Location-dependentsediment sorting in gravellymegaripples from the Rio Grande, centralNew Mexico byDavid W. Love, JamesT. Boyle, Sylveen Robinson, and Mark Hemingway, New Mexico Bureau o{ Minesand Mineral Resources, Socorro, NM

Introduction the conveyancechannel, to determine water was clearand the tops of some of the An ongoing study of modern fluvial sed- whether a megaripples'sposition within a megarippleswere near the surfaceof the water iments in New Mexico has revealedthat grain- progressionmay be locatedby analyzing its when surveying and sampling were con- sizesorting in streamsis dependenton sample grain-size distribution, and to determine ducted (Fig. 3). Little was in sus- locationwithin portions of bedformsand on sorting mechanismstaking place along pension. Sand was transported only where bedform position within downstream pro- megaripples. megarippleswere erodedby flow in the thal- gressionsof similar forms. Theseresults have weg. Most parts of the megarippleswere no many potential geologicapplications. Description of channel conditions longer migrating downstream. One of the sets of bedforms studied is The conveyancechannel east of Socorrois Straight-crestedmegaripples with wave- straight-crested,gravelly megaripplesof the about 23 m wide at the top and 4 m deep, lengthsaveraging 2.8 m were measuredand Rio Grande conveyancechannel near So- with an averagegradient of 0.016 (Fig. 2). sampledalong a 30 m long reach where re- corro, New Mexico. Megaripplesare ripple- During the week of April 8, 1984,flow av- working by a low-flow thalweg had not al- shapedbedforms having wavelengthslarger eragedan estimated86 m'/sec(based on flow tered the bedform configuration. After than 60 cm and a range in amplitudeslarger estimationtechniques of Williams, 1978).On samplingwas completed,the reachwas sur- than ripples. In streams,megaripples form April 14,flow in the conveyancechannel was veyed using a Leitz TM 20 theodolite and a under lower-flow-regimeconditions and rhi- diverted back into the Rio Grande channel rod measuring to the nearesthundredth of gratedownstream by progressivedeposition upstream. During the next four days, the a foot (about 3 mm). The megarippleshad along their slip faces(Fig. 1). The purposes flow dropped to an estimated 3 m3/sec,so long, low-angle stoss sides, steep lee sides of this paper are to relate grain-sizedistri- that the channelwas approfmately 10m wide (at the angle of repose),and crestsabout 12 butions to the form and location of mega- and 0.2m deep.Flow was maintainedin part cm above the adjacenttroughs downstream ripples and troughs left by waning flow along by seepagefrom the banks. On April 19, the (Fig. a). No ripples occurred on the stoss

flow velocity zoneof no diff usion

crest oce toce interior 6i6s\de trough mobilebed ovolonchereverse reversef low fl ow FIGURE l-Anatomy of megaripples, water flow (in blue), and sediment movements (in black) involved in sorting processes Modified from Jopling (1963) and Reineck and Singh (1980). Index map shows approximate study location.

FIGURE 2-Low flow in conveyancechannel of Rio Grande near Socorro on FIGURE 3-Megaripples in conveyancechannel of Rio Grande. Thalweg in April 19, l9&1. Note tops of megaripplesnear water surface.Channel is lower middle portion of picture is reworking sand and obliterating mega- approximately10 m wide. nPPles.

