This paper examines the place of ice and snow in the process of Euro-Canadian colonisation of what are today the Yukon and Northwest Territories. Using oral histories and accounts of Indigenous life experiences from the late nineteenth and early twentieth centuries, the paper opens with an examination of the ways in which ice and snow, freeze-up and break-up, were inextricable from wider social and cultural relationships between Indigenous northerners and other-than-human nature. Intensified trade and missionary activity after the mid-nineteenth-century along the Mackenzie and Yukon rivers and their tributaries created new colonial geographies, rhythms, and knowledge. These paid close attention to the character of ice, the timing of freeze-up and break-up, and the transportation possibilities of the open water season especially. By examining the histories of a scarlet fever epidemic in 1865 and an influenza epidemic in 1928, this paper uses the role of ice and its transformations in shaping the movements of pathogens to trace emerging northern colonial ecologies between 1860 and 1930.