A very large debris flow was triggered during a period of record-breaking high temperatures in upper Capricorn Creek, within the Mount Meager Volcanic Complex, a part of the Garibaldi Volcanic Belt of the southern Coast Mountains. The debris flow deposit impounded Meager Creek, creating an 800 m long landslide-dammed lake. The total event volume was 1.2 × 106 m3. The debris flow was followed by three days of almost continuous hyperconcentrated flow surges, which caused significant fluvial aggradation in the Meager Creek flood plain below the Capricorn Creek confluence. Within a few days of the formation of the landslide dam, a spillway notch had been cut through the deposit, thereby preventing the occurrence of a catastrophic dam break. The landslide, which triggered the debris flow, originated in deep volcanic colluvium having a previous history of progressive slope deformation, a consequence of glacial downwasting since the Neoglacial maximum. This paper highlights an important landslide response to recent glacial retreat and suggests that similar events could reoccur within Capricorn Creek, as well as at other sites where steep colluvial and weak bedrock slopes have been glacially debuttressed.