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Quantification of climate forcing of glacial hydrological systems at the decadal scale is rare because most measurement stations are too far downstream for glacier impacts to be clearly detected. Here we apply a measure of daily hydrograph entropy to a unique set of reliable, high-altitude gauging stations, dating from the late 1960s. We find a progressive shift to a greater number of days with diurnal discharge variation as well as more pronounced diurnal discharge amplitude. These changes were associated with the onset of rapid warming in the 1980s as well as declining end of winter snow depths as inferred from climate data. In glaciated catchments, lower winter snow depths reduce the magnitude and duration of snowpack buffering and encourage the earlier onset of glacier ice exposure, with associated lower surface albedo and more rapid melt. Together, these processes explain the increase in the observed intensity of diurnal discharge fluctuations.

Plain Language Summary River basins that have a high proportion of ice cover are particularly sensitive to climate warming. Daily variations in insolation and temperature typically lead to fluctuations in snow and/or ice melt and thus a daily rise and fall in river flow. Snow, and the glaciers themselves, can buffer this rise and fall. For six high mountain Alpine basins, we show that daily discharge fluctuations are changing due to climate warming at the decadal scale, with both increasing daily discharge maxima and reducing daily discharge minima. These changes reflect decreased snow accumulation at the end of winter, reducing the buffering and increasing the onset of rapid glacier melt.