An analysis of field data for the lower reach of Columbia Glacier, Alaska, U.S.A., indicates that, during the time of the field work, glacier-speed variations in the tidal frequency range are dependent on tidal period variations in sea-level at the terminus, and diurnal period variations in melt-water input at the surface. A simple model based on a perturbation expansion of the flow equation provides a suitable framework in which to understand the observations. Variations in sea- level cause variations in the normal stress at the terminus and are transmitted up the glacier by way of the longitudinal stress. The variations in melt-water input affect glacier speed, presumably through changes in basal water pressure. The data set does not allow a detailed analysis of the relation between melt-water input, basal water pressure, and speed.