Conductivity-temperature-depth (CTD) and mooring data from La Grande under-ice plume, northeast James Bay, 2016-2017

Properties of the under-ice river plume associated with the regulated La Grande River were studied throughout the winters of 2016 and 2017. River runoff acts as a source of both momentum and buoyancy when it enters saline oceanic water. Initially, the buoyancy of fresh river water is often sufficient to overcome the forces that drive vertical mixing, such that a stratified river plume is formed on the surface. The direction of flow, thickness, and size of a river plume in the coastal domain are determined by external factors, such as coastal geometry, discharge rate, tides, the Coriolis effect, coastal background circulation, and wind-driven mixing. However, there remain few field observations of river plumes under the sea ice. Immobile landfast sea ice that forms along high-latitude coasts during winter blocks direct wind mixing, resulting in a larger spatial extent of under-ice river plumes compared with open water conditions under similar levels of river discharge. Underneath continuous level sea ice, an under-ice plume can spread largely unmixed until it reaches the floe edge, after which the river water presumably mixes quickly with ambient seawater and disperses into the ocean surface layer. A few observational studies in Hudson Bay have included time-series observations of under-ice river plumes but generally the influence of winds and storms on under-ice plume behaviour remains poorly known. This research focuses on the La Grande River (LGR) plume on the northeast coast of James Bay using data collected during the winters of 2016 and 2017. The data follow up on early work in the 1980s and early 1990s when winter discharge from LGR was increased by the first phases of hydroelectric development of LGR. The purpose of the study was to update our understanding of the size and configuration of the LGR winter plume under present-day discharges that exceed 5000 m3/s during typical winters.