Effects of a 'Natural' Flood Event on the Riparian Ecosystem of a Regulated Large-river System: the 2011 flood on the Missouri River, USA

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Large Infrequent Disturbance, Restoration, Dams, Populus Deltoides, Cottonwood, Floodplain Forest, Vegetation Change


Flow regulation has significantly altered hydrological, geomorphic, and ecological processes on the Missouri River. Cumulative effects are evident in declines in cottonwood (Populus deltoides) recruitment and in altered forest age structure and composition. Record runoff in 2011 exceeded reservoir capacity on the Upper and Middle Missouri, leading to a 500-year recurrence interval flood in terms of volume, with the highest peak discharge in 59 years and flood durations of up to 3 months. We assessed the effects of this ‘large infrequent disturbance’ by comparing pre-flood (2006–2009) to post-flood (2012) changes in riparian forests along two unchannelized segments of the Missouri River. Live shrub and tree density declined sharply within young forest stands (<30 years). Higher proportions of non-native (Elaeagnus angustifolia) and upland (Juniperus virginiana) trees showed evidence of recent mortality than did the native cottonwood. Sandbar area increased sharply from 2006 to 2012 and area of young forests declined, with particularly sharp declines in sapling stands that had established following the previous post-dam record flow releases in 1997. Cottonwood recruitment was widespread in 2012, but nearly all seedling patches occurred on sandbars in the active channel rather than on overbank sites, with moderately high (61–77%) seedling mortality over 2012–2013. Physical and operational constraints within the regulated Missouri River limited the restorative effects of the 2011 flood and will likely limit future forest recovery. Process-based riparian restoration would require restoring flow and sediment regimes that more closely mimic historical conditions, as well as overcoming the physical legacies of decades of flow regulation.

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