Flooding, Structural Flood Control Measures, and Recent Geomorphic Research along the Red River, Manitoba, Canada

Abstract

The Red River, Manitoba, Canada, is a low-gradient, meandering river that traverses the broad, flat Red River Valley on the north-eastern portion of the Great Plains of North America. The shallow stream-cut valley occupied by the river has insufficient capacity to contain large discharges, which allows higher magnitude flows to overtop the valley sides and spread up to 40 km across the adjacent clay plain. Major flooding impacts communities and rural areas, including the City of Winnipeg, and has caused significant flood disasters in the nineteenth and twentieth centuries. Since 1950, an array of structural flood control measures has been constructed (and some later upgraded) to mitigate flooding, including two diversion canals, a flood control dam, dyking (linear and ring dyking), and elevated earthen pads under structures. Multidisciplinary research initiated following the 1997 Red River flood provided a geomorphic context to the flood problem in support of decision making towards enhancing the flood-protection infrastructure. Based on flood signatures in the growth rings of bur oak trees (Quercus macrocarpa Michx.), the historic flood-of-record in 1826 is interpreted to be the largest Red River flood since at least 1648. An assessment of the decrease in river gradient arising from regional differential uplift revealed that the broad, shallow flood character is intrinsic to the landscape of the Red River Valley and that the contemporary rate of uplift is causing an insignificant change to the extent of flooding. An investigation of the evolution of the genetic floodplain indicates that fluvial geomorphic processes are not significantly enlarging or infilling the shallow stream-cut valley at rates relevant to altering the modern flood problem. Although flood management along the Red River is heavily dependent on structural measures, the design discharge of the integrated flood control works protecting Winnipeg has recently been enhanced to an up to 700-year return period, which reduces the flood hazard substantially.