Fluvial style

Rivers patterns (fluvial styles) have been classified in the classical paper by Leopold & Wolman (1957) as braided, meandering and straight. This classification is essentially based on the shape of the river channel, as it appears on aerial photographs. Later, Smith (1983) added anastomosed rivers as a distinct fluvial style.

Figure 1 Braided river: the Slims River in Kluane National Park, Yukon, Canada (photo by H.J.A. Berendsen).

Braided rivers (Figure 1) are characterized by multiple channels that are often laterally unstable. Channels enclose bars, consisting of sand or gravel. Meandering rivers (Figure 2) typically have one channel that is sinuously curved.

Figure 2 Meandering river: Williams River in Alaska (photo by N.D. Smith).

Straight rivers (Figure 3) have a low sinuosity index (the ratio: channel length/distance across the channel belt). They generally lack the typical features of meandering rivers, like point bars. Anastomosing rivers (Figure 4) are characterized by multiple channels that are laterally stable. The channels enclose flood basins.

Figure 3 The Waal River in the Netherlands is originally a low-sinuosity meandering river. It has become 'straight' as a result of human interference (meander cutoff, construction of groynes). Photo by H.J.A. Berendsen.
Figure 5 Classification of fluvial styles by Makaske (1998). Anastomosing channel patterns may consist of individual channels that can be braided, meandering or straight.
Figure 4 Anastomosing river: the Columbia River in British Columbia, Canada (photo by H.J.A. Berendsen).

This subdivision in four distinct fluvial styles should be seen as end-members in a continuum of fluvial channel patterns (Miall 1996). Fluvial style may change in a downstream direction, and also depends on stage. Makaske (1998) showed, that anastomosing channel patterns may consist of individual channels that can be braided, meandering or straight (Figure 5).

Figure 6 The Mackenzie River in northern Canada is on a large scale characterized by anastomosing channel patterns. Individual channels are braided, meandering, straight or anastomosing. This casts some doubt on attempts to characterize channel patterns, based on only a few variables such as discharge and gradient as shown in Fig. 7 and 8.
Figure 7 Channel pattern primarily depends on discharge, sediment load and slope (Bridge 1996, 2003).

In the real world, many intermediate fluvial styles occur (Figure 6). Attempts have been made to 'predict' fluvial style, based on an understanding of the controls that determine channel type (Van den Berg 1995). Meandering occurs in all fluid systems as a result of turbulence, internal shear and bank and bed friction. As a result, straight channels tend to develop meanders. Observations and measurements of modern rivers, as well as experiments in flumes, have shown that the channel pattern is primarily dependent on discharge, sediment load, and slope (Figure 7), although discharge and slope seem to be the most important (Figure 8). Figures 7 and 8 have conceptual value only, and cannot be used to predict fluvial style (Van den Berg 1995).

Figure 8 Channel pattern plotted in a discharge versus slope diagram (Smith 1993).

In the Rhine-Meuse delta, all the different fluvial styles can be recognized (Figure 9). During the Late Weichselian braided rivers prevailed (Figure 10). These changed to meandering in the beginning of the Holocene, when discharge decreased, and sediment load became less and contained more fines. In the western part of the delta, the rapid rise of sealevel led to an anastomosing channel pattern, consisting of predominantly straight channels. A characteristic of this channel pattern is the abundance of crevasse splays. Crevasse splays are formed as a result of breaches in the natural levees during floods (Figure 11). They resemble dike breach deposits.

Figure 9 Alternation of fluvial styles in the Rhine-Meuse delta (Berendsen & Stouthamer 2001).
Figure 10 Late-Weichselian braided river deposits occur in the eastern part of the Rhine-Meuse delta (after Pons, 1966, from Berendsen 2005).
Figure 11 Crevasse splay of the Columbia River, Canada (left, picture by H.J.A. Berendsen) and the Mississippi River (right, picture by T.E. Törnqvist).

