Depositional Model of Point Bars in the Lower Wabash River

ABSTRACT A process-response model of point-bar sediments in the lower Wabash River of Illinois and Indiana was constructed by integration of (1) results (reported elsewhere) of repetitive hydraulic surveys of meander bends and (2) distributions of sedimentary structures and sediment size in Holocene alluvium. Vertical sequences of alluvium exposed in cutbanks and in point-bar trenches demonstrated preservation of bedform-texture patterns recognized from the hydraulic program. The depositional model is defined by distributions of three depositional facies around meander bends of disparate relative curvature. A fully developed facies assumed fully developed zones for current velocity and for bed-material size (velocity magnitude and mean size increasing from the inner bank toward the outer bank) A transitional facies incorporated transitional zones for both parameters (neither increasing outward from the inner bank). An intermediate depositional facies contained a fully developed velocity zone but a transitional size zone and was of significant extent only in gently curved bends. The latter two depositional facies differ greatly from standard facies models of meandering streams, constitute the bulk of surficial point-bar sediment in very sharply curved bends and in gently curved bends, and can be preserved preferentially by common types of bend migration. Preservation of the depositional facies of each meander bend depends upon the type of deformation (migration) the bend underwent. Limited observations from other freely meandering streams suggest that the Wabash depositional model may be widely applicable.