Sixty years of channel adjustments to dams in the two segments of the Missouri National Recreational River, South Dakota and Nebraska

First posted September 20, 2022 For additional information, contact: Director, Columbia Environmental Research CenterU.S. Geological Survey4200 New Haven RoadColumbia, MO 65201Contact Pubs Warehouse The Missouri National Recreational River (MNRR) consists of two segments managed by the Park Service on border South Dakota and Nebraska. Both river are unchannelized maintain much their pre-dam channel form, but upstream dams have caused reductions in peak flow magnitudes sediment supply. 39-mile segment is located between Fort Randall Gavins Point Dams, transitioning from a riverine process domain to distributary delta headwaters Lewis Clark Lake. 59-mile segment, an entirely domain, downstream Dam, most main dam River, highly altered navigation extending more than 1,000 kilometers St. Louis, Missouri. seeks preserve outstandingly remarkable natural, cultural, recreational values MNRR. There particular need understand bank-erosion processes guide management decisions related controls.Changes shape, as measured topographic cross sections surveyed every 5–10 years since mid-20th century, document bed incision (bed-elevation lowering) domains, mix aggradation delta, Channel greatest where mean thalweg (deepest point section) 3.5 meters, net greater 5 meters was observed at section 93 Dam. Analysis also indicates that rates bed-elevation change 1960 were lowest Rates higher lower near Dam confined bank revetment both banks has coarsened substantially completion dam. Several large floods recent decades, including post-dam record flood event 2011, scoured deposited high-elevation sandbars segments, especially segment. cross-sections 2011 substantial erosion deposition, low magnitude considerable lake. Surveys taken after indicate trend rearrangement recovery following floods, with highest parts bed, sandbars, eroding lowering while deepest channel, which increased elevation.Inundation modeling results narrower valley percentage plain being inundated flooding relative wider valley. Likewise, capacity resulted modern corridor inset into flood-plain surface. flood, rarely inundated. boundaries over time channel-migration five times larger Bank primarily been post-1894 deposits; comparably very zones isolated local hot spots used establish predictions for 10 20 future based past movement MNRR segments. Long-term planform trends adjustment slowing may be approaching new equilibrium, spatial variability contribute uncertainty. Additional monitoring morphology would needed confirm this analysis.