The export of coarse particulate organic matter (CPOM) from mountain catchments seems to be strongly linked to rising discharge, but the mechanism leading to this is unclear. We show that log jams in a steep headwater stream are an effective barrier for CPOM export. Exceptional discharge events play a dual role: First, they destroy existing jams, releasing stored material. Second, they intensify channel--hillslope coupling, thereby recruiting logs to the channel, around which new jams can form.