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The Impact of Valley Confinement from Growth Processes and Previous HitMagnitudeNext Hit of Point-Bar Type Reservoirs; Missouri River

Gregory W. Kliem, John M. Holbrook, James L. Blankenship, Bianca Leddy, and Anthony Rios
Earth & Environmental Science, University of Texas Arlington, Arlington, TX

The Missouri River Valley east of Kansas City, Missouri is characterized by an increase in valley width and transition from bedrock to alluvial control (4 to 10km width), followed by abrupt valley narrowing and bedrock constriction to 7.25 km width. The reach offers an opportunity to examine the variability of meandering behavior and resultant reservoir Previous HitmagnitudeTop under conditions of varying constriction. We mapped alluvium of the reach utilizing aerial photographs and 159 hand-auger boreholes (10cm sampling). The fluvial deposits include active (mixed sand/mud) and passive (clay) low-permeability channel-fill deposits averaging 200 meters wide and 5 meters deep; that compartmentalize large bar deposits composed of fine-to-medium-grained reservoir-quality sand. Where valley width first expands downstream there is an increase in lateral migration rate and resultant meander amplitude increases. Downstream meanders encounter abrupt valley narrowing becoming 50% larger and tighter, making hairpin-type meander loops (1 km wide by 3 or more km long) as meanders pile against the constriction and shrink through the choke point. In addition, meander migration rate approximately doubles. At initial valley widening beyond constriction bar size expands from several hundred meters back to kilometer-scale lengths as sinuosity recovers. Meander migration and abandonment increase substantially from even the prior doubling (≥20 m/year), generating at least 5 apparent loop abandonments in the last 1500 years. The increases in meander rate and amplitude on either side of the constriction, and the resultant changes in reservoir geometry are interpreted to record increase in local sediment supply as a result of valley constriction and changes in the free movement of sediment. Similar variations in reservoir geometry may be possible where valleys are noted to constrict from subsurface data.


AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas