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Sedimentary processes are known to help facilitate tidewater glacier advance, but their role in modulating retreat is uncertain and poorly quantified. In this study we use repeated seafloor bathymetric surveys and satellite‐derived terminus positions from LeConte Glacier, Alaska, to evaluate the evolution of a morainal bank and related changes in terminus dynamics over a 17‐year period. The glacier experienced a rapid retreat between 1994 and 1999, before stabilizing at a constriction in the fjord. Since then, the glacier terminus has remained stabilized while constructing a morainal bank up to 140 m high in water depths of 240–260 m, with rates of sediment delivery of 3.3 × 105 to 3.8 × 105 m3 a−1. Based on repeated interannual surveys between 2016 and 2018, the moraine is a dynamic feature characterized by push ridges, evidence of active gravity flows, and bulldozing by the glacier at rates of up to meters per day. Beginning in 2016, the summertime terminus has become increasingly retracted, revealing a newly emerging basin potentially signaling the onset of renewed retreat. Between 2000 and 2016, the growing moraine reduced the exposed submarine area of the terminus by up to 22%, altered the geometry of the terminus during seasonal advances, and altered the terminus stress balance. These feedbacks for calving, melting, and ice flow likely represent mechanisms whereby moraine growth may delay glacier retreat, in a system where readvance is unlikely. Key Points: A new morainal bank up to 140 m high grew in 17 years at LeConte Glacier (southeast Alaska), during a glacier stillstand at a constrictionRepeated multibeam surveys have highlighted dynamic processes of moraine sediment delivery, failure, and reworking by iceIn cases where moraine development does not facilitate glacier readvance, it may still influence the retreat history [ABSTRACT FROM AUTHOR]

Fjords on the West Antarctic Peninsula (WAP) serve as sediment traps, preserving histories of glacial sediment supply. Regional warming trends are expected to change sediment supplies, altering water quality, depositional history, and ecosystem drivers. Our ability to assess magnitudes of these changes is limited by sparse data on modern sediment accumulation. Twelve new cores and four existing cores from Andvord Bay were used to characterize variability in sediment accumulation rates. These range from 1.5 to 7.9 mm/year (0.12 to 0.56 g·cm−2·year−1). Spatial differences and a weak down‐fjord gradient in rates suggest diverse sediment sources, including from outside the fjord. This data set provides a comprehensive assessment of sedimentation during the past century, indicating little change in rates due to recent WAP warming, and sets a benchmark for assessing climate‐related changes in sediment delivery and ecosystem drivers (e.g., burial disturbance) in the fjord over coming decades. Plain Language Summary: Tidewater glaciers commonly carve deep fjords, which fill with sediment as the glaciers retreat. Rates of fjord sediment accumulation increase from polar to temperate latitudes because glaciers become more erosive due to increases in air (and water) temperatures and more abundant subglacial meltwater. Presently, subpolar glaciers on the West Antarctic Peninsula (WAP) are weakly erosive, and adjacent fjords host high levels of biological diversity and abundance. Sediment delivery and seabed disturbance are expected to increase with regional climate trends; unfortunately, we have sparse information on present sediment accumulation rates. In this study, we collected a large set of cores and constructed a detailed picture of sediment properties in Andvord Bay on the WAP. Sediment accumulates at slow rates of millimeters per year, conditions which foster great biologic productivity in the WAP coastal zone. These data provide a critical foundation for assessing the sedimentation disturbance regime in Andvord Bay. Key Points: Sediment accumulation rates in Andvord Bay (Antarctica) are only millimeters per year and are spatially variable, reflecting slow delivery from diverse sourcesLimited evidence exists of sporadic gravity flows adjacent to the two primary tidewater glaciersSeabed radioisotope profiles do not indicate any substantial changes in sediment accumulation rates over the past century [ABSTRACT FROM AUTHOR]