Fluid flow is a diffusion process. For homogeneous turbulence, dispersion occurs normally about the center of the process, the rate of growth depending on the diffusion coefficients Ki.Measurements of Ki for longitudinal and lateral diffusion are presented for the Lewis River, a proglacial stream in north-central Baffin Island. In the river, flows up to 250 m3/s, fully turbulent, are characterized by 'roller eddies'. Observations were made by electro-chemical recording of longitudinal dispersion of salt solutions and by photography of lateral dispersal of dye plumes. It is shown that the K-values depend on the scale of flow, while the overall pattern of diffusion depends on the flow and channel geometry of the stream.The geomorphological consequences of diffusion include the phenomenon of momentum transfer in the channel and resultant shear stresses brought to bear on the walls, and the pattern of sediment transport.