  Narrow, elongated regions of very low polarized intensity -- so-called canals
-- have recently been observed by several authors at decimeter wavelengths in
various directions in the Milky Way, but their origin remains enigmatic. We
show that the canals arise from depolarization by differential Faraday rotation
in the interstellar medium and that they represent level lines of Faraday
rotation measure RM, a random function of position in the sky. Statistical
properties of the separation of canals depend on the autocorrelation function
of RM, and so provide a useful tool for studies of interstellar turbulence.
