Debris flow deposits of Chicago Creek and the sediment, pollen, and macrofossil records of Seeley Lake were studied to elucidate the Holocene history of the northwest flank of the Rocher Déboulé Range near Hazelton, British Columbia.The Chicago Creek drainage has experienced numerous rockfalls, debris slides, and debris flows. A large debris flow covering approximately 300 ha occurred about 3580 ± 150 BP. This flow was two to three orders of magnitude larger than historic debris flows in this drainage. It traveled about 3 km down Chicago Creek and dammed the outlet stream of Seeley Lake. A debris deposit along lower Chicago Creek is interpreted as the product of debris torrents that formed during or soon after the damming of Seeley Lake. Its surface exhibits soil development (rubification and profile development) comparable to that on the large debris flow, suggesting equivalent age.Pollen and plant macrofossils are described from a core taken in Seeley Lake. This core spans the period from ca. 9200 BP to the present. A disturbance event in 3380 ± 110 BP, correlative with the large Chicago Creek debris flow, is recorded by a clastic sediment layer and changes in the microfossil and macrofossil assemblages.The Chicago Creek debris flow and debris torrent ca. 3500 BP may be the catastrophic event recorded in the story of the Medeek, an oral history or "ada'ok" of the Gitksan people of Hazelton.