This paper examines the velocity field in an idealised heterogeneous open channel during normal flow conditions, corresponding to two different subcritical flow conditions (discharges of 0.040 m/s and 0.056 m/s, with correspond-ing Froude numbers 0.51 and 0.57). The bed of the channel is formed of two full-length, longitudinal strips of equal width, a smooth section constructed from polyvinyl chloride and a rough section constructed from gravel (d=10 mm). The results indicate that the turbulence appears to propagate vertically from the rough bed and horizontally over the rough-smooth boundary. There is also evidence to suggest that the turbulent momentum transfer is maximised at the rough-smooth boundary. Secondary flow structures are seen to exist due to the rough-smooth boundary and provide another mechanism for momentum transfer. These transfer mechanisms are important in understanding the effect of a heterogeneous channel bed on open-channel flow.