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Abstract At tidewater glaciers, the ocean supplies heat for submarine ice melt and the glacier supplies freshwater that impacts ocean circulation. Models that employ buoyant plume theory are widely used to represent the effects of subglacial discharge on both glacier melt and freshwater export, but a scarcity of observations means that these models are largely unvalidated. The challenges and inherent risks of working near actively calving glaciers make it difficult to collect in situ observations. This study, conducted at Xeitl Sít’ (LeConte Glacier) in southeast Alaska, reports the first observations of velocity and geometry of the upwelling core of a subglacial discharge plume. This subglacial discharge plume rises along an overcut ice face, with vertical velocities in excess of 1 m s⁻¹, and a plume shape consistent with subglacial discharge emerging from a narrow outlet. Buoyant plume theory, as commonly applied, fails to replicate the observed entrainment, underestimating the plume's volume flux by more than 50%. Large eddy simulations reveal that over half of this mismatch can be attributed to the overcut slope of the ice, which enhances entrainment. Enhanced mixing near the grounding line may account for the additional entrainment. Accurate representation of plume geometry and entrainment is critical for understanding plume‐driven melt of the terminus and the initial mixing of glacial meltwater as it is exported into the ocean. 

Abstract Frontal ablation, the combination of submarine melting and iceberg calving, changes the geometry of a glacier's terminus, influencing glacier dynamics, the fate of upwelling plumes and the distribution of submarine meltwater input into the ocean. Directly observing frontal ablation and terminus morphology below the waterline is difficult, however, limiting our understanding of these coupled ice–ocean processes. To investigate the evolution of a tidewater glacier's submarine terminus, we combine 3-D multibeam point clouds of the subsurface ice face at LeConte Glacier, Alaska, with concurrent observations of environmental conditions during three field campaigns between 2016 and 2018. We observe terminus morphology that was predominately overcut (52% in August 2016, 63% in May 2017 and 74% in September 2018), accompanied by high multibeam sonar-derived melt rates (4.84 m d −1 in 2016, 1.13 m d −1 in 2017 and 1.85 m d −1 in 2018). We find that periods of high subglacial discharge lead to localized undercut discharge outlets, but adjacent to these outlets the terminus maintains significantly overcut geometry, with an ice ramp that protrudes 75 m into the fjord in 2017 and 125 m in 2018. Our data challenge the assumption that tidewater glacier termini are largely undercut during periods of high submarine melting.