Permafrost-Related Erosional Effects of Water Level Regulation and Climate Change along the Shorelines of Aishihik and Canyon Lakes, Southwest Yukon

Lake shoreline erosion is caused by wind-generated waves ( mechanical erosion) and, in areas of permafrost, can be compounded by thawing ground ice (thermal erosion). In southwest Yukon, erosion patterns along the shorelines of Aishihik and Canyon Lakes reflect the combined effects of climate change and lake level regulation for hydroelectricity, which began in 1975. An investigation characterizing shoreline erosion and aiming to determine the relative contributions of lake level regulation and climate change was undertaken to support an environmental and socio-economic effects assessment associated with hydroelectric facility relicensing. The investigation involved three phases: (i) compilation and review of pertinent topographic, geological, and hydrological information; (ii) helicopter- and ground-based field reconnaissance; and (iii) mapping of shoreline types, relative severity of erosion and recession from former beaches at select sites. Three types of shorelines were identified based on their distinct responses to lake level regulation and climate change. Type 1 shorelines exhibit a moderate slope of coarse-grained sediment and may contain ice-poor permafrost; they are relatively insensitive to climate change and quickly self-stabilize when exposed to higher lake levels. Type 2 shorelines typically exhibit an abruptly steep embankment of ice-rich, fine-grained sediments; they are vulnerable to recession through long-term retrogressive thaw and slumping in response to climate change and lake level regulation. Type 3 shorelines are typically low-relief and composed of fine-grained, ice-rich sediments; they are most sensitive to higher lake levels due to inundation-induced thaw. Erosion along the Type 2 and Type 3 shorelines of Aishihik and Canyon Lakes appears to be driven primarily by climatic effects, despite an anomalous peak in lake level at the onset of regulation that locally initiated and exacerbated erosion.