May 1985 Nao MexicoGeolog" N-. tlow direction J IEVEI

O 5m ffi 2X verlicolexoggerolion

FIGURE 4-Longitudinal profile of megaripples and water surface TABLE l-Sieve aperture sizes in mm used in sediment analyies. sidesof the megaripples,except in the thal- laminae.Unfortunately, time and expensedid 1.00 weg/ where the megarippleswere 38.05 beginning not allow more sophisticatedtechniques for 26.67 0.85 to be reworked and fine sand was being sampling cross sectionsof sedimentary 18.35 0.71, transported.The exacttiming of depositioi structures(such as taking epoxy box cores) IJ. JJ 0.605 of the megaripples is not known, but prob- to be used. 9.52 0.500 ably the formation and preservation of the In the lab, the samples were dried and 8.00 0.430 bedformslagged behind flow conditions (cf. sievedusing sievesizes listed in Table1. The 6.30 0.355 Allen, 1982).The slip faces probably were entire samplewas sieved to a size of 1 mm 5.66 0.303 depositedjust beforeflow waned to the point in order to have an adequatesample of large 4.76 0.250 4.00 0.272 of no , whereas the in- clasts. Sampleswith abundant portions of 3.35 0.180 terior portions of the megarippleswere de- sand less than 1 mm were sDlit to about 50 2.83 0.150 posited during active transport conditions. g and sieved.Then the proportion of the 2.36 0.725 Although the timing and conditionsfor dep- total was calculated.Duplicate splits of some 2.00 0.707 osition of the megaripplesremain partially samnleswere sievedto test the adeouacvof 7.68 0.090 undetermined, the fact that they were pre- this procedure. 1.40 0-075 served during waning flow suggestsihat Mass-frequency diagrams were plotted 1.18 0.063 similar in geologic following Bagnold's (1941)procedures. In the 0.04s contextsmay be examined and interpreted diagrams that illustrate grain-size distribu- in a similar manner. tions (Figs. 5-9), both axes are logarithmic from unstream to downstream to illustrate Sample treatment becauseof the wide rangesin grain sizeand any lorigitudinal shifts. More subtle trends masspercentages. The irregularintervals be- are revealedby comparing adjacentparts of Sampleswere taken from the megaripples tween sieves are standardized by dividing by sampling portions of bedforms progres- megaripples/troughs for different popula- the mass percent in each size classby the tions/line slopes and by examining relative sively upstream so that the samples re- differencebetween logarithms of the diam- mainedundisturbed and uncontaminatedby percentagesof different parts of the popu- etersof the largestand smallestgrains in the lations. These differences and trends are sedimentsagitated during the samplingpro- size class.The diagramsshow the massfre- cess.Thus, summarizedbelow. troughs downstream from each quencyof different-sizegrains in sucha way megaripplewere sampledfirst, then the lower that proportions of one population may re- slip faces,upper slip faces,the armor at the Distributions from analogousportions of main similar even though other populations megaripples crestof the megaripples,and, finally, the in- are added or subtractedfrom the total in other terior portion of each megaripple. Not all parts of the distribution. In the olots that As shown in Fig. 5, troughs show a broad, megaripples within the sequencewere sam- iollow, somesieves caught consistently less skewedpeak between 0.25 and 8-10 mm, a pled, primarily becausethey were all very mass than the sieves adiacent to them be- decreasein grainslarger than about L0 mm, similarin overallcharacteristics. We were not causeof deviationsin aperturesize. For ex- a paucity of grains between 0.075and 0.180 convinced that differenceswould be found ample, the 0.5-mm sievenearly always had mm, and a secondpeak at 0.063mm. Grains between adjacent megaripples, and, there- less sand in it than did the 0.43-mm sieve, between 0.85 and I mm increasein mass fore, we expected only slight differencesbe- indicatingthat someapertures of the 0.5-mm percent downstream, whereas grains be- tween the upstream and downstream ends sieve are too large and let too many grains tween 0.355and 0.85 mm decreasein mass of the sequence.Where the megarippleswere through to the 0.43-mmsieve. These incon- percentdownstream. dominated by gravel, about 1 kg sampleswere sistencieshave not been correctedin the plots, Grains armoring the crest of megaripples taken in an areaof 200-400cm'. The upper- but are clearly apparent as a V-shaped have distributions somewhat similar to most few cm were scoopedbv hand into a depressionin the grain-sizecurves at 0.5 mm. troughs, becausethey have a generalbroad la,rgeplastic bag. The interiors of megarip- Somecurves are discontinuousat the coarse peak from 0.25to 8 