Mountain channels closely coupled to landslide-prone hillslopes often exhibit bed surface grain sizes coarser than transportable by annual high flows. Coarse particles within poorly sorted sediment delivered to channels by mass-wasting processes may not be readily transported as bed load and can consequently form lag deposits that influence the morphology, hydraulic roughness, and sediment storage within mountain channel networks. A tracer study and comparison of supply and bed grain size distributions from a valley-spanning landslide in the North Cascades of Washington state were used to derive relations between shear stress and the probability of particle entrainment and erosion from the sediment pulse. Rapid bed surface armoring formed a relatively immobile lag deposit within 2 years. Covering of 20% of the bed by lag boulders with <5% probability of entrainment was sufficient to retard vertical incision and force considerable channel widening during a flood with an 8- to 152-year recurrence discharge on locally gauged streams. Our results imply that numerical models of sediment pulse evolution that do not explicitly incorporate the influence of lag formation may substantially overestimate long-term dispersion rates. The grain size distribution and lithology of a sediment input relative to the flow competence of the receiving channel are important factors influencing the rates and mechanisms of sediment pulse dispersion and the sediment capacitance provided by coarse-grained sediment pulses in mountain drainage basins.