Fourier analysis reveals that winter electrical self potential (SP), water pressure (PW), and electrical conductivity (EC) time series collected beneath Haut Glacier d'Arolla, Switzerland, are forced by earth and atmospheric tides. Forcing is dominant during periods of expanding bedrock, consistent with glacier substrate deformation periodically driving water from the ice body into the bed. This may modify the strength of subglacial sediments, and could therefore influence glacier drainage and motion on a global scale.