Expansion of woody vegetation on a Missouri River reservoir delta‐backwater

Construction of dams in the mid‐20th century reduced channel dynamism, sediment transport, and regeneration of riparian trees on the Missouri River. Opportunities for forest regeneration, however, may occur where the Missouri River or tributaries deposit sediments in reservoir headwaters, forming deltaic and associated upstream backwater areas. One such delta‐backwater occurs at the confluence of the Niobrara and Missouri rivers upstream from Lewis and Clark Lake in southeastern South Dakota and northeastern Nebraska. We investigated spatiotemporal patterns of tree recruitment through vegetation sampling in 2017–2018, dendrochronology, and analysis of aerial photography from 2006 to 2016. We sampled woody vegetation in 47 plots and took tree‐ring samples from one to two trees within each plot to determine establishment dates. Woody vegetation was dominated by Populus deltoides (cottonwood) and Salix amygdaloides (peachleaf willow) in the tree layer and Salix interior (sandbar willow) in the shrub layer. Tree establishment occurred primarily in 2010–2012 and was distributed throughout the study area, including at the delta‐reservoir interface. LiDAR data showed that most of the delta‐backwater experienced a 0.5‐m increase in elevation in 2005–2011, a period that included high flows in 2010 on the Niobrara River and 2011 on the Missouri River. Woody vegetation decreased 8% from 2006 to 2012, but doubled from 2012 to 2016, colonizing areas of previous sandbar and herbaceous vegetation. These woodlands represent early successional habitat that is in decline elsewhere along the river. Understanding vegetation dynamics within reservoir delta‐backwaters may be vital for forecasting changes in regulated riverine landscapes, assessing biodiversity values of these ecosystems, and informing sustainable reservoir management.