2022 CSDMS meeting-044

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Marsh deposition has a first-order control on channel properties and kinematics in an experimental river delta

Kelly Sanks, (she/her),Tulane University New Orleans Louisiana, United States. ksanks@tulane.edu

Low-lying regions of river deltas (marshes, swamps, and tidal flats) are an important part of the delta life cycle. Marsh sedimentation is characterized by non-riverine, low-bulk density material, and interacts with the riverine sediment delivered by distributary channel networks. These two forms of sedimentation interact to produce the channel properties and kinematics observed in the system. Here, we aim to understand this interaction by comparing two physical delta experiments, one with marsh deposition (treatment) and one without (control). We show that the addition of the marsh proxy (kaolinite) alters the channel properties and kinematics of a river delta. Notably, the channels are longer in the treatment experiment and the shoreline roughness is enhanced. The treatment channels have the same width from the entrance to the shoreline, while the control channels get narrower as they approach the shore. Flow is concentrated in the channels in the treatment experiment, as it has about one-fourth the amount of overbank flow as the control experiment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the aerial portion of the delta top, a phenomena observed in global deltas. However, in the control experiment, the channel beds generally exist above relative sea level. The channels in the treatment experiment have a slope break, on average, which is also seen in some global deltaic rivers (e.g., Mississippi River Delta). The difference in channel properties created by the addition of the marsh proxy in the overbank region helps offset the channel bed aggradation rates in the treatment experiment, as the overbank region aggrades faster (relative to the channel) than in the control. This difference in in-channel and far-field aggradation shows a longer channel in-filling time for the treatment as compared to the control. Although the lateral channel mobility statistics are similar in both experiments, there is more area on the delta top in the treatment experiment that is rarely visited by a channel. This suggests that the marsh proxy may buttress the channels in the treatment experiment. Ultimately, marsh sedimentation on the delta top plays a key role in the channel properties and kinematics of a river delta, producing channels which are more analogous to channels in global river deltas, and which cannot be produced solely by increasing cohesion in an experimental river delta.