mm and a small peak at ples were sampledby clearingaway a[l sur- end becauseno clastswere caught on some about 0.063 mm (Fig. 5). The broad peak face clasts from a broader area and then sieves. greaterthan 0.25mm tends to be flatter (more scooping the sample from the megaripple by of a plateau) than the broad peak of the hand. The dominantly sandy slip faceswere Results troughs, and it tends to show slightly lesser sampledusing film canistersthat hold about The grain-sizedistributions from the amounts of the coarserclasts (curve has a 40 g of sand when full; one to two canisters megaripplesdepend on where sampleswere negative slope at coarsersizes). Grains be- of sand were taken from each of the upper takenwithin eachmegaripple and associated tween 1.4 and 4.76 mm decreasein mass and lower slip faces. trough downstream, and on the location of percent downstream, whereas grains be- Sedimentsforming the megaripples and the megaripple and trough set within the tween 0.25and 1.4mm increasein massper- troughswere extremelyloosely packed. \A/hile sequence.The differencesin grain size are cent downstream, with the last megaripple walking acrossthe channel, a 55-kg (143lb) reflectedin the overall distributions at a given having a definite peak between 0.5 and L.18 person could sink up to 60 cm into the un- location within each megaripple, in trends mm (this peak may be due in part to sam- compactedsandy .Although slip downstream along each megaripple/trough pling the underlying megaripple interior). No faceswere present on the lee sides of the sequence,and in proportions of gravel frac- trend is apparentin the amount of fine sand megaripples,no internal sedimentarystruc- tions and various sand fractions within in- forming the peak at about 0.063mm. tures were seen during sampling, probably dividual megaripples and along the entire Samplesfrom the interiors of megaripples becauseslumping occuied immediaietyafter sequence.The grain-sizedata are organized show a similar broad peak from 0.25 to 6.3 sampleswere taken, and becauseof the lack in Figs. 5-9 for each location along mega- mm and a precipitousdecline in the amount of differentiation of grain sizes into discrete ripples/troughs,and the data are "stacked"

Nm Mexico Geology May 1985 o o o o c 2 c o N a N o o I o o o o o c c o c o o o o o- o- I E E E o o o o J J ) o.l roo Groinsize in mm Groinsize in mm Groinsize in mm FIGURES-Grain-size distributions of troughslo- FIGURE6-Grain-size distributions of crest armor FIGURE7-Grain-size distributions of megaripple cateddownstream from megaripples A, B, C, and of megaripples A, B, C, and J. For clarity, curves interiors. No curves have been offset. Curve A is J For clarity,curves B, C, and J havebeen offset B, C, and J have been offset vertically by the re- the same as that for crest armor A because no verticallyby the respectiveaddition of 0.3,0.6, spectiveaddition 0.3, 0.6, and 0.9 to their Y-val- separatesample was obtained. and0.9 to theirY-values. ues of sand less than 0.25 mm (Fig. 7). In com- parison with samples of crest armor, inte- o riors have fewer clastslarger than 6.3 mm. a) c There appears to be a decreasein fine sand c o peaks o N fraction downstream, but secondary N at 0.107or 0.063mm or both occurln all j'Jb ol o interior samples. o Upper slip faceshave a narrower range of o o grain sizes,commonly exhibiting a peak be- o o cn C tween 0.25and L mm (Fig. 8). From the up- o streammegaripple (J) to the next to the last o @ megaripple (B) in the sequence/the distri- bution shiftsto a finer peak (from 0.605-0.85 3 -r to 0.303-0.43mm), but the last megaripple E E (A) is similar to the first. Moreover, samples o ) ) from different parts of the upstream mega- -2 ripple (activeand inactive) are quite different oor o.l r ro roo (Fig. Groinsrze tn mm in proportions of coarseand fine grains "dffi"J.:lt 8: Ja, active,and Jb, inactive). FIGURE8-c,",^-,;" uppersrip FIGURE9-Grain-size distributionsof lower slip Lower slip faces are much better sorted facesof megaripplesA, B, C, andJ. CurvesA, B, facesof megaripplesA, B, C, andJ CurvesB, C, than either the troughs or the rest of the C, and fa arenot offset;Jb is offsetby 0.5Y-units and J are offsetvertically by the respectiveaddi- megaripple locations. They have a single, to avoidoverlap Jais from activeslip facein thal- tion of 0.2,0.4, and 0.6 to theirY-values. relatively narrow peak, slightly skewed weg;fb is from inactiveslip face 2 m from thalweg. toward the fine side (Fig. 9). The peak of the ferences downstream. The lower slip faces distribution shifts downstream from 1.18- arenot presentin slip faces.Secondary modes containprogressively less mass greater than 1,.4to 0.7t-0.85 mm. The amount of grains