A moving standard deviation (MSD) technique is developed to infer the onset and cessation of ephemeral streamflow using temperature data from the upper 2.25 m of streambed sediments. During periods of streamflow, shifting of the predominant thermal-transport mechanism within the sediments from conduction to advection produced changes in the amplitude of the vertically propagating diurnal temperature waves. Analytical expressions describing propagation of conductive and advective diurnal temperature waves through streambed sediments are presented for identifying depths with the largest changes in the diurnal temperature wave amplitude between periods of flow and no flow. The MSD statistical technique was developed to identify the thermal amplitude changes from bed sediment thermographs and to infer streamflow timing. The accuracy of the MSD technique is quantified using direct streamflow and streambed water content measurements. Accuracy of the technique was most sensitive to the MSD window length and the threshold parameter separating periods of conductive and advective heat transport. An alternative calibration procedure was developed using temperature measurements alone. The average error for streamflow timing was approximately 400 min for each event. The results show that temperature sensors may be deployed at a range of sediment depths depending on streamflow stage and soil thermal and hydraulic properties, and that the MSD procedure can provide an objective and repeatable means to quantify streamflow timing.