The Calamus River lies within the Sandhills Region of Nebraska (see location map). In order to develop a three-dimensional facies model for low-sinuosity river deposits, Bridge, et. al (1986) studied a low sinuosity reach of the Calamus River.

In order to document facies in the modern river channel, detailed maps of channel bed topography were made using plane table and alidade, and channel bed sediments were sampled using a box-corer. Box coring allowed for examination of sediment textures and sedimentary structures. Older Calamus River deposits are preserved in abandoned channels below the river floodplain. These sediments were sampled by vibracoring; cores were extracted using tripod and chain hoist.

Based on sieve analysis of channel bed sediment samples, a map showing the mean grainsize of bed sediments was produced. Coarsest sediments are found adjacent to cutbanks on the outside of slightly curved channel segments. Mean grainsizes are finest on the downstream ends of midstream islands and point bars.

Map and air photo studies of the study reach showed that several bends had migrated laterally and then undergone chute cut-off. Chute channels remain active for decades, creating cutoff islands. Studies of abandoned channels shows that the upstream ends are higher and drier than downstream ends. The upstream ends of cut-off channels fill in with deposition from bedload and, in some cases, from deposition on scroll bars that migrate into the entrance of the cut-off channel. Deposition on the downstream end of the cut-off island helps fill in the downstream end of the cut-off channel. Eventually, the upstream end of the cut-off channel fills completely. Filling of the abandoned channel is then accomplished mainly by deposition of suspended material (mainly organic debris in the Calamus) that enters the channel during floods, and by slumping of material from the cutbank of the abandoned channel.

These observations allowed Bridge, et. al (1986) to construct a three-dimensional facies model for Calamus River deposits. This is shown in three cross-sections, the first through the upstream end of an abandoned channel and adjacent active channel, the second through the middle portion, and the third through the downstream end. In the figure the arrows above each cross section indicate paleocurrent orientations in the deposits. The cross-sections show bedding geometry (lateral accretion surfaces) produced as the original curved channel segment migrates to the left. Following cutoff, the bend begins to fill with coarser-grained bedload deposition in the upstream end and the downstream-most part of the bend. Much of the middle and downstream parts of the bend fill much more slowly by deposition of suspended sediment (mainly organic debris in the Calamus; shown as black on the figure) and to a lesser extent, from slumping of the cut bank. The channel filling deposits fine upward. Meanwhile, the flow is taken up by the active channel. This channel may migrate to the right, producing the lateral accretion deposits shown.

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Reference: Bridge, J.S., Smith, N.D., Trent, F., Gabel, S.L., and Bernstein, P. (1986) Sedimentology and morphology of the low-sinuosity Calamus River, Nebraska Sandhills. Sedimentology, v. 36, pp. 851-870.

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