Late Holocene history of Squamish River north of Brackendale, British Columbia

Laminated silt, organic-rich silt, and peat exposed in the banks of Squamish River north of Brackendale, British Columbia, provide evidence for a lake impounded behind Cheekye Fan during the late Holocene. The lake gradually filled with sediment as Squamish River advanced its delta and floodplain southward toward the fan. Radiocarbon ages on detrital and growth-position plant fossils provide evidence that the lake reached up to 9 km upstream of Cheekye Fan about 3400 years ago and persisted until about 2300 years ago. Geomorphic observations and the distribution of fine-grained facies indicate that both Cheakamus and Squamish rivers contributed sediment to the lake; consequently, different depositional environments existed near Cheekye Fan than farther upstream. Debris flows that travelled down Cheekye River to Cheakamus River provided fine-grained sediment that accumulated in the deepest part of the lake just north of the fan. Farther upstream, organic-rich silt was deposited in fens and marshes surrounding the lake. Squamish River is confined by steep banks of cobble-boulder gravel 1.5–2 km north of the present Cheakamus-Squamish confluence, suggesting that coarse sediment transported by Cheakamus River controlled the lake outlet and, accordingly, lake level. As sediment delivery from Cheakamus and Cheekye rivers declined through the late Holocene, the lake outlet was progressively lowered and the lake drained. Sedimentation in Squamish Valley upstream of Cheekye Fan is also influenced by landslides at Mt. Cayley, 45 km upstream. A large landslide at Mt. Cayley about 1100 years ago produced a sediment pulse that propagated downstream and caused the floodplain above Cheekye Fan to aggrade. Sand buried the remaining fens that covered part of the low-gradient floodplain, leading to their replacement by coniferous and riparian forest. The model that I use to interpret Squamish River sedimentary environments can be applied to other low-gradient floodplains influenced by alluvial fans.