CUMULATIVE GEOMORPHIC PERTURBATIONS LEAD TO IN INCREASED FLOOD HAZARD, MODERN LOWER MISSOURI RIVER

Recent flooding on the Lower Missouri River – especially in 2011 and 2019 – owes much of its severity to ongoing geomorphic adjustments to river management that began in the 19^th century and accelerated through the mid 20^th century. Despite being downstream from the largest reservoir system in North America, this recent flooding has resulted in billions of $US in damages. The long-term geomorphic perturbation caused by channelization and diminished sediment supply due to trapping in reservoirs has created a low-conveyance zone along approximately 200 km of river between Omaha, Nebraska and St. Joseph, Missouri. Increased water-surface elevations for a given discharge in this constricted segment have increased the frequency of overbank flooding and associated levee breaks. Energy concentrated through levee breaks results in acute geomorphic perturbations involving deep erosion of agricultural lands and extensive deposition of sand splays advected from the main channel. The geomorphic set-up to increased flood hazard due to this history of geomorphic perturbations has been exacerbated by more recent patterns of floods that have either overwhelmed the storage capacity of the Missouri River reservoir system (as in 2011) or have occurred downstream of the reservoir system (as in 2019). Recognition of the history and geography of cumulative perturbations on large rivers may provide insights into alternative flood-risk reduction strategies under an increasingly uncertain future.