at 0.063mm also decreasefrom troughs to 0.605mm downstream, but other megarip- between0.063 and 0.71mm increasesdown- slip faces.In two of the progressionsfrom ple portions do not show significanttrends. stream,affecting the slopeof the fine side of lower slip facesto troughs, the troughs have the distribution, but no separatepeak of fine a deficiencyof clastsin the rangeof the peak Discussion sand occurs. Grains between 1,.4and 2.83 of the lower slip faces. Rather than merely comparing grain-size mm decreasedownstream, but the lastlower distributionsof adjacentsamples, discussion slip face (A) has an increasein granulesbe- Other locational indicators of the above results focuseson the sorting tween 3.35 and 4 mm. In comparing curves, it was noted that mechanismsresponsible for producing the points where two curves cross(termed cross- distributions and on interpreting the distri- Progressivedownstream changesin over points) may shift significantly depend- butions based on the proposed sorting distributions from adiacentportions ing on location within the megaripple mechanisms. of megaripples sequence.For example,the crossoverpoints Downstream progressionsalong individ- between crest armor and adjacent trough Sorting mechanisms ual megaripplesalso revealtrends in the fol- rangedownstream from 5.66mm atl to 4.76 Numerous factors determine the grain size lowing sequence:1) trough to 2) megaripple mm at C to 2.36mm at B to 1.4-1.68mm at and sedimentary structureswithin eachreach crest to 3) interior to 4) upper slip face to 5) A. Similarlv. the amount of mass in certain of a stream. In general, dominant grain size lower slip faceto 6) trough. As Figs. 5-9 show, sizeintervals may show trends downstream. along streambeds and characterof flow (such distributions becomebetter sorted and uni- As Table 2 shows, the mass of clasts larger as velocity, depth, stream powet and shear modal in progressionsfrom troughs to lower than 2 mm in troughs decreasesdown- stress) determine bedforms. For examPle, slip faces. The coarse modes (>8 mm) of stream,but the massof all clastslarger than stream beds with a preponderance of grains troughs decreasein crestsand interiors and 0.5 mm in troughs shows no significantdif- larger than 0.5-0.7 mm do not develop rip-

May 1985 Nar MexicoGeology TABLE 2-Examples of longitudinal ranges in weight percent of selectedlarge clast sizes from portions mobilebecause of equalshear stress to move *armor of megaripples along the conveyancechannel of the Rio Grande; and interior sampled together all sizes(cf. Andrews, 1983)or that the grains in megarippleA; b, only Jb, inactive upper slip face,represented here. were differentially exposed to shear stress size and placement of sur- percent > 2 mm percent > 0.6 mm in depending on (Parker Klingeman, megaripple in troughs troughs Iower slip upper slip armor interior rounding grains and 1982).The plateausof nearly equal amounts 75.3 83.3 99.8 a/.5' 83.7 77.4 of grains are much wider (8 mm is 32 times C 75.1, 91.3 97.7 72.8 75.4 65.6 as lig as 0.25 mm) than p,redictedby An- B 73.6 91.8 86.4 28.7 /6.J / /.2 drewi (roughly 14 times from 0.3xd50 to 83.0* 72.2 97.3 81.9 52.3 4.2xd50).This discrepancymay indicatethat the larger grains are moving under a nearly ples; rather, as flow is increased,such beds Interpretationsof distributions based on constaXtliw valueof criticll dimensionless progressfrom being stationary to planar mo- sorting mechanisms shear stress.Lower shear stressin troughs bile beds to megaripples to upper-flow-re- would force accumulationof otherwise mo- gime planebeds, and finally to antidunes (as These mechanismsof grain movement in bile large grains, accounting for the slight summarized in Reineck and Singh, 1980). megaripplessuggest that the interiors of the "hump" in the massof coarsegrains (Fig. 5). Under lower-flow-regime conditions of water bedforms should be made up of upper and If megaripplesand troughs migrate, stoss and sedimentdischarge, sediment transport lower slip faces,and that pavement on crests sidesof megaripplesmust be progressively and sorting mechanismsin gravelly mega- and stosssides of megaripplesshould be de- erodedaway as the form shifts downstream' ripples depend primarily on 1) mobile-bed rived in part from reworking megaripple in- The differences between crest armor and phenomenasuch as armor developmentand teriorsalong with addition of grains that passed trough armor may be explained either by overpassing,and on 2) flow separition at the over previous bedforms. These implied rela- progressiveaccumulation (lag) of coarsegrains brink of the bedforms (Fig. 1). tions appearto be substantiatedby the grain- br 5v additionsof coarsegrains in transit As a