t news and views River meanders in a tray

Gary Parker Meanders are a feature of some of the world's noblest rivers, but laboratory models have failed to meander convincingly. A new experiment succeeds, promising us a refined understanding of this familiar but vexing phenomenon.

bench, has been dragged along by the pens that an energetic amateur succeeds migrating river bend for centuries, Inwhere the pursuit specialists of science have it repeatedly sometimes failed. hap inscribing its half-moon signature on the Such is the case with the experimental repli floodplain. cation of river meandering on a small scale The Fly River is an alluvial river — that reported by Charles E. Smith in the October is, one in which the bed and banks have issue of Geomorphology1. been freely constructed by the interaction As seen from the air, meandering between flowing water and sediment. Well- rivers offer a feast for the eye (Fig. 1). The established theory suggests that meander Figure 1 The meandering Fly River, Papua New bends describe a smooth, orderly spatial ing is an instability phenomenon, so that Guinea (flow from top to bottom). The near oscillation that never quite repeats itself. any deviation from the straight state is self- absence of any human development on the Although the Fly River shown in Fig. 1 perpetuating2. And yet a crucial part of the floodplain means that the sedimentary structures created by channel migration can be appears to be frozen in time, a closer look theory, the mechanism that maintains reveals the history of its leisurely migrations channel coherence, is only imperfectly clearly seen. Note the nearly parallel successions across the surface of its floodplain. understood. of long, gently curved lines known as scroll bars that mark previous channel banks, the Repetitive rows of lacey lines on the Meandering alluvial rivers typically have abandoned oxbows created where bends have floodplain, known as scroll bars, mark well-vegetated floodplains that are rich in been cut off, and the short crescent features previous positions of the channel banks. fine-grained cohesive sediment. The self- associated with the concave bank bench visible Meandering rivers do not seek some reinforcing combination of fine sediments ideal sinuosity or amplitude2; instead, and vegetation seems to make the flood- as the indentation in the sharp bend in the lower the bends migrate laterally and down plain resistant to erosion, and helps right of the photograph. As discussed here, stream, interacting with their neighbours. slow down the rate of lateral erosion at Smith1 has created tiny relatives of these characteristics of meanders in his garage. Eventually a bend can become so tight that the outside of a bend to the point that it is cut off and left on the floodplain as a deposition at the opposite inside bend can (Photograph courtesy W. E. Dietrich; channel relict oxbow lake that gradually fills with keep pace. In this way the river channel can width is on the order of 250 m.) sediment. Particularly remarkable in Fig. 1 maintain its coherence as it migrates. In the is a semicircular indentation on the up absence of floodplain stabilization, a river depicted in Fig. 2. Indeed, Murray and stream, outside bank ofa sharp bend. The often devolves into the less coherent (but Paola3 have proposed that braiding is the indentation, known as a concave bank equally interesting) braided state, as default morphology of alluvial streams, and that meandering occurs only when braiding tendencies are suppressed. Many researchers have attempted to model meandering in the laboratory at reduced scale. The bends form readily and spontaneously, only to be followed by channel widening and a devolution to the braided state before the bends achieve appreciable sinuosity. Until now, the best attempt4 involved the use of clay as an agent to stabilize sediment deposits on the inside of bends, but even in this case the bends were only mildly sinuous. Smith is a software specialist who devotes his spare time to the modelling of meanders in a 'river tray' in his garage. His meandering channels have a width of only about 4 cm. They evolve from a straight alignment to channels that are as sinuous as any seen in nature. Concave bank benches, bend cutoffs and features analogous to scroll bars form readily in spite of the vast Figure 2 The braided Aichilik River on the north slope of the Brooks Range, Alaska, USA (flow from difference in scale and the facjjhat the flow top to bottom). This channel form contrasts with the meanders shown in Fig. 1. (Photograph is barely turbulent;' this suggests that the courtesy C. Paola; the width of the braided valley flat is on the order of 500 m.)

