Longitudinal and cross-sectional profiles of alluvial channels are altered by the presence of seepage. However, the available literature suggests that seepage has not been considered in analyses of bank profiles of threshold alluvial channels. In the experimental study reported here, downward seepage was applied to parabolic channels based on tractive force theory in order to determine its effect on the longitudinal and cross-sectional profiles. It was observed that sediment transport in the threshold channels increased when downward seepage was applied. The parabolic shape did not hold, but channels formed a new cross-sectional shape with a flat bed and two curved banks and achieved stability. The reason for the increased sediment transport in a threshold channel after the application of downward seepage was sought by analysing the time-averaged velocities. An empirically derived exponential expression was developed for the bank profiles of threshold alluvial channels affected by seepage. A relationship among hydraulic parameters for a stable channel with downward seepage is also suggested on the basis of the experimental observations.