Nearly 100 days of hourly glacier motion, hydrology and hydrochemistry measurements on Kennicott Glacier, Alaska, USA, demonstrate the complicated relationship between water and motion at the glacier bed. Our observations capture the transient glacier response to seasonal and daily melt cycles, and to a jökulhlaup that prompts a sixfold increase in glacier speed. Sliding is promoted whenever the water inputs to the glacier exceed the capacity of the subglacial hydrologic system to transmit the water. Sensitivity of sliding to daily meltwater inputs varies strongly through the season, implying that the state of the hydrologic system governs the sensitivity of basal sliding. A numerical model constructed to explore these relationships reveals: the roles of the effective pressure; the exponent to which this is taken in the ‘sliding law’ (0.1 <γ< 0.6); glacier macroporosity (φ < 2%); and the ‘cavity-generating capacity’ of the glacier bed, which encapsulates the sizes and spacing of bed roughness elements. Temporal changes in the effective pressure associated with evolution of both water inputs and subglacial water transmission capacity can explain the varying strength of diurnal velocity fluctuations of Kennicott Glacier. Spatial patterns of glacier macroporosity and of basal roughness govern variation in sensitivity of sliding to water inputs.