result of fluid shear stressalong the size distributions illushated in Figs. 5-9. froni upstream. Basedon tfie results of An- bed, mobile (i.e., all grainsmove episodically The interiors of megaripplesappear to be drews (1983) and Parker and Klingeman and cannotbe consideredlag) gravelly beds made up of sedimentsfrom the slip faces, (1982),armor is not due to lag. Therefore, develop an armor or pavement at the bed- althoughsome clastscoarser than those of the coarsegrains must come from uPstream, water interface that results in nearly equal the slip facesalso occur.These coarse clasts probably from erosion and overpassingof transport rates for all sizes of clasts in the in the interior of the megaripplesmay be due stosssides of megaripples.The relative bedload(Andrews, 1983;Parker and Klinge- to flow conditions at an earliertime of dep- abundanceof coarsegrains in troughs com- man, 1982);winnowing of finer sizesby dif- osition; the sampled slip facesformed later 'weakpared to crestsmay be due to the relatively ferential bedload transport apparently does during waning flow. Megaripple interiors C overall currents in the zone of reai- not take place. Grains larger than about 4-5 andJ aresimilar to upper slip faces,whereas tachment (Fig. 1). Large grains may not be times the median diameter of the bedload A and B appear to contain components of moved out of this area as readily as smaller are most exposedto shearstress and tend to both upper and lower slip faces.In part, this grains.- overpass thi bed (Everts, 7973; Allen, 1983; may be a problem of sampling at equal depths The abrupt decreasein mass percent of Andrews, 1983).Fine grains may also over- or at equivalentpositions in adjacentmega- grains less than 0.25 mm in the armor of pass, but they are more likely to become ripples. irests and troughs suggestsan efficientsort- transportedin partial or full suspension.As The excellentdegree of sorting of the lower ing processsuch as suspensionof finer grains, grains are remobilized from the pavement, slip facesis unexpectedconsidering the pro- but secondary peaks commonly occur at other grains drop into the "holes" and be- posed mechanismof intermittent avalanch- 0.090-0.107and 0.063mm in the armor of comeless mobile. Andrews (1983)and Parker ing of grainsdown the slip face.In bedforms the troughs and crests.This sorting of sec- and Klingeman (1982)imply that grain-size studiedin other streams,the lower slip faces ondary modes takes place despite the fact distributions of pavement should reflect are the least well sorted locations. If ava- that no ripples form alongbeds with average nearlyequal amounts of a largerange in sizes lanching was the chief sorting mechanism, grain sizelarger than about0.6 mm. Bridge of grains.Parker and Klingeman(1982) noted coarsergrains should be expectedat the base (t98t; explored the production of grain-size that in gravelly streams with plane beds, of the slip face. Instead, the shape of the distributions with abrupt decreasesin the bedload and subpavementgrain-size distri- grain-size distributions suggestssorting amountof fine grainsdue to suspension.Most butions tend to be similar, while the pave- during unidirectionalflow similar to that of of his theoreticaldistributions show an ab- ment at the interface is coarser.However, sand in ripples and plane beds. Therefore, rupt decreasein amountsof grainsfiner than with megaripple bedforms other sorting the sand of the lower slip face mav be re- u fiiu"tr size determined by-the strength of mechanismsappear to influence the grain- sortedand accumulatedby reverse'flowin bed shear stress.For example,Bridge's cal- size distributions of the subpavement so that the wake of the megaripples. Under this culated grain-size distribution for a mean it is no longer similar to the bedload. mechanism,avalanching coarse grains either shearstress of 4 dynes/cm'yields a curvethat FIow separationat the brink of megarip- may be trapped occasionallyat the base of abruptly declinesin massof grainsless than ples takes some grains beyond the brink to the slip face or may roll to the trough and 0.21mm in diameter. settle through zones of differential turbu- continue downstream. The medium to coarse Bridge (1981)proposed two alternativesto lenceto the trough or stossside of the next sand,on the other hand, may accumulateat explainthe presenceof fine grainsin gravelly megarippledownstream (jopling, 1964). the baseof the slope where ihe reversecur- deposits.He suggestedthat some fines may Grains settling beyond the brink are sorted rents begin to rise. Finer sand would con- bepresent due to entrapmentand infiltration according to forward momentum, settling tinue in suspensionor perhaps would be