Declaration of competing interest
The lower 800 km of the Missouri River has been incising for decades. However, the 2011 flood scoured more sediment than the previous 12 yr combined. The river rebounded in the following two years, re-depositing over 70% of the sediment scoured during the 2011 flood. Historic evidence suggests that past Missouri River floods may have followed similar scour-rebound patterns. This paper analyzes 16 channel surveys from 1987 to 2014, including 50,723 total cross sections to document the morphological history of the lower 800 km of the Missouri River over the last thirty years, including flood disturbance and long-term trends. The last thirty years on the Missouri River included two basic morphologic regimes: steady incision during low-to-moderate flow periods and rapid scour-rebound responses to floods. Surveys collected during the 2011 flood demonstrate that the river scoured throughout the flood (including the falling limb) and did not begin a rebound phase until after the flood. The analysis also identifies sediment sinks associated with each of these scour regimes (flood scour and incision between floods) that exceed the total volume eroded from the bed during those periods. Results suggest that floodplain deposition may induce the supply limitation that drives flood scour on this large sand-bed river. Additionally, aggregate mining removes substantially more sediment than the total river incision between floods and the reaches with maximum scour during the 1993 and 2011 floods correspond to the zones of aggregate mining.
Flood disturbance and recovery are critical components of river morphology (Schumm, 1976; Wolman and Gerson, 1978; Lewin, 1989; Knighton, 1998; Simon and Rinaldi, 2006; Phillips and Dyke, 2016) and ecology (Resh 1982; Ward and Stanford, 1983; Lake, 2000). Fluvial form and lotic communities reflect the influence of periodic, powerful events separated by extended periods of low-to-moderate flows. Rivers respond to disturbance events in a variety of ways, on multiple time scales. Additionally, river responses to disturbance-recovery cycles can be complicated by a variety of natural and anthropogenic influences driving long-term incision (e.g., Downs et al., 2013). Parsing transient disturbance-recovery impacts from long-term morphological trends can be particularly challenging on large sand-bed rivers, where noise from rapidly-migrating bedforms can drown out the signal of actual morphological change at individual cross sections. Moreover, remote sensing technologies such as LiDAR, photogrammetry, and structure from motion cannot detect morphological change under meters of turbid water. Currently, only underwater sonar can measure bed shape and change in these deep, turbid systems, which is time consuming and costly over large spatial domains.
Because of the difficulty of large scale morphological studies on large sand-bed rivers, these systems are underrepresented in morphological literature (Hudson, 2002), including the disturbance-recovery literature, despite growing suspicion that their morphologic behaviors differ from those documented in smaller rivers (Latrubesse, 2008). Repeated surveys on the Missouri River present an opportunity to investigate the relative importance of disturbance-recovery cycles and long-term incision on a large sand-bed river.