The dissolution of rocks by rainfall commonly generates streamwise parallel channels, yet the occurrence of these natural patterns remains to be understood. Here, we report the emergence, in the laboratory, of a streamwise dissolution pattern at the surface of an initially flat soluble material, inclined and subjected to a thin runoff water flow. Nearly parallel grooves of width about one millimeter and directed along the main slope spontaneously form. Their width and depth increase continuously with time, until their crests emerge and channelize the flow. Our observations may constitute the early stage of the patterns observed in the field.