NATl'RI- VOL395 lOSI-I'TI-MHER 1998 ..lews and views

role oft^biilenee-m--meander mechanics is constant width. Smith's technique offers for encodes Sonic hedgehog (Shh) do not less critical than previously supposed. the first time an experimental analogue that develop midline structures of the neural Although bend shape is somewhat different essentially reproduces this behaviour of its plate, leading, at worst, to profound from that seen in the field, the experimental own accord. It thus opens a window into cyclopia. A single, medial eye is located meanders are clearly close relatives of field study of the interaction of riverbank above a nose containing a single nostril". meanders. The key to the success of the dynamics with meander dynamics. But although Shh is necessary for experiments seems to be the use of a Now, in the mind's eye sprinkle the specification of the neural plate, other mixture of dia_ojnaceous_eaiih (a natural floodplain of the Fly River with the various results indicate that it may not be enough. sediment rich in diatom skeletons) and engineering works of human civilization — Cloning of the genes responsible for two kaolinite clay as the model sediment. This bridges, ports, roads, pipeline crossings — cyclopic zebrafish mutant phenotypes now mixture provides just the degree of that are constructed under the misunder suggests that members of the transforming cohesion that allows stabilization of the standing that the river does not move. If growth factor-fj (TGF-3) family may also deposits on the inside of bends while Smith's approach can teach us how rivers be involved in these processes. slowing but not preventing bank erosion. move, it can help the engineer better design The TGF-3 superfamily is a large group Further study of these small bends for and live with that eventuality. of secreted polypeptide growth factors. should tell us more about how a much Gary Parker is at the St Anthony Falls Laboratory, Mouse embryos containing two mutated larger meandering stream, such as the University of Minnesota, Mississippi River at Third copies of the TGF-|3-related gene nodal Fly River, maintains its integrity as a Avenue, Minneapolis, Minnesota 55414, USA. cannot form mesoderm6. Likewise, mice highly sinuous, single-thread channel as it e-mail: parke002@tc. utnn.edu with mutations in the Smad2 gene, which is wanders across its floodplains. In existing 1. Smith, C. E. Geomorphology 25, 19-30 (1998). predicted to transduce signals downstream theoretical treatments of meandering, 2. Parker. G. & Andrews, E. D. /. Fluid Merit. 162, 139-156 of an as-yet-unidentified nodal receptor, (1986). deposition at the inner bank is simply 3. Murray, A. B. 8c Paola. C. Nature 371, 54-57 (1994). gastrulate abnormally and lack mesoderm assumed to keep pace with erosion at the 4. Jin, D. 8< Schumm, S. A. in lit Int. Geomorphology Conf. (ed. (Fig. I)7,8. In a particularly insightful outer bank of a bend, so as to maintain Richards. K. S.) 680-691 (Wiley, Chichester. 1986). experiment, Nomura and Li' previously analysed mice that were heterozygous for Developmental biology mutations in both nodal and Smad2. One-third of the embryos had profound Casting an eye over cyclopia cyclopia, suggesting that nodal signalling Patrick Blader and Uwe Strahle may also be required to specify the midline of the neural plate. But for more clues about this, we must shift to the mutants National Academy of Sciences, provide an identified in the zebrafish. had a single, central eye. Today, cyclopia insight into the mechanisms that establish Zebrafish embryos with mutations in Inis ancient an extreme Greek characteristic mythology theof a cyclopsclass of the ventral regions of the brain and spinal the cyclops (eye) gene have defects in human birth-defect syndromes known as cord. specification of the ventral brain and holoprosencephaly. Although the morphol Cells in the mesoderm layer at the floorplate. As the name suggests, these ogical abnormalities that underlie holo midline of the embryo secrete signals that defects result in a cyclopic phenotype". prosencephaly are well characterized — a establish midline identity in the overlying Sampath et al.2 and Rebagliati et aV now lack of, or defect in, development of the neural plate. Among these signals are show that the eye gene encodes a TGF-3 ventral brain — the biochemical defects members of the hedgehog (Hh) family of signalling molecule with 69% identity to that underlie this malformation have proteins, which can induce a variety of the mouse nodal protein". Moreover, remained elusive. But the analysis of cells, including the neural midline. This Feldman et al.] show that a second, milder cyclopic zebrafish (Danio rerio) mutants by region gives rise to the ventral brain of the cyclopic mutation, squint (sqt), is an Feldman et aV and Sampath et al.2 (on embryo, and the floorplate and motor enhancer of the eye phenotype. sqt/eye pages 181 and 185 of this issue), and by neurons in the spinal cord*1. Mouse double mutants lack mesoderm, sqt Rebagliati et al} in Proceedings of the embryos with mutations in the gene that encodes another nodal-related protein1. As with mouse nodal, eye and sqt are expressed at the correct place and time, in keeping Floorplate Floorplate with a possible role in controlling the fates Hedgehog A A Ndr Hedgehog* of midline cells in the neural plate1,210. Interestingly, eye mutants lack the floor Midline Midline Ndr plate but develop motor neurons9, mesoderm mesoderm suggesting that the Shh signalling pathway ModeH Model2 is still active. Because mutations in either Midline the Shh or nodal signalling pathways lead Figure 2 Alternative models of how nodal and to defects in the ventral neural tube, Figure I Structures formed during nodal-related factors (Ndr) may act in perhaps nodal modulates the target-cell embryogenesis. Early embryogenesis is specification of the floorplate. a, Ndr secreted response to Shh signals. Thus, whereas characterized by a series of morphogenetic from notochord and prechordal plate cells act both Cyc and Shh would be required to movements, known as gastrulation, which directly in combination with proteins of the induce the floorplate, Shh alone would be establishes the three primary germ layers — the hedgehog family on floorplate precursor cells, enough to induce motor neurons. ectoderm, mesoderm and endoderm. At the end b, Ndr act indirectly in floorplate specification. However appealing this model is, it of this process, the mesoderm (the future The function of the Ndr is confined to the axial leaves some questions. The evidence that muscles and bones) comes to rest sandwiched mesoderm (notochord and prechordal plate), a nodal-like signal needs to be received between the ectoderm (the future nervous where they may be involved in processing of by the neural plate for induction of the system) on the outside of the embryo, and the hedgehog signals or in controlling the expression floorplate and ventral-brain fates remains endoderm (the future gut) on the inside. of additional, as-yet-unidentified signals. circumstantial. Mesoderm at the anterior 112 NATURE I VOL 395110SF.PTEMBER 1998 X

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