MEASURING THE ABUNDANCE AND DIMENSIONS OF NATURAL RIVER LEVEES ACROSS THE CONTIGUOUS USA WITH 3DEP LIDAR

Natural levees form at the channel margin during overbank floods as a product of sediment exchange between the river channel and the adjacent floodplain. However, levees do not form on all rivers, and it remains challenging to predict levee location and size. Factors typically considered to be important for levee formation—like flood regime or channel sinuosity—can vary bend to bend and season to season, rendering generalization across floodplains challenging. To better understand the conditions that promote levee growth across many river bends, we evaluated levee abundance and dimensions at n > 500 randomly selected river reaches, using USGS 3DEP lidar data available across the contiguous USA. These reaches span a range of environments, climates, and land-use practices, and we have also collected data to quantify the river valley dimensions, meandering rate, and discharge variability. We take a functional approach to classifying levees: any sediment deposit within one channel width of the channel margin that is also higher in elevation than the surrounding floodplains, is labelled as a levee. We also take a reach-averaged approach. Levee existence or absence is determined by averaging floodplain elevations relative to the river channel and averaging over a river reach ≥ 10 channel widths long. We hypothesize that levee abundance will scale with three main parameters: discharge variability, floodplain width, and lateral migration rate. Rivers with highly variable flow may generate swift overbank flows that disperse sediment across the floodplain, preventing accumulation close to the channel. Floodplain width also affects overbank flows. Narrow floodplains fill quickly with floodwaters, diminishing flow velocity across the channel margin. Channels that migrate rapidly across their floodplains abandon their levees while the levees are still small, preventing substantial channel-margin deposits from accumulating.