Stump Lake is strategically located to provide a sediment record of very large debris flows that travel down Cheekye River to lower Cheekye fan from the west flank of Mount Garibaldi, a Quaternary volcano in the southern Coast Mountains of British Columbia. Cores collected from Stump Lake span the last 11,500 years and consist largely of gyttja. However, a diamicton layer and an associated detrital organic layer, which is capped by a clay lamina, record a large debris flow derived from the headwaters of Cheekye basin. Radiocarbon ages indicate that this event occurred about 6,900 years ago. We estimated the peak discharge of the flow by measuring cross-sections of the Cheekye River channel adjacent to Stump Lake and by calculating flow velocity using a Newtonian flow model. Reasonable values of peak discharge, calculated in this way, were input into an empirical equation linking discharge and debris-flow volume, developed for non-granitic debris flows. The estimated minimum volume of the debris flow is 3 to 5 × 106 m3. Another large debris flow (ca. 3 × 106 m3), which occurred about 800 years ago, is not recorded in Stump Lake sediments and thus was too small to reach the lake. The Stump Lake sediment sequence indicates that no debris flows larger than several million cubic meters have reached lower Cheekye fan in post-glacial time.