RESPONSE OF SQUAMISH RIVER TO ACTIVITY ON THE CHEEKYE FAN

Cheekye Fan, 58 km north of Vancouver, Canada, is one of the largest Holocene fans in British Columbia. Numerous large debris flows from Mount Garibaldi, a Plio-Pleistocene stratovolcano near the north end of the Cascade Volcanic Belt, have built the fan to its present extent. The fan impinges on Squamish River, which drains an area of 3328 km² in the southern Coast Mountains and flows into Howe Sound at Squamish. The gradient of Squamish River over a 12-km reach above Cheekye Fan is anomalously low due to the control on base level exerted by remnants of the fan where it has been dissected by Squamish River. Although the fan is now incised, episodic debris flows continue to affect local base level and river planform in Squamish Valley. Long, continuously exposed banks of sediment in the banks of the river provide a history of channel sedimentation and floodplain aggradation over the past 3500 years. Laminated organic and inorganic silts and peat are exposed for more than 11 km upriver of Cheekye Fan and record either valley-wide sedimentation in a swampy slackwater environment or more localized sedimentation in backwater meander cutoff settings. These fine-grained sediments occur up to 4 m above the present channel of Squamish River and are overlain by rhythmically bedded sands and silts deposited by an aggrading and meandering Squamish River over approximately the past 700 years. Collectively, the evidence shows that there has been a substantial change in Squamish River planform and sedimentary environments in the late Holocene. We relate the slackwater silts to the known history of debris flow activity and growth of Cheekye Fan. We also seek to understand the cause of recent and continuing incision of the fan, which has resulted in incision of the Squamish River channel above the fan.