  Narrow depolarized canals are common in maps of the polarized synchrotron
emission of the Milky Way. Two physical effects that can produce these canals
have been identified: the presence of Faraday rotation measure ($\RM$)
gradients in a foreground screen and the cumulative cancellation of
polarization known as differential Faraday rotation. We show that the behaviour
of the Stokes parameters $Q$ and $U$ in the vicinity of a canal can be used to
identify its origin. In the case of canals produced by a Faraday screen we
demonstrate that, if the polarization angle changes by $90\degr$ across the
canal, as is observed in all fields to-date, the gradients in $\RM$ must be
discontinuous. Shocks are an obvious source of such discontinuities and we
derive a relation of the expected mean separation of canals to the abundance
and Mach number of supernova driven shocks, and compare this with recent
observations by \citet{Haverkorn03}. We also predict the existence of less
common canals with polarization angle changes other than $90\degr$.
Differential Faraday rotation can produce canals in a uniform magneto-ionic
medium, but as the emitting layer becomes less uniform the canals will
disappear. We show that for moderate differences in emissivity in a two-layer
medium, of up to 1/2, and for Faraday depth fluctuations of standard deviation
$\lesssim 1 \mathrm{rad}$, canals produced by differential rotation will still
be visible.
