FREQUENCY OF DEBRIS FLOWS ON THE SHEEP MOUNTAIN FAN, KLUANE LAKE, YUKON-TERRITORY

The debris flow track on the east side of Sheep Mountain, Yukon Territ ory, consists of an upper accumulation area (1350-1900 m elevation), a constriction caused by a hard band of rock which acts as a dam (1350- 1150 m), a box canyon with talus along its sides (880-1150 m), and the debris flow fan below 880 m. Tree ring counts indicate that there hav e been no major flows through the box canyon during the last centruy. However, the presence of White River ash (1220 BP) below eight debris flow deposits in a section in the fan suggests an average frequency of one flow every 150 years. Up to 5 m of diamicton was deposited during each event. Weak paleosols or stone lines separate the diamictons, th e paleosols indicating an appreciable break in time between each event . The present surface soil on the fan is less well developed than thes e paleosols. Thus this fan represents the higher volume, lower frequen cy debris member of a spectrum of active debris flows over icy permafr ost, and as such, completes the range of variation of these active sys tems (see table 5). It differs from the fossil debris flow fans in tha t it lacks the deep red Altithermal soil.