  We study depinning of vortex chains in channels formed by static, disordered
vortex arrays. Depinning is governed either by the barrier for defect
nucleation or for defect motion, depending on whether the chain periodicity is
commensurate or incommensurate with the surrounding arrays. We analyze the
reduction of the gap between these barriers as function of disorder. At large
disorder, commensurability becomes irrelevant and the pinning force is reduced
to a small fraction of the ideal shear strength of ordered channels.
Implications for experiments on channel devices are discussed.
