On long hillsides one can examine systematic downslope changes in the ground surface as Horton overland flow gathers into depressions and eventually incises the surface to form channels. Microtopography plays an important role in this process. We have sampled the microtopography of two long hillslopes in a savanna region of southern Kenya, and defined the spectral characteristics of its roughness at various distances from the drainage divide. The microtopography is fractal with a dimension that decreases systematically downslope, and the overall roughness varies between and along hillslopes in response to: (i) the weathering characteristics of the underlying bedrock; (ii) the type and density of patchy vegetation, and (iii) the tendency for wash to incise the surface with increasing intensity as runoff discharge increases downslope. The downslope decrease in fractal dimension reflects the progressive development of low-frequency roughness, here referred to as “swaley” microtopography, which the wash develops even far upslope of the channel head. The statistical analysis of surface roughness motivates a discussion of the role of microtopography in the interaction between wash and diffusive sediment transport processes that ultimately determines the critical distance from the divide at which channels begin.