Stanwell–Fletcher Lake is a cold monomictic Arctic lake, essentially isothermal at about 1.5 °C, with almost complete ice cover throughout the year. The annual heat input is about 13 000 cal cm−2, much less than that of dimictic Arctic lakes. The lake is isochemical, with very low ionic concentrations.The lake basin was formed during the Pleistocene by glacial scouring of soft sediments from a graben. As the ice retreated the basin filled with sea water, and became a fjord. Subsequent isostatic uplift raised the marine shorelines to 150 m above present sea level; the uplift curve indicates emergence of the fjord sill and termination of estuarine conditions about 3500 years ago. Tidal influx persisted for about 1100 years before the lacustrine phase started.Comparisons with modern fjords and coastal meromictic lakes suggest that most of the salt left the lake during its lacustrine phase. The mechanism was probably entrainment of salt water into a deep freshwater current flowing over the chemocline due to convective circulation in summer.