Flooding impacts and status of riverine mesohabitats, channel complexity, and habitat turnover along the Missouri River

Background/Question/Methods

The magnitude of the 2011 flood on the upper Missouri River surpassed any flows in the last 59 years and the duration of the flood (>2 months in many locations) is likely unprecedented in the recent history of the river. This unusually long duration, in combination with cumulative prior effects of long-term flow regulation and flood control on the riverine ecosystem, could lead to major impacts to floodplain habitats. The purpose of this study is to examine the flooding impacts and status of riverine habitats and metrics of channel complexity and habitat turnover for non-reservoir segments of the Missouri River. We interpreted and digitized channel and floodplain features for 2006 and 2012 county mosaic orthophotography (NAIP) for 7 segments of the Missouri River (Fort Peck, MT to Kansas City, KS). Total area of each mesohabitat (e.g. sandbars, backwaters, islands, etc.) and wetted perimeter and area were calculated for each segment and time frame studied in ArcGIS (ESRI). Turnover rates were calculated and compared with historical turnover rates as a way to measure potential effects by flooding. This flooding-impact study is unprecedented in its geographic extent, covering 1,315 kilometers (815.3 miles) of the Missouri River (35%).

Results/Conclusions

Overall, there has been an increase in mesohabitat area along all study segments of the Missouri River between 2006 and 2012. The deposition of sand throughout the Missouri River resulted in an increase in sandbar area of 12 to 22 hectares per kilometer for all segments except for the lowest channelized reach of the Missouri River. Channel degradation/incision possibly led to an overall decrease in wetted area within the inter-reservoir segments (2-14%). However, an increase in wetted area (16-26%) did occur along the channelized segments, but this was mostly due to an increase in floodplain lakes resulting from levee breaching (1-1.6 ha/km). Aquatic mesohabitat area did increase (0.5 to 3.8 ha/km) within the inter-reservoir segments and segment 10 which is relatively natural and free flowing and designated as a National Wild and Scenic River. Wetted perimeter increased (4-34%) for the channelized segments, but varied for the other river segments. The 500-yr flood in 2011 caused erosion and deposition of sediment and creation of new habitat along the Missouri River resulting in an overall increase in riverine heterogeneity which may influence species recruitment, richness, and abundance.