MORPHOSEDIMENTARY EFFECTS OF EXCEPTIONAL DISCHARGE IN AN ENTRENCHED, LOW-SINUOSITY, SANDY TO GRAVELLY RIVER IN SOUTHEASTERN NEBRASKA, USA

May-June 2015 high-stage flows on the Little Blue River (LBR)—a major flood followed by two lesser flows—set a record for daily mean discharge, modified channels and bars, and led to rare overbank deposition of sand and gravel. The LBR is a small, single-thread stream entrenched by ~4 m and dominated by bank-attached alternate bars, although there are a few meanders with point bars. During the 2015 events, unit bars with 10-40 cm-high fronts migrated extensively over the alternate bars, and elongate pools a few meters in length developed at bar-to-bank attachment points as unique depositional environments. Drapes of sand were deposited locally on the steep channel walls. Multiple crevasses, 6-70 m in width, were eroded in the banks of the LBR; some of these crevasses developed funnel-like walls of large woody debris on their channel-ward sides. Large (≤ 85 cm in height) dunes of coarse sand and gravel migrated upward out of the channel on ramped surfaces extending through the crevasses. Sand and gravel were then transported onto the floodplain by flows that attained ~4 m in depth during the first event. Overbank deposition, locally exceeding 1 m, produced: (1) elongate (≤ 50 m in length) chute splays extending downstream from bank crevasses, along poorly-defined, and mostly nonerosive, temporary chute channels, and (2) channel-fringing “washover aprons,” which formed along the floodplain edge of narrow riparian woods by virtue of deposition from: (a) local bank-overtopping flood flow trending perpendicular to net downstream flow, and (b) overbank flood flow trending parallel or subparallel to net downstream flow. Chute splays consist chiefly of sand to gravel, overwhelmingly in the form of pebble and cobble-topped, sinuous-crested to lunate dunes 12-40 cm in height. Little mud was deposited in these settings by the first major overbank-flood event, and mud drapes within chute splay deposits are minor and very thin. Nevertheless, thicker (as much as 13 cm in a wooded meander neck) and more prominent mud drapes were deposited by waning flow in the subsequent flow events. “Washover aprons” are distinctive, complexly stratified, amalgamated deposits of bank-perpendicular sediment shadows (behind riparian trees) and downstream-migrating dunes. Our results provide a new perspective on coarse-grained overbank sediments.