Long–term trends and status of riverine mesohabitats, channel complexity, and habitat turnover along the Missouri River from Fort Benton‚ MT to Kansas City‚ KS from 1890 to 2006

Background/Question/Methods

Human modifications to the Missouri River have decreased the river’s heterogeneity, limited habitat variability, and greatly affected ecosystem processes.  In addition, nutrient cycling and ecosystem benefits associated with the terrestrial-aquatic interface and moving littoral zone may be greatly diminished with altered flow regime and the disconnection of the river with its floodplain.  The purpose of this study is to examine the long-term trends 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 the 1890s maps (MRC), 1950s aerial photography (USDA), and 2006 county mosaic orthophotography (NAIP) for 8 segments of the Missouri River (Fort Benton, 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 to compare turnover from 1890s to 1950s with the turnover rates of 1950s to 2006 as a way to measure potential impacts by flow regulation and human alteration.  This study is unprecedented in its geographic extent, covering 1,566 kilometers (973 miles) of the Missouri River (42%).

Results/Conclusions

Overall, there has been a substantial decrease in mesohabitat area along all study segments of the Missouri River between the 1890s and 2006. Unvegetated sandbars exhibited the highest rates of decrease ranging between 67-100% for all segments. Aquatic mesohabitat area, wetted perimeter, and wetted area greatly decreased within the channelized reaches (74-87%, 48-63%, 44-52% respectively), but varied among the inter-reservoir reaches.  Land to water (erosion) and water to land (accretion) turnover rates have both substantially decreased across all study segments (48-87%) except for a 59-mile reach of the river which is relatively natural and free flowing and designated as a National Wild and Scenic River.  The channelized segments exhibited the highest decrease in turnover rates (84-87%).  The decreases in turnover rates indicate that the dynamic nature of the Missouri River is fading.  The natural physical processes that have sustained the spatial and temporal heterogeneity of this river have been diminished by human alteration and management.