An important question is, whether fluvial style can be recognized in the fossil record, in other words: are the end members of fluvial style accompanied by a distinct sedimentation record? In the Rhine-Meuse delta this seems to be the case, but whether this is universal, is still a matter of debate. In general, it seems to be impossible to infer fluvial style from a single core (Bridge 2003). However, if different fluvial styles occur in the same area, and the channel pattern can be mapped, then it appears that the distinct fluvial styles have different geometries of the channel sandbodies (Figure 12), and straight anastomosed channels are characterized by complicated overbank deposits (Figure 13, Weerts 1996).

Figure 13 Block diagram of an anastomosing river system, and cross section, showing complicated overbank deposits (Weerts 1996).
Figure 12 Channel pattern and sandbody geometry. Different fluvial styles are associated with different sandbody geometries (after Törnqvist).

In the Alblasserwaard, the alternation of fluvial styles can also be recognized in the deposits (Figure 14): the Late Weichselian braided river deposits consist of a wide sandbody, without intercalation of clay. On top of this, anastomosed river deposits occur, characterized by narrow sandbodies and complicated overbank deposits, consisting of sandy and silty clay (Figure 14). These deposits were formed by the Schaik and Gorkum systems (Figure 14). They are overlain by a thick peat layer. At the surface floodbasin deposits occur, consisting of clay. The clay layer thins laterally from the narrow meandering Spijk channel belt.

Figure 14 Fluvial deposits in the Alblasserwaard are associated with changes in fluvial style (Weerts 1996).

Literature

  1. Berendsen, H.J.A. (2005), Fysisch-geografisch onderzoek - Thema's en methoden. Fysische geografie van Nederland. Assen: Koninklijke Van Gorcum. Vierde, geheel herziene druk, met CD-ROM.
  2. Berendsen, H.J.A., & E. Stouthamer (2001), Palaeogeographic development of the Rhine-Meuse delta, The Netherlands. Assen: Van Gorcum. 270 p.
  3. Bridge, J.S. (2003), Rivers and floodplains. Forms, processes and sedimentary record. Oxford: Blackwell Publishing.
  4. Makaske, A. (1998), Anastoming rivers - Forms, processes and sediments. Netherlands Geographical Studies 249, 287 p. KNAG/Faculteit Ruimtelijke Wetenschappen Universiteit Utrecht.
  5. Miall, A.D. (1996), The geology of fluvial deposits. Berlin: Springer Verlag, 582 p.
  6. Pons, L.J. (1957), De geologie, de bodemvorming en de waterstaatkundige ontwikkeling van het Land van Maas en Waal en een gedeelte van het Rijk van Nijmegen. 's Gravenhage: Verslagen van Landbouwkundige Onderzoekingen 63.11, Ph.D. Thesis, Wageningen. Bodemkundige Studies 3.
  7. Pons, L.J. (1966), De Bodemkartering van het Land van Maas en Waal en een gedeelte van het Rijk van Nijmegen. Verslagen van Landbouwkundige Onderzoekingen 646, Wageningen: Pudoc. De Bodemkartering van Nederland, deel 22, Wageningen: Stiboka.
  8. Smith, D.G. (1983), Anastomosed fluvial deposits: modern examples from Western Canada, In: Collinson, J. and Lewin, J., Eds. Modern and ancient fluvial systems: Oxford: Blackwell (Special Publication of the International Association of Sedimentologists v. 6, p. 155-168.
  9. Törnqvist, T.E. (1993), Fluvial sedimentary geology and chronology of the Holocene Rhine-Meuse delta, The Netherlands. Netherlands Geographical Studies 166, 169 p. KNAG/Faculteit Ruimtelijke Wetenschappen Universiteit Utrecht.
  10. Van den Berg, J.H. (1995), Prediction of alluvial channel pattern of perennial rivers. Geomorphology 12, p. 259-279.
  11. Weerts, H.J.T. (1996), Complex confining layers. Architecture and hydraulic properties of Holocene and Late Weichselian deposits in the fluvial Rhine-Meuse delta, The Netherlands. Netherlands Geographical Studies 213, 189 pp. KNAG/Faculteit Ruimtelijke Wetenschappen Universiteit Utrecht.