Abstract
River plumes form when freshwater from rivers enters the denser, saltier ocean, generating buoyant water masses that shape coastal circulation, stratification, and mixing. These plumes transport land-derived materials — such as sediments, nutrients, and pollutants — while creating sharp density gradients that drive complex dynamical processes. At the leading edge of these systems, density contrasts define the plume front, a highly dynamic region that governs transport, mixing, and overall plume evolution. Despite significant progress in understanding plume behavior, key gaps persist, particularly in how plume fronts evolve in terms of structure, turbulence, and momentum balance. This dissertation investigates the evolution of river plume fronts through high-resolution, front-following observations of the Merrimack River plume. A novel multi-platform sampling strategy was developed, integrating an Uncrewed Underwater Vehicle (UUV, T-REMUS), an aerial drone, shipboard measurements, and surface drifters to dynamically track and sample the plume front in three dimensions. Over 32 frontal crossings during half a tidal cycle provided an unprecedented dataset, capturing spatial and temporal variability in density, velocity, acoustic backscatter, and turbulence dissipation rates. The findings reveal that the plume front progresses through three distinct phases — Advective, Transition, and Arrested — each marked by specific changes in frontal structure and dynamics. During the Advective phase, the front propagates rapidly offshore, exhibiting strong convergence, downwelling, and turbulent kinetic energy dissipation rates (in the order of 10−3 W/kg). As the front enters the Transition phase, a rotor-like circulation develops, intensifying vertical shear and turbulent dissipation further from the front. In the Arrested phase, the front slows and stalls due to a weakening barotropic pressure gradient. The strong frontal convergence initially maintains a direct connection between the front and the plume interior by efficiently advecting source water forward. However, as the plume evolves into the Arrested phase, the front increasingly behaves as an independent feature. Changes in frontal depth trigger the generation and release of two trapped-core solitary waves, which play a crucial role in the front’s evolution. These waves effectively detach and transport a portion of the frontal mass, modifying the frontal structure and progression. Our momentum balance analysis revealed that this mass loss significantly contributes to halting the front’s propagation, altering its dynamics. Additionally, we found that the along-front momentum balance is strongly influenced by wind forcing, which interacts with Coriolis and tidal forcing to modulate frontal evolution. This interplay between frontal structure, solitary wave generation, and external forcing underscores the complexity of plume front dynamics and their sensitivity to both internal and external physical drivers. Studying the evolution of plume fronts provides an ideal framework for advancing our general understanding of oceanic frontal dynamics by allowing sampling and modeling at accessible temporal and spatial scales. Understanding the dynamic processes governing their evolution is essential for advancing both scientific knowledge and effective coastal management. Our findings offer new insights into the complex interplay of physical processes at river plume fronts, contributing to a broader understanding of coastal dynamics and gravity currents.