Ecological Changes of the Lower Niobrara River Valley and Lewis and Clark Lake Delta from Past to Present

Over half of the world’s large river systems are impacted by dams. These may prevent sediment flow downstream, resulting in accumulation in the upstream reservoir. On the Missouri River, deltas and associated backwater-affected areas occur in nine locations across six reservoirs. One of these, the Lewis and Clark Lake delta-backwater, is influenced by sediment inputs from both the Missouri River and the Niobrara River. While the extension of the prograding delta into the reservoir has been well documented, there has been little research on upstream (backwater) effects of the delta on the lower Niobrara River. This study examined ecological changes on the Lewis and Clark delta-backwater and lower Niobrara River, particularly the riparian vegetation response to long-term (1985-2020) and recent (2017-2020) geomorphic changes in this area, with special emphasis on the effects of the 2019 Spencer Dam breach flood. Remote sensing was used to analyze historical changes in land cover in the prograding Lewis and Clark Lake delta (LCD), the backwater-affected area upstream on the Missouri River (LCB), and on the lower 62 km of the Niobrara River (NIO1-NIO3). Forest area increased by 198%, grassland decreased by 10%, and the area of coniferous woodland (principally red cedar, Juniperus virginiana) increased by 146% from 1985-2020 across the study area. Historically, hydro-riparian vegetation has increased in the lowlands and coniferous/red cedar has increased in the uplands. More recently (2017-2020), however, grassland increased by 20% and bare ground decreased by 64% across the study area and the area of forest declined sharply in the lowlands of NIO3, LCD, and LCB. Field vegetation sampling indicates that hydro-riparian vegetation, like willows and cottonwoods, have established on areas of the lower Niobrara River near the confluence, particularly over the last 30 years. Sediment deposition of up to 4 m has occurred over the last 65 years near the confluence, with net aggradation extending at least 14.5 km upstream on the lower Niobrara River. Operation of Gavins Point Dam and the resulting sedimentation have altered the lower Niobrara River and Lewis and Clark Lake ecosystems temporally and spatially by setting into motion major sedimentation and vegetation changes