  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
about 1 mm wide 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.
