Distinguishing flood frequency and magnitude in the morphodynamics and sedimentology of rivers: insights from the South Saskatchewan River, Canada

The impact of a 1 in 40 year flood on the morphology and sedimentology of the sandy braided South Saskatchewan River, Canada was assessed. Comparison of 2004 - 2007 repeat GPR surveys and the production of DEMs of difference allowed quantification of the initial and long-term 2005 flood impact on reach morphology and sedimentology. Main results show that even though a significant initial morphological impact was caused due to the flood through net erosion and channel incision across Bar A, subsequent low-magnitude high-frequency floods were able to rework morphology due to the ability to transport the medium sized sand bed load. In the subsurface, no distinct flood signature has been left, as flood deposits are similar to the scale and composition of deposits produced by low-magnitude high-frequency floods. Consequently, little evidence of such a flood event will be preserved in the sedimentary record. The research has also highlighted some important findings with respect to linking morphological processes to sedimentary deposits. In particular they have suggested the revision of depositional models for braided rivers, and further research on the relationship between bedform geometry and flow depth in natural rivers. The results have wider applications to other sand bed braided rivers and may aid interpretation and modelling of such deposits on a wider scale