Landsliding during a magnitude 7.7 earthquake in 1929 in northwestern South Island, New Zealand, delivered up to ∼400 000 m3/km2 of sediment to stream channels. First- to third-order channels were buried to depths of as much as 10 m with sedimentary rock debris for distances ranging from <100 to ∼2000 m. Little sediment has been transported out of 65 km2 of the 92-km2 study area because of the coarse grain size of the sedimentary rock debris and trapping of sediment in numerous landslide-dammed lakes. Sand- and pebble-sized debris originating from landslides on a 6.6-km2 area of granite has been transported downstream no more than ∼10 km. At least 50% to 75% of this granitic debris is retained in the fourth-order 92-km2 catchment 50 yr after the earthquake. The long retention times of sediment and the volume of debris supplied by landslides during a large earthquake suggest that earthquake-induced landsliding is the principal sediment-supply mechanism in the region.