Meander degeneration in bed-load proximal streams: repeated chute cut-off due to bar-head gravel accretion - a hypothesis.

Abstract

  In the upper Bóbr- and Strzegomka drainage basins (Middle Sudetes) recent streams flow mostly in meandering channels natural reaches of which provide conditions permitting the investigations into modern fluvial processes. All rivers studied are proximal, bed-load meandering streams somewhat entrenched (0.3-3.5 m) in deforested, loamy, valley floor alluvia. The slope of reaches investigated varies from 0.003 to 0.01. In flood, the streams were able to transport boulders, sand, silt, clay, large trees, and smaller plant fragments. Generally, any flood may develop supercritical flow over most of reaches studied. It has been found that typical of gentler-sloped valleys are well-developed meandering patterns, while in steep-sloped ones a more complicated (mixed) pattern usually exists including alternating meandering-, straight-, and braided reaches. It has been also found that old-stege meanders of the regular meandering patterns tend to grow in phase faster than in amplitude, which results in neck cut-off degeneration of meander loops. In contrast, in steep-sloped valleys meandering channels frequently degenerate through the process of repetitive chute cut-off. It seems that at least in the reaches studied the process of chute cut-off degeneration is governed by the rate and direction of overgrowth of bar head gravel. The process results in the deflection of thalweg towards a low, inner bank (and relatively fine-grained point bar sequences), where a new chute channel is being soon eroded by-passing the gravel jam. The process of chute cut-off degeneration may be very rapid compared with that leading to the formation of a new meander. Generally, the degeneration may be regarded as indicative of instability of meandering channels under conditions of coarse bed-load and high water- and sediment discharge. Pebble fabric as an additional method in studying the processes of bar head gravel accretion as well as fossil counterparts of cut-off channels are also briefly discussed.