A nonlinear theory for the saturation of the HF-induced parametric decay instability is formulated in terms of the normal modes of a field-aligned plasma wave duct. Coupling coefficients between modes are derived, and a saturation spectrum is computed numerically for a typical (Δn/n0~0.04,&lgr;⊥~4.5m) irregularity. The computed spectrum shows that unstable wves may propagate at larger angles to the B field than would be the case in a regular medium. The relevance of this work to the experiment of Muldrew and Showen (1977), concerning th height of the HF-induced plasma line at Arecibo, is discussed. A possible explanation is also found for the sharpness of the 'decay line' feature in comparison with the 'broad bump' in the observed backscatter spectrum.