We consider a basin made of an impermeable soil, eroded by water flow (not by weathering). Our approach is based on standard deterministic models, within a `streamlet' picture where the flow is always directed downhill. The central assumption is that erosion occurs only when the surface shear stress (due to water flow) is above a certain threshold <sub>c</sub> . Some features of the landscape are simplified by the existence of <sub>c</sub> , and do not depend on detailed assumptions on the behaviour above <sub>c</sub> . In particular, if the initial landscape is a U-shaped valley, we can construct the `line of attack' - i.e. the border of the eroded regions - by a simple prescription.