Influence of loss of gradient from postglacial uplift on Red River flood hazard, Manitoba, Canada

The north-flowing, low-gradient section of the Red River in Manitoba has lost ∼60% of its valley gradient since 8 ka cal. BP. An existing hydraulic model of the modern Red River flood zone was used to examine the change in flood extent and depth of a discharge equivalent to the 1997 Red River flood (3970 m³/s) for scenarios of gradients at 8, 6, 4 and 2 ka cal. BP as well as 2 ka in the future. The modelling indicates a broad, shallow flood zone for all of the gradient scenarios, with extent and depth increasing over time. Between the 8 ka cal. BP and present-day scenarios, the flood zone increased from 1186 km² to 1531 km² (∼29%) with depth increasing along four east-west cross-sections by 0.69 m (∼61%), 0.91 m (∼82%), 0.56 m (∼64%) and 0.48 m (∼86%). The flood extent and depths increased by a further 18 km² (∼5%) and 0.04-0.06 m (2-5%), respectively, by 2 ka in the future. Most of these changes to the flood zone occurred between 8 and 2 ka cal. BP, reflecting an exponential loss of gradient. A rise in flood depth equivalent to that which occurred between 8 ka cal. BP and the present-day, is assessed as increasing the long-term flood hazard; in contrast, the slight rise in depth between the present-day and 2 ka in the future does not.