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)(2007)(2008)(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.

In 2011, a large, long‐duration flood occurred on the regulated Missouri River following six decades without flooding. This study evaluated the effects of the flood on riparian forest structure and composition. In 2012, 168 forest sites sampled in 2006–2009 were resampled on five floodplain segments between Montana and Missouri, with 80 sampled again in 2013–2014. Changes in riparian forest area over 2006–2012, by age class and segment, were assessed using aerial imagery and GIS. Repeated‐measures analysis of variance was used to examine (a) the initial effects of the flood (preflood to 2012) on tree and shrub stem densities, (b) postflood (2012–2014) changes in stem density, and (c) species‐level responses for cottonwood (Populus deltoides), eastern red cedar (Juniperus virginiana), and Russian olive (Elaeagnus angustifolia). Across the study area, forest area declined 6–38% among age classes, with the greatest declines in the youngest classes. Tree density declined 19–49% across segments from preflood to 2012 but did not change significantly from 2012 to 2014. Shrub density declined 52–89% across segments from preflood to 2012, with 73–78% declines in the two youngest age classes, but increased by 42% from 2012 to 2014. Cottonwood and Russian olive, but not red cedar, showed partial recovery in the shrub/sapling layer, increasing from 2012 to 2014. Although flooding is important for floodplain forest health, the 2011 flood had mixed effects, with significant mortality of native floodplain trees and shrubs and only limited cottonwood recruitment. The initial decline in invasive species (Russian olive and red cedar), however, suggests that flooding may be an effective management tool.