Evolution of the Moose Lake Delta, British Columbia: implications for Holocene environmental change in the Canadian Rocky Mountains

Reconstruction of the geomorphic history of the Moose Lake delta is used to infer environmental change in the uppermost Fraser River watershed, British Columbia. Delta-top changes are interpreted from surface channel morphology, riverbank exposures, hand auger data, vibracores and other borehole records. River channel and overbank sediments are related to source area contributions in terms of granule lithology and fine (<63 μm) fraction geochemistry. A substantial area of the northeast delta-top floodplain dates from the early Holocene, and is characterized by fragmentary, low-discharge paleochannels with bed material characteristic of the uppermost Fraser River sub-basin. Floodplain development since ca. 4.0 ka BP has been limited to the western and southern areas of the delta and records persistently higher sediment loads, aggradation and larger, more active channels. These changes reflect overall shifts from warm, dry Hypsithermal to cooler, wetter Neoglacial climatic conditions defined regionally. The composition of both laterally and vertically accreted floodplain sediments plus channel morphology and floodplain stratigraphy record the changing influence of partially glaciated source areas on the Holocene sediment flux. Specifically, periods of climatic deterioration and possibly glacier expansion lead to relatively greater yields of carbonate-rich sediment from the higher elevation Moose River sub-basin. Storage and reworking of sediments on the Moose Lake delta-top floodplain influence interpretation of the more distal lacustrine record. Over the last 4000 years, gradual increases in lacustrine sedimentation rate of up to ∼18% are inferred to result from delta progradation.