Date: 2025-01-01

Creator: Benjamin Wong Halperin

Access: Open access

Large impoundment dams have well-documented impacts on hydrologic and geomorphicfunction. Numerous tools and metrics have been developed over time to characterize theseimpacts, but they remain disparate, are often applied in a small number of studies, and rarelyapplied in concert with each other. Utilizing the open-source programming language R, Iassemble a suite of metrics known as DAMS – the Dam Analysis and Metrics Suite – thatcombines several pre-existing metrics for characterizing dam impacts into one script. Thesemetrics include the Indicators of Hydrologic Alteration to characterize hydrologic change; themean streambed elevation to characterize vertical change in the river; and sediment mass balanceand flood magnitude reduction. By combining these schemas, DAMS provides a flexible andcomprehensive way to characterize the impact of dams on hydrology and geomorphology.I apply DAMS to two dams in diverse geographic settings: the Buford Dam on theChattahoochee River in Georgia and the Harris Station Dam on the Kennebec River in Maine.Both are hydroelectric dams with long stream gage records before and after dam construction. Ifound that the Buford Dam has caused a decrease in high flows in the Chattahoochee River aswell as a change in the seasonality of flows. I found that the Kennebec River has seen anincrease in high and peak flow volume after the construction of the Harris Station Dam, but thisincrease is less than comparable unimpounded rivers. The geomorphic data the ChattahoocheeRiver is fairly limited and cannot be access for the Kennebec River at all, meaning that DAMSwas unable to tell a complete story about how these rivers changed due to impoundment,highlighting the need for increased monitoring on all of the United States’ rivers.

Date: 2020-01-01

Creator: Siena Brook Ballance

Access: Open access

Phytoplankton underpin marine trophic systems and biogeochemical cycles. Estuarine and coastal phytoplankton account for 40-50% of global ocean primary productivity and carbon flux making it critical to identify sources of variability. This project focuses on the Kennebec River and Harpswell Sound, a downstream, but hydrologically connected coastal estuary, as a case study of temperate river influence on estuarine nutrient regimes and phytoplankton communities. Phytoplankton pigments and nutrients were analyzed from water samples collected monthly at 8 main-stem rivers stations (2011-2013) and weekly in Harpswell Sound (2008-2017) during ice-free months. Spatial bedrock and land use impacts on river nutrients were investigated at sub-watershed scales using GIS. Spatial analysis reveals a 10-fold increase in measured phytoplankton biomass across the Kennebec River’s saltwater boundary, which demonstrates ocean-driven phytoplankton variability in the lower river. The biomass pattern is accompanied by a transition in phytoplankton community structure with respect to which groups co-occur (diatoms, chlorophytes, and cryptophytes) and which are unique (dinoflagellates in Harpswell). Upstream, the timing of each community depends on land-use proximity and seasonal discharge. In Harpswell Sound, the nutrient regime and phytoplankton community structure vary systematically: first diatoms strip silicate, then dinoflagellates utilize nitrate, followed by chlorophytes and cryptophytes that utilize available phosphate. These findings reveal, for the first time, patterns in phytoplankton communities and nutrient dynamics across the fresh to salt water interface. Ultimately the Kennebec River phytoplankton communities and nutrient regimes are distinct, and the river is only a source of silicate to Harpswell Sound.

Date: 2021-01-01

Creator: Eugen Florin Cotei

Access: Access restricted to the Bowdoin Community

Date: 2025-01-01

Creator: Inga Christhild Dovre

Access: Open access

Large dams intrinsically change the geomorphic and hydrologic forms and processes of the rivers they reside in, yet the complexity of their impacts in different settings is not fully understood. In the Upper Midwest, Saylorville and Red Rock Dam are constructed on the Des Moines River, in a region undergoing drastic alterations due to climate and land use change. In this study, I seek to quantify these alterations, using the principles of Lane’s Balance to evaluate dam impacts on river aggradation and degradation. I use historical aerial imagery to measure channel width and other geomorphic properties from the 1930s to the 2020s. Stream gauge data from the United States Geological Survey is used to construct a temporal and spatial hydrologic understanding of the Des Moines River during this time period. I find that channel width is increasing significantly, driven by both the trapping of sediments by dams and the climate-driven increase in annual mean flows. These width increases are variable spatially, with the inter-dam reach widening slower than the reach downstream of both dams. These results demonstrate the control of other factors such as climate and land use on the geomorphic changes seen downstream of dams, illustrating the importance of considering regional characteristics of watersheds in geomorphic analysis.