In 2018, the Canadian Coast Guard approved a science team to sample in tandem with its ice-breaking and ship escorting operations. This collaboration provided the first mixing observations during winter that covered the largest spatial extent of the St. Lawrence Estuary and the Gulf of St. Lawrence ever measured in any season. Contrary to previous assumptions, we demonstrate that fluvial nitrate inputs from upstream (i.e., Great Lakes) are the most significant source of nitrate in the estuary.