Morphosedimentary features from a major flood on a small, lower-sinuosity, single-thread river: The unknown quantity of overbank deposition, historical-change context, and comparisons with a multichannel river

Abstract

This study details overbank sedimentation on a single-thread, bedload stream, the Little Blue River (southeastern Nebraska and northeastern Kansas, USA), which experienced exceptional floods during May–June 2015. Downvalley elongate splays left deposits as thick as 1.2m and attained areas as great as 156ha. Splays consisted of fields of large, gravel-covered dunes 0.1 to 0.3m in height and extensive sand sheets with current ripples or current ripples atop low-relief, long-wavelength dunes. Erosion occurred chiefly in the heads of splays. Levee-floodplain channel complexes were more architecturally complicated, with channel–channel-fringing deposits deposited within ~110m of the banks of the channel. These complexes consisted of two amalgamated components: (1) sediment shadows (vegetation-induced sedimentary structures), as long as 6m and as high as 0.6m at their heads, which were produced by the interaction of direct overbank flow with rooted trees; and (2) indistinct, flat-topped bars (some of which were built around eroded remnants of sediment shadows) shaped chiefly by downvalley flood flow. Changes on in-channel bars, the erosion of bank crevasses as large as 70m in width, and the deposition of large woody debris also occurred during the floods.The earliest aerial photographs (1938) of the study area, taken after multiple low-flow years, show no evidence for recent overbank deposition. Increases in channel-belt area, the number of bars, and the sizes of bars in the Little Blue River took place after 1938, and most rapidly during 1938 to 1950, when annual discharge was increasing. Annual discharge has decreased since ca. 1990, and yet there has been a long-term increase in annual peak from since the 1930s. We surmise that increasing annual peak flows have led to larger floods and, since 1938, serial overbank deposition, as well a wider channel and numerous, large compound bars.