Flood Frequency Variability During the Past 80 Years in the Slave River Delta, NWT, as Determined from Multi-Proxy Paleolimnological Analysis

A paleolimnological approach was employed to reconstruct variations in the frequency of spring break-up flooding in the Slave River Delta during the past ~80 years based on multi-proxy analyses (geochemistry, diatoms, plant macrofossils) of a sediment core from a shallow, flood-prone lake in the active delta. Results reveal oscillating decadal-scale intervals of high and low flood frequency. The post-1960 reconstruction of Slave River flood frequency parallels variations in measured Slave River discharge and corresponds to the flood history derived from observations of land users. Notably, the interval of lowest water levels inferred from a peak sedimentary abundance of Sagittaria cuneata seeds pre-dates upstream regulation of the Peace River in 1968. Multi-proxy records reveal that the onset of river regulation coincided with a period of increased flood frequency beginning in the early 1960s and ending in the early 1980s. It is therefore unlikely that river regulation is the primary factor causing declines in the frequency of spring break-up floods at the study site. Furthermore, the flood record developed in the Slave River Delta parallels similar changes in flood frequency from an oxbow lake in the northern Peace sector of the upstream Peace-Athabasca Delta. This suggests that climate-driven change in the runoff regime of the upper Mackenzie River Basin is likely the principle driver of variability in flood frequency in both deltas. Continued reductions in snowpacks and headwater runoff are therefore likely to reduce the frequency of flooding in the Slave River Delta.

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