Abstract

The Ottawa River Basin (ORB) experienced extreme spring floods in recent years. To improve flood management, an understanding of long-term flood frequency is necessary. Currently, the instrumental discharge data is short and geographically sparse. This thesis aims to fill this gap by reconstructing multi-centennial flood histories using oxbow-lake sediments. To produce such a reconstruction, sediment cores were collected from oxbow lakes of the ORB: Half Moon and Scullion (Noire River), and Brach (Rouge River). Sediment cores were studied using X-ray fluorescence (XRF), magnetic susceptibility (MS), 14C and 210Pb dating. Results show that detrital input dominates sediment composition. Site-specific geochemical proxies detected flood sediments: Si/Ca at Lac Half Moon, K/Ca at Scullion Lake, and K/Si at Lac Brach. At Lac Half Moon, floods were more frequent during ~1750-1800 CE (Little Ice Age and pre-industrial periods) than in the current warming, suggesting that cooler climates and modern regulation may influenced flood frequency. Correlation with reconstructed summer temperatures was positive, while that with climate oscillations (AMO, ENSO, NAO) was weak, showing that large-scale climate oscillations may modulate background conditions rather than directly causing floods. Scullion Lake experienced a high flood frequency during ~750-850 CE (Dark Ages Cold Period). Although flood activity in Lac Brach was relatively stable, floods were slightly more frequent during the Little Ice Age. These results show that floods in the ORB may be affected by catchment characteristics and sediment dynamics. Therefore, generalization should be avoided, and multiple proxies across the basin should be used to understand long-term flood patterns.