between larger grains, particularly as flow velocity, and strength of turbulence. Small depositedon the upper slip face. wanes,and other fines may be present due grains are carried away in suspension.Grains Armor could have been derived either by to mechanicalcrushing of some clasts be- near the bed in the zone of differential tur- 1)reworking megarippleinteriors, removing tween larger clasts.Mechanical crushing to bulencemay be caught in a zone of reverse large amounts of finer clasts,and accumu- producefines in the range of the grainsseen flow, being transported toward the slip face Iating coarserclasts as lag, or by 2) additions in the megaripplesappears to be extremely of the megaripple upstream. Other grains of coarserclasts in a mobile bed that moved unlikely. Some trapping of fines may occur aredeposited at the brink and avalanchedown over the troughs and up stoss sides of the becausethe bedforms are loosely packed. the slip face.Grains caught in avalanchesare megaripples.The nearly flat "plateaus" be- However, if one assumesthat the 0.25 mm sorted by differential shearing and relative tween 0.25 and 8 mm of the grain-sizedis- grains (the smallest grains in great abun- dispersivepressure/ as well as by turbulent tributions of the armor (Fig. 6) could indicate dance)contTol the sizeof intersticesbetween suspensionof smaller grains. eitherthat clastsin that sizerange were equally

NezoMexico Geology May'1985 grains,the largestspace between grains would the reachto achieveequal rates of movement were like from the portions that are pre- be about 0.100mm with cubic packing and from bedform to bedform. The possibility of served in geologic sectionsas well as their 0.039 mm for close rhombohedral packing Iarger-scaleperiodicity in deposition along relative longitudinal position within a pro- (derived from formulae in Pettijohn, 7957). the conveyancechannel cannot be evalu- gressionof bedforms. Under rhombohedral packing, void fillers ated. ACKNOWLEDGMENTS-Weare grateful to the couldbe between0.104 and 0.056mm in size, New Mexico Bureau of Mines and Mineral but the void-filling grainscould not filter be- Conclusions Resourcesfor supporting this study. We thank tween grains. In any case, smaller grains In gravellymegaripples along the convey- Ron Broadhead,John MacMillan, and Steve should be favored over larger grains in an ance channel of the Rio Grande, grain-size Cather for their helpful comments and crit- entrapmentprocess, and such definitepeaks distributions and some of their d'erivative icisms.We thank CheriePelletier for drafting at about 0.063mm should not be expected. parameterscan be used to locate samples the figures. We are especiallythankful to Deb Therefore,some sorting processof fine sand within a longitudinal sequenceof bedforms. Shaw for editing and reducing the length of along the bed must be taking place. The Each part of a megaripple (crest, interior, the manuscript, and for helping to produce mechanismappears to takeplace during mo- upper slip face, lower slip face, or trough) the figures. bile conditions rather than after movement has a diagnosticgrain-size distribution. Dis- References of all coarsegrains has ceasedbecause some tributions from eachportion of megaripples subpavementsamples exhibit Allen, J R L , 1982, Computational models for dune secondary changeslightly downstreamso that locating time-lag----calculationsusing Stein'srule for dune height: peaks. Fine grains may be sorted into sec- a samplewithin a train of megaripplesis pos- SedimentaryGeology, v.20, pp 165276. ondary peaks by reverse flow up the slip sible.Along a reach of only 10 megaripples, Allen, J R L , 1983, Gravel overpassing on humpback face,but this mechanismwould not explain the crests,lower slip faces,and troughs of bars supplied with mixed sediment-examples from secondarypeaks in armor. The the lower Old Red ,south Britain: Sedimen- small modes upstreammegaripples are coarserthan their tology, v 30, pp 285-294 of fine sandin armor suggestthat secondary downstream counterparts. Similarly, the Andrews, E. D, 1983,Entrainment of qravel from nat- currentssort fine sand moving between larger crossoverpoints between grain-sizecurves urally sorted river-bedmaterial: Geol6gical Society of grains, but the nature of the sorting pro- of crests and troughs shilt to finer sizes America, Bulletin, v 94, pp 1225-7231 cessesproducing secondary Bagnold, R A., 1941, The physics of blown sand and peaks of sand downstream.Although the mass percent of desertdunes: Methuen, London, 265pp remainsto be examinedfurther. all clastslarger than 0.6 mm in troughs shows Bridge, | 5.,1981,,Hydraulic interpretation of grain-size The downstream trends in qrain-sizedis- no