed a similar two-phase response, with rapid deposition in the first three weeks after dam removal followed by bed degradation to the pre-removal elevation profile within a year. I have continued the field methods of Pearson et al. (2011) for the past two survey periods, June 2011 and July 2012. Using five years of comprehensive field data, I conduct a hindcast to compare the sediment erosion and deposition patterns predicted by DREAM-1 to the observed downstream response of the Souhegan River. I model the changes in bed elevation for the downstream and upstream channel reaches at intervals that correspond with the dates of four longitudinal profile surveys and seven annual cross-section surveys. Results of the hindcast show that DREAM-1 predicts channel elevation accurately within one meter and with average discrepancy of ±0.35 m when compared to average channel bed elevations of each cross-section. DREAM-1 successfully simulates two phases of upstream channel response, rapid impoundment erosion followed by a longer period of gradual sedimentation change. However, DREAM-1 erodes to base elevation within 11 weeks after dam removal (erosion of the 88% impoundment sand), leaving little sand for transport during the later survey periods. This overestimation of impoundment erosion is likely the product of limitations of the model, specifically the simplification of channel cross-sections with constant width throughout the simulation. The model assumes uniform lateral sediment transport in the impoundment and does not capture the variation in width due to incision and channel widening. This hinders the ability of the model to simulate some details of the sediment budget developed by Pearson et al. (2011) and extended with recent surveys. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Thesis advisor: Noah P. Snyder / Thesis advisor: Gail Kineke / Fluvial terraces form in both erosional and depositional landscapes and are important recorders of land-use, climate, and tectonic history. Terrace morphology consists of a flat surface bounded by valley walls and a steep-sloping scarp adjacent to the river channel. Combining these defining characteristics with high-resolution digital elevation models (DEMs) derived from airborne light detection and ranging (lidar) surveys, several methods have been developed to identify and map terraces. This research introduces a newly developed objective terrace mapping method and compares it with three existing DEM-based techniques to determine which is most applicable over entire watersheds. This work also tests multiple methods that use lidar DEMs to quantify the thickness and volume of fill terrace deposits identified upstream of dam sites. The preliminary application is to the Sheepscot River watershed, Maine, where strath and fill terraces are present and record Pleistocene deglaciation, Holocene eustatic forcing, and Anthropocene land-use change. Terraces were mapped at four former dam sites along the river using four separate methodologies and compared to manually delineated area. The methods tested were: (1) edge detection using MATLAB, (2) feature classification algorithms developed by Wood (1996), (3) spatial relationships between interpreted terraces and surrounding natural topography (Walter et al., 2007), and (4) the TerEx terrace mapping toolbox developed by Stout and Belmont (2013). Thickness and volume estimates of fill sediment were calculated at two of the study sites using three DEM-based models and compared to in situ data collected from soil pits, cut bank exposures, and ground penetrating radar surveys. The results from these comparisons served as the basis for selecting methods to map terraces throughout the watershed and quantify fill sediment upstream of current and historic dam sites. Along the main stem and West Branch of the Sheepscot River, terraces were identified along the longitudinal profile of the river using an algorithm developed by Finnegan and Balco (2013), which computes the elevation frequency distribution at regularly spaced cross-sections normal to the channel, and then mapped using the feature classification (Wood, 1996) method. For terraces upstream of current or historic dam sites, thickness and volume estimates were calculated using the two best performing datum surfaces. If all analyzed terraces are composed of impounded sediment, these DEM-based results suggest that terraces along the main stem and West Branch of the Sheepscot River potentially contain up to 1.5 x 106 m3 of fill. These findings suggest powerful new ways to quickly analyze landscape history over large regions using high-resolution, LiDAR DEMs while relying less heavily on the need for detailed and costly field data collection. / Thesis (MS) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

In 2001 the World Commission of Dams concluded that the economic, social and environmental cost of dams has been unacceptably high. As a result, dam removal is emerging as a promising option in addressing these concerns. However, dam removal is a contentious issue sharply divided between biophysical and socioeconomic interests. The purpose of this thesis is to conduct an explanatory case study of Eden Mills, Ontario to investigate the process of dam removal consideration. It was determined that i) safety ii) economics iii) social value and iv) environmental impact were critical variables influencing the decision-making process. Following site analysis, key-informant interviews and a design vignette survey it was determined that Eden Mills pursue dam removal as the social value of the millpond no longer justified the sustained economic and biophysical costs associated with the dam. This process theory can be applied to other dam removal scenarios to facilitate the decision making process.