From meander bend to oxbow lake: morphodynamics and sedimentology of chute cutoffs

Chute cutoffs are common features of meandering channels. The development of a chute cutoff locally shortens and straightens a meandering river channel, excavating a large volume of floodplain sediment as the chute channel deepens and widens. Bar development at the upstream and downstream ends of the cutoff channel leads to abandonment of the bend and formation of an oxbow lake, which, together with oxbow lake sedimentation, contributes to the production of a complex, highly three-dimensional floodplain sedimentary architecture. Moreover, the oxbow lakes that result from meander cutoffs enhance habitat diversity within riparian corridors. Thus, chute cutoffs play an integral role in the geomorphology, sedimentology, and ecology of the channel-floodplain system of meandering rivers. The primary objective of this thesis is to advance understanding of chute cutoff morphodynamics through a process-based interpretation of the co-evolution of morphology, flow structure, and sedimentology in chute cutoff channels prior to bend abandonment. This objective is achieved through a combined field and laboratory approach, using observations from a detailed field study of two chute cutoffs at Mackey Bend, Wabash River, IL/IN, USA, and results from laboratory experiments in a physical model of a chute cutoff system. Analysis of the morphologic evolution and sedimentology at the field site (Chapter 4) indicates that chute cutoffs undergo an initial phase of rapid widening, during which sediment is deposited at the cutoff mouth. This period of rapid chute widening gradually leads to a phase of bar deposition in the upstream limb of the bend and along the inner bank of the chute channel, and reorganization of sediment deposited at the cutoff mouth. The compound bars at Mackey Bend are constructed by deposition of unit bars and dunes, as well as deposition of fine grained sediment during periods of high backwater caused by high flow on the nearby Ohio River. Flow structure associated with the two chute cutoffs is characterized by: (1) deceleration of flow (and possible flow stagnation) in the main channel, moving from upstream to downstream past the entrance to the chute cutoff channel; (2) strong curvature of flow into the chute cutoff channel, which induces strong secondary circulation and advection of the core of high velocity toward the outer bank of the cutoff channel; (3) flow separation along the inner bank of the cutoff channel(s); (4) convergence of flows from the cutoff channel and the main bend at the downstream end of the chute cutoff channel (cutoff mouth), which promotes the development of counter-rotating helical cells in the converging flows; (5) flow stagnation and superelevation of the water surface at the upstream junction corner of the cutoff mouth; and (6) deceleration of flow, and possible flow separation, at the downstream junction corner of the cutoff mouth. The hydrodynamics of chute cutoffs were also investigated in a physical model with a rectangular channel geometry, simplified planform shape, and immobile bed and banks. In this physical model, the cutoff channel was equal to the width of the main channel. Experimental results show that the three-dimensional structure of flow is analogous to a scenario in which the upstream limb of the bend is completely plugged with sediment. In this situation, all of the discharge is routed through the cutoff channel, and a large, vertically-oriented gyre develops in the downstream limb of the bend, similar to the pattern of flow recirculation that occurs in a side embayment along an open channel. Areas of high turbulent kinetic energy are located along: (1) the shear layer between flow entering the cutoff channel and near-stagnant water in the upstream limb of the bend, (2) the boundary of the flow separation zone in the cutoff channel, and (3) the shear layer between flow moving out of the cutoff channel and stagnant, or slowly moving, flow within the downstream limb of the bend. Values of turbulent kinetic energy in the abandoned bend, particularly for the case in which most of the discharge is moving through the cutoff channel, are much lower than those in a meander bend with no cutoff channel. The integrated results from field and laboratory studies allow formulation of a model of chute cutoff morphodynamics, where the entrance and exit of a chute cutoff channel behave as a bifurcation and confluence, respectively, and bar development primarily occurs: (1) in the upstream limb of the main channel, (2) along the inner bank of the cutoff channel, and (3) in the mouth of the cutoff channel. Once the upstream limb of the bend is completely plugged with sediment and no flow moves through the bend, the side embayment model describes the morphodynamics of plug bar deposition, where sediment is entrained into the downstream limb of the bend by a large-scale gyre. The bend becomes an oxbow lake once the downstream limb of the bend is completely sealed with sediment

Decimeter-scale in situ mapping of modern cross-bedded dune deposits using parametric echo sounding (PES): a new method for linking river processes and their deposits

Collecting high-resolution data to quantify the sedimentary architecture within contemporary alluvial channels remains one of the outstanding challenges in fluvial sedimentology. Here, we present data collected using a new geophysical method, the parametric echo sounder (PES), which can meet this challenge. From surveys over a field of sand dunes in the Río Paraná, Argentina, we demonstrate the unique ability of PES to image the subsurface structure within active channels at a decimetric resolution. These data reveal the bounding surfaces between bars and dunes, as well as the foresets and reactivation surfaces within them. This provides quantitative in situ data for recent work that suggests cross-strata preserved by dunes may be more related to flow depths less than the commonly assumed bankfull level. These surveys demonstrate that PES can provide hitherto unobtainable data from alluvial channels, and presents significant opportunities for more detailed coupled studies of fluvial processes and their deposits