  We consider layered superconductors with a flux lattice perpendicular to the
layers and random columnar defects parallel to the magnetic field B. We show
that the decoupling transition temperature Td, at which the Josephson coupling
vanishes, is enhanced by columnar defects by an amount ~B^2 relative to Td.
Decoupling by increasing field can be followed by a reentrant recoupling
transition for strong disorder. We also consider a commensurate component of
the columnar density and show that its pinning potential is renormalized to
zero above a critical long wavelength disorder. This decommnesuration
transition may account for a recently observed kink in the melting line.
