Connecting Fluvial Levee Deposition to Flood‐Basin Hydrology

Levees are commonly found along every kind of river system, yet there are no widely accepted models for where along the channel they form and what controls their shape. In this study, we investigated whether levee growth is driven by sediment transfer from the channel adjacent to the levee or by inundation dynamics in the flood basin. To test these ideas, we conducted empirical analyses and numerical modeling of levees on the fine‐grained, meandering Muscatatuck River, IN. Using LiDAR data, we found no statistical relationship between the levee and the adjacent channel planform, which suggests levees are not genetically related to their adjacent channel. On the contrary, modeling experiments of a simplified Muscatatuck River show that levee initiation can be genetically related to the adjacent channel because bed shear stress on the floodplain is low where channel curvature is high. But after levees initiate, the genetic connection to the adjacent channel is obscured because levee shape is modified by inundation dynamics. For instance, tall mature levees are not inundated regularly and instead obstruct floodplain flow, creating flow shadows on the downstream side. Sediment is preferentially deposited in the flow shadow, which moves the location of maximum deposition from the levee crest to the toe. This causes levees to prograde down‐valley, which reshapes the levee and genetically disconnects it from the channel. We propose that this morphodynamic mechanism of levee growth is characteristic of fine‐grained rivers in narrow floodplains where flood basins can act as conveyance channels that transport sediment down‐valley before deposition. Key Points Empirical analyses of LiDAR data on the Muscatatuck River, Indiana, USA, show that levee size and shape are unrelated to channel planform Numerical modeling reveals that levee initiation is related to the channel planform but subsequent growth is influenced by inundation dynamics in the flood basin On tall levees, the levee toe grows faster than the levee crest because it is inundated more frequently, and this causes levees to prograde down‐valley over time