DOES A HISTORICAL FLOOD IN AN INTRA-DAM RIVER REACH MAKE A DIFFERENCE? A CASE STUDY OF THE 2011 FLOOD IN THE GARRISON REACH OF THE MISSOURI RIVER (Invited Presentation)

Research focused on inter-dam river reaches is relatively new and unique as interactions between the upstream dam and downstream reservoir exhibit particular longitudinal channel behavior and responses to disturbances. This study analyzes the impact of the exceptional flood of 2011 on the Garrison Reach of the Missouri River. The flood was the largest and longest duration flood on the Missouri River since flow regulation. We analyzed pre- and post-flood cross-sections and longitudinal large wood distribution along the Garrison Reach, and post flood aeolian sand transport on three sandbars and islands. Long-term trends (1948 to present) in cross sectional area indicate that the upper and middle sections of the reach approached a new steady state approximately 30 years after the upstream and downstream dams were constructed. The channel gained capacity (degraded) near the upstream dam but lost competence (aggraded) near the slackwaters of the downstream reservoir, forming a delta. In-channel large wood (LW) distribution showed no clear trend before reaching the effects associated with low velocity near the delta, where LW accumulated. Aeolian sand transport, while not historically measured, likely decreased in most of the reach due to the vegetation of previously bare sandbars and islands. Field observations indicated that sand transport increased in the downstream end of the reach as new sandbars were created near and in the reservoir. The flood of 2011 increased channel capacity through most of the reach with substantial deposition on bar, islands, and the floodplain in the middle to lower reach. These surfaces experienced heightened aeolian sand transport for approximately a year before vegetation established. The in-stream LW population more than doubled with the majority of wood accumulating in the delta. Most flood impacts, including sediment flux, wood flux, and storage of both components, was muted after 2013 as annual dam-regulated processes and revegetation obscured evidence of the flood. The results of this study indicate that channel trends created by the inter-dam regime were only temporarily disrupted by this historic flood (if at all). Future restoration of inter-dam reaches will require extraordinary efforts to change the altered state of such rivers to a preferred state.