significanttrends downstream,the mass distributions using a physical model for bedload trans- port: of Sedimentary ,v pp tributions from analogonsporlio.ts of mega- percentof clastslarger than 2 mm does de- 't1.24Journal 51, 1109- ripples along such a relatively short reach creasedownstream. In lower slip faces,the Everts, C H , 1973,Particle overpassing on a flat granular are difficult to explain, but may be consid- massof clastslarger than 0.6 mm decreases boundary: American Society of Civil Engineers, Pro- ered in terms of 1) changesin flow condi- downstream. ceedrngs,Journal of the Watemays, Habors, and Coastal tions, 2) pulsesof sediment, and 3) relative Sorting mechanismsthat produce differ- Engineering Division, v. 99, no WW4, Paper 10125, mobility of grains.The actualexplanation may ent grain-sizedistributions along megarip- pp 425-438 Jopling, A. V., 1953, Hydraulic studies on the origin of be a combinationof thesefactors. Deoosition ples apparently are related to mobile-bed bedding: ,v 2, pp 1,1,5-121 (anderosion) at eachpoint alongeach mega- formation of armor along troughs, stosssides, Jopling,A. Y., "1954,Labontory study of sorting processes ripple/trough must be controlled by local flow and crestsand to flow separation,avalanch- related to flow separation:Journal of Geophysical Re- conditions and availability of grains. The ing, and reverseflow along slip faces.Sus- search,v 16,pp 3403-3418. Parker,G , and Klingeman, P. C ,7982, On why gravel sampledreach is only a small portion of the pension of fines less than 0.25 mm appears bed streams are paved: Water ResourcesResearch, v conveyance channel containing megarip- to take placefrom troughs to crestsof mega- 1.8,pp. 1409-1423 ples. No obvious changes in channel ge- ripples. Fine sand is alsosorted into second- Pettijohn,F. I ,7957, Sedinentary rocks:Harper and Row, ometry or size of bedforms occur along the ary modesthat may indicatethe presenceof New York, 718 pp Reineck,H E , and Singh, I 8., 1980,Depositional sed- reach,so changesin averageflow conditions secondary currents between larger gravel imentary environments: Springer-Verlag, New York, along the reachappear to be unlikely causes clasts.The reasonsfor trendsin distributions 549pp of changesin sediment sorting. However, downstream over a distance of only 30 m Williams, C. P., 1978,Bank-full dischargeof rivers: Water the reach is in a gently curving portion of may include changes in flow conditions, ResourcesResearch, v. 1.4,pp 1,141,-1,1,54 Tl the channel (Fig. 2) and flow could change pulsesof sediment, and/or relative mobility along the curve. Possiblythe waning stages of the largestgrains. of deposition changed progressivelydown- The grain-size distributions illustrated streamso that deposition,particularly along aboveshow the importance of knowing the slip faces,continued downstream after it had location of sampleswith respect to bed- ceasedupstream. forms. Only the grain-sizedistributions of The uniformity or periodicity of sediment the lower slip facesappear to have relatively transport cannot be assessedwithout mea- simple probability distributions. The varia- surementsof flow and sediment discharge tion in grain-size distributions along bed- during formation of the megaripples. Dif- forms demonstratesthat many fundamentallv ferentpulses of grainsmay have reacheddif- differentdistributions occur in fluvial de- ferent portions of megaripples at different -posits. times during deposition, providing more The techniqueof relating grain-sizedistri- coarsegrains to upstream megaripplesthan butions to locationwithin bedformssuggests to downstream megaripples, or providing severalpotential directions for further studv. more grains in the range of 0.85-8 mm to Where grain-size data are gathered from dif- downstreamtroughs after passingupstream ferent bedforms formed under known flow bedforms. conditions, it may be possibleto relate the The trough farthest downstreamcontains distributions to flow conditions more quan- all but the coarsestclasts, so most sizes of titatively.Alternatively, more data may dem- grains have been transported through the onstratethat severalsets of flow conditions reach.The largestgrains must be lessmobile canproduce similar distributions or that one than smallergrains so that movement of large setof flow conditionscan producedissimilar grains from trough to trough is slowed. distributions. If quantification of flow con- Therefore, upstream troughs should have ditions is possibleusing grain-sizedistribu- more large claststhan downstreamtroughs, tions from similar bedforms. it also mav be until large clastshave migrated throughout possibleto determinewhat entire bedforms

30 May 1985 New Mexico Geology