  Landscapes that are rhythmically dissected by natural drainage channels exist
in various geologic and climatic settings. Such landscapes are characterized by
a length-scale for the lateral spacing between channels. We observe a
small-scale version of this process in the form of beach rills and reproduce
channelization in a table-top seepage experiment. On the beach as well as in
the experiments, channels are spontaneously incised by surface flow, but once
initiated, they grow due to water emerging from underground. Field observation
and experiment suggest the process can be described in terms of flow through a
homogeneous porous medium with a freely shaped water table. According to this
theory, small deformations of the underground water table amplify the flux into
the channel and lead to further growth, a phenomenon we call ``Wentworth
instability''. Piracy of groundwater can occur over distances much larger than
the channel width. Channel spacing coarsens with time, until channels reach
their maximum length.
