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River meandersin a trayGary ParkerMeanders are a feature of some of the world's noblest rivers, butlaboratory models have failed to meander convincingly. A newexperiment succeeds, promising us a refined understanding of thisfamiliar but vexing phenomenon.

In the pursuit of science it sometimes happens that an energetic amateur succeedswhere specialists have repeatedly failed.

Such is the case with the experimental replication of river meandering on a small scalereported by Charles E. Smith in the Octoberissue of Geomorphology1.

As seen from the air, meanderingrivers offer a feast for the eye (Fig. 1). Thebends describe a smooth, orderly spatialoscillation that never quite repeats itself.Although the Fly River shown in Fig. 1appears to be frozen in time, a closer lookreveals the history of its leisurely migrationsacross the surface of its floodplain.Repetitive rows of lacey lines on thefloodplain, known as scroll bars, markprevious positions of the channel banks.Meandering rivers do not seek someideal sinuosity or amplitude2; instead,the bends migrate laterally and downstream, interacting with their neighbours.Eventually a bend can become so tight thatit is cut off and left on the floodplain as arelict oxbow lake that gradually fills withsediment. Particularly remarkable in Fig. 1is a semicircular indentation on the upstream, outside bank ofa sharp bend. Theindentation, known as a concave bank

bench, has been dragged along by themigrating river bend for centuries,inscribing its half-moon signature on thefloodplain.

The Fly River is an alluvial river — thatis, one in which the bed and banks havebeen freely constructed by the interactionbetween flowing water and sediment. Well-established theory suggests that meandering is an instability phenomenon, so thatany deviation from the straight state is self-perpetuating2. And yet a crucial part of thetheory, the mechanism that maintainschannel coherence, is only imperfectlyunderstood.

Meandering alluvial rivers typically havewell-vegetated floodplains that are rich infine-grained cohesive sediment. The self-reinforcing combination of fine sedimentsand vegetation seems to make the flood-plain resistant to erosion, and helpsslow down the rate of lateral erosion atthe outside of a bend to the point thatdeposition at the opposite inside bend cankeep pace. In this way the river channel canmaintain its coherence as it migrates. In theabsence of floodplain stabilization, a riveroften devolves into the less coherent (butequally interesting) braided state, as

Figure 2 The braided Aichilik River on the north slope of the Brooks Range, Alaska, USA (flow fromtop to bottom). This channel form contrasts with the meanders shown in Fig. 1. (Photographcourtesy C. Paola; the width of the braided valley flat is on the order of 500 m.)

Figure 1 The meandering Fly River, Papua NewGuinea (flow from top to bottom). The nearabsence of any human development on thefloodplain means that the sedimentarystructures created by channel migration can beclearly seen. Note the nearly parallel successionsof long, gently curved lines known as scroll barsthat mark previous channel banks, theabandoned oxbows created where bends havebeen cut off, and the short crescent featuresassociated with the concave bank bench visibleas the indentation in the sharp bend in the lowerright of the photograph. As discussed here,Smith1 has created tiny relatives of thesecharacteristics of meanders in his garage.(Photograph courtesy W. E. Dietrich; channelwidth is on the order of 250 m.)

depicted in Fig. 2. Indeed, Murray andPaola3 have proposed that braiding is thedefault morphology of alluvial streams, andthat meandering occurs only when braidingtendencies are suppressed.

Many researchers have attempted tomodel meandering in the laboratory atreduced scale. The bends form readilyand spontaneously, only to be followed bychannel widening and a devolution to thebraided state before the bends achieveappreciable sinuosity. Until now, the bestattempt4 involved the use of clay as an agentto stabilize sediment deposits on the insideof bends, but even in this case the bendswere only mildly sinuous.

Smith is a software specialist whodevotes his spare time to the modelling ofmeanders in a 'river tray' in his garage. Hismeandering channels have a width of onlyabout 4 cm. They evolve from a straightalignment to channels that are as sinuous asany seen in nature. Concave bank benches,bend cutoffs and features analogous toscroll bars form readily in spite of the vastdifference in scale and the facjjhat the flowis barely turbulent;' this suggests that the

NATl'RI- VOL395 lOSI-I'TI-MHER 1998

..lews and viewsrole oft^biilenee-m--meander mechanics isless critical than previously supposed.Although bend shape is somewhat differentfrom that seen in the field, the experimentalmeanders are clearly close relatives of fieldmeanders. The key to the success of theexperiments seems to be the use of amixture of dia_ojnaceous_eaiih (a naturalsediment rich in diatom skeletons) andkaolinite clay as the model sediment. Thismixture provides just the degree ofcohesion that allows stabilization of thedeposits on the inside of bends whileslowing but not preventing bank erosion.

Further study of these small bendsshould tell us more about how a muchlarger meandering stream, such as theFly River, maintains its integrity as ahighly sinuous, single-thread channel as itwanders across its floodplains. In existingtheoretical treatments of meandering,deposition at the inner bank is simplyassumed to keep pace with erosion at theouter bank of a bend, so as to maintain

Developmental biology

constant width. Smith's technique offers forthe first time an experimental analogue thatessentially reproduces this behaviour of itsown accord. It thus opens a window intostudy of the interaction of riverbankdynamics with meander dynamics.

