The nature and timing of postglacial valley fill incision, Fraser River, Big Bar to Watson Bar, British Columbia

Since deglaciation, Fraser River through the southern British Columbia (BC) interior has undergone episodic aggradation and incision to create a series of distinct stepped terraces. This study employed optical dating to date different terrace levels in the Big Bar and Watson Bar reaches of Fraser River to calculate the rate of postglacial incision through glacial valley fill. This incision rate was developed to explore correlation of Fraser River incision with terrace-forming drivers such as climate, local base level change, and glacioisostatic adjustment. The oldest age in this study corresponds to the outburst flood of glacial Lake Fraser at 11.3 ± 1.5 ka, consistent with other independent ages for the event. From this event, Fraser River incised through ~180 m of glacial valley fill to reach its present-day level, where it now flows on bedrock. The average incision rate during the last ~11 ka was 15 mm/a, though this study speculates that during the last ~11 ka, Fraser River incision rates may have varied, roughly following postglacial climatic phases imprinted on the paraglacial cycle. For example, fast incision rates (30 m/a) were present through the cooler and wetter middle Holocene (7-4 ka) due to the reduction of upland paraglacial sedimentation and high flow power due to wetter conditions. Local aggradation resulted upstream of landslides and large paraglacial fans that temporarily increased local base level. Fraser River incised to bedrock sometime in the last 4 ka. This is the first detailed chronological study of postglacial Fraser River terraces.