Summary Laboratory experiments were conducted to evaluate the transport capacity and the selective transport of thin flows. The aggregate size as well as the grain size of the eroded and transported loamy materials could best be related to the shear velocity of the flow, whereby it was observed that, when the shear velocity was greater than 3.0–3.5 cm/s, transport and erosion of loamy materials becomes aselective. Sediment concentration however could best be related to the product of mean and shear velocity. The transport capacity of thin flows for a well sorted Tongrian sand (D 50 ≅ 100 μm) starts rising rapidly at a shear velocity of 3.15 cm/s. It could thus be expected that the erosive power of a thin flow would increase rapidly once a shear velocity of 3.0–3.5 cm/s is exceeded and that this value might be a valuable threshold for the onset of rill erosion. This is confirmed by field observations.