Now, in the mind's eye sprinkle thefloodplain of the Fly River with the variousengineering works of human civilization —bridges, ports, roads, pipeline crossings —that are constructed under the misunderstanding that the river does not move. IfSmith's approach can teach us how riversmove, it can help the engineer better designfor and live with that eventuality.Gary Parker is at the St Anthony Falls Laboratory,University of Minnesota, Mississippi River at ThirdAvenue, Minneapolis, Minnesota 55414, USA.e-mail: parke002@tc. utnn.edu1. Smith, C. E. Geomorphology 25, 19-30 (1998).2. Parker. G. & Andrews, E. D. /. Fluid Merit. 162, 139-156

(1986).3. Murray, A. B. 8c Paola. C. Nature 371, 54-57 (1994).4. Jin, D. 8< Schumm, S. A. in lit Int. Geomorphology Conf. (ed.

Richards. K. S.) 680-691 (Wiley, Chichester. 1986).

Casting an eye over cyclopiaPatrick Blader and Uwe Strahle

In ancient Greek mythology the cyclopshad a single, central eye. Today, cyclopiais an extreme characteristic of a class of

human birth-defect syndromes known asholoprosencephaly. Although the morphological abnormalities that underlie holoprosencephaly are well characterized — alack of, or defect in, development of theventral brain — the biochemical defectsthat underlie this malformation haveremained elusive. But the analysis ofcyclopic zebrafish (Danio rerio) mutants byFeldman et aV and Sampath et al.2 (onpages 181 and 185 of this issue), and byRebagliati et al} in Proceedings of the

National Academy of Sciences, provide aninsight into the mechanisms that establishthe ventral regions of the brain and spinalcord.

Cells in the mesoderm layer at themidline of the embryo secrete signals thatestablish midline identity in the overlyingneural plate. Among these signals aremembers of the hedgehog (Hh) family ofproteins, which can induce a variety ofcells, including the neural midline. Thisregion gives rise to the ventral brain of theembryo, and the floorplate and motorneurons in the spinal cord*1. Mouseembryos with mutations in the gene that

Floorplate FloorplateHedgehog A A Ndr Hedgehog*

Midlinemesoderm

ModeH

Midline Ndrmesoderm

Model2Midline

Figure I Structures formed duringembryogenesis. Early embryogenesis ischaracterized by a series of morphogeneticmovements, known as gastrulation, whichestablishes the three primary germ layers — theectoderm, mesoderm and endoderm. At the endof this process, the mesoderm (the futuremuscles and bones) comes to rest sandwichedbetween the ectoderm (the future nervoussystem) on the outside of the embryo, and theendoderm (the future gut) on the inside.

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Figure 2 Alternative models of how nodal andnodal-related factors (Ndr) may act inspecification of the floorplate. a, Ndr secretedfrom notochord and prechordal plate cells actdirectly in combination with proteins of thehedgehog family on floorplate precursor cells,b, Ndr act indirectly in floorplate specification.The function of the Ndr is confined to the axialmesoderm (notochord and prechordal plate),where they may be involved in processing ofhedgehog signals or in controlling the expressionof additional, as-yet-unidentified signals.

encodes Sonic hedgehog (Shh) do notdevelop midline structures of the neuralplate, leading, at worst, to profoundcyclopia. A single, medial eye is locatedabove a nose containing a single nostril".But although Shh is necessary forspecification of the neural plate, otherresults indicate that it may not be enough.Cloning of the genes responsible for twocyclopic zebrafish mutant phenotypes nowsuggests that members of the transforminggrowth factor-fj (TGF-3) family may alsobe involved in these processes.

The TGF-3 superfamily is a large groupof secreted polypeptide growth factors.Mouse embryos containing two mutatedcopies of the TGF-|3-related gene nodalcannot form mesoderm6. Likewise, micewith mutations in the Smad2 gene, which ispredicted to transduce signals downstreamof an as-yet-unidentified nodal receptor,gastrulate abnormally and lack mesoderm(Fig. I)7,8. In a particularly insightfulexperiment, Nomura and Li' previouslyanalysed mice that were heterozygous formutations in both nodal and Smad2.One-third of the embryos had profoundcyclopia, suggesting that nodal signallingmay also be required to specify the midlineof the neural plate. But for more cluesabout this, we must shift to the mutantsidentified in the zebrafish.

Zebrafish embryos with mutations inthe cyclops (eye) gene have defects inspecification of the ventral brain andfloorplate. As the name suggests, thesedefects result in a cyclopic phenotype".Sampath et al.2 and Rebagliati et aV nowshow that the eye gene encodes a TGF-3signalling molecule with 69% identity tothe mouse nodal protein". Moreover,Feldman et al.] show that a second, mildercyclopic mutation, squint (sqt), is anenhancer of the eye phenotype. sqt/eyedouble mutants lack mesoderm, sqtencodes another nodal-related protein1. Aswith mouse nodal, eye and sqt are expressedat the correct place and time, in keepingwith a possible role in controlling the fatesof midline cells in the neural plate1,210.Interestingly, eye mutants lack the floorplate but develop motor neurons9,suggesting that the Shh signalling pathwayis still active. Because mutations in eitherthe Shh or nodal signalling pathways leadto defects in the ventral neural tube,perhaps nodal modulates the target-cellresponse to Shh signals. Thus, whereasboth Cyc and Shh would be required toinduce the floorplate, Shh alone would beenough to induce motor neurons.

However appealing this model is, itleaves some questions. The evidence thata nodal-like signal needs to be receivedby the neural plate for induction of thefloorplate and ventral-brain fates remainscircumstantial. Mesoderm at the anterior

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