in
Location-dependent
sorting
sediment
gravelly
megaripples
fromtheRioGrande,
centralNewMexico
byDavidW.Love,JamesT.
Socorro,
NM
Boyle,
Resources,
Sylveen
Robinson,
andMarkHemingway,
NewMexico
Bureau
o{ MinesandMineral
Introduction
An ongoing study of modern fluvial sediments in New Mexico has revealedthat grainsizesorting in streamsis dependenton sample
locationwithin portions of bedformsand on
bedform position within downstream progressionsof similar forms. Theseresults have
many potential geologicapplications.
One of the sets of bedforms studied is
straight-crested,gravelly megaripplesof the
Rio Grande conveyancechannel near Socorro, New Mexico. Megaripplesare rippleshapedbedformshaving wavelengthslarger
than 60 cm and a range in amplitudes larger
than ripples. In streams,megaripplesform
under lower-flow-regimeconditionsand rhigratedownstreamby progressivedeposition
along their slip faces(Fig. 1). The purposes
of this paper are to relate grain-sizedistributions to the form and location of megaripples and troughs left by waning flow along
t h e c o n v e y a n c ec h a n n e l , t o d e t e r m i n e
whether a megaripples'sposition within a
progressionmay be locatedby analyzing its
grain-size distribution, and to determine
s o r t i n g m e c h a n i s m st a k i n g p l a c e a l o n g
megaripples.
Description of channel conditions
The conveyancechanneleastof Socorrois
about 23 m wide at the top and 4 m deep,
with an averagegradient of 0.016 (Fig. 2).
During the week of April 8, 1984,flow averagedan estimated86 m'/sec(basedon flow
estimationtechniquesof Williams, 1978).On
April 14,flow in the conveyancechannelwas
diverted back into the Rio Grande channel
upstream. During the next four days, the
flow dropped to an estimated 3 m3/sec,so
that the channelwas approfmately 10m wide
and 0.2 m deep.Flow was maintainedin part
by seepagefrom the banks. On April 19, the
z o n eo f n o d i ff u s i o n
water was clear and the tops of some of the
megarippleswere near the surfaceof the water
when surveying and sampling were conducted (Fig. 3). Little sediment was in suspension. Sand was transported only where
megarippleswere erodedby flow in the thalweg. Most parts of the megarippleswere no
longer migrating downstream.
Straight-crestedmegaripples with wavelengthsaveraging2.8 m were measuredand
sampledalong a 30 m long reach where reworking by a low-flow thalweg had not altered the bedform configuration. After
samplingwas completed,the reachwas surveyed using a Leitz TM 20 theodolite and a
rod measuring to the nearesthundredth of
a foot (about 3 mm). The megarippleshad
long, low-angle stoss sides, steep lee sides
(at the angle of repose),and crestsabout 12
cm above the adjacenttroughs downstream
(Fig. a). No ripples occurred on the stoss
flow velocity
oce
toce
c r es t
6i6s\de
interior
t r o ug h
r e v e r s ef l o w
m o b i l eb e d
o v o l o n c h er e v e r s e
flow
FIGURE l-Anatomy
of megaripples, water flow (in blue), and sediment movements (in black) involved
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
April 19, l9&1. Note tops of megaripplesnear water surface. Channel is
approximately10 m wide.
May 1985 Nao MexicoGeolog"
in sorting processes Modified
FIGURE 3-Megaripples in conveyancechannel of Rio Grande. Thalweg in
lower middle portion of picture is reworking sand and obliterating meganPPles.
N-.
tlow
direction
J
I EV E I
O
5m
ffi
2 X v e r l i c o le x o g g e r o l i o n
FIGURE 4-Longitudinal
profile of megaripples and water surface
sidesof the megaripples,exceptin the thalweg/ where the megarippleswere beginning
to be reworked and fine sand was being
transported.The exact timing of depositioi
of the megaripples is not known, but probably the formation and preservation of the
bedformslaggedbehind flow conditions (cf.
Allen, 1982).The slip faces probably were
depositedjust beforeflow waned to the point
of no sediment transport, whereas the interior portions of the megarippleswere deposited during active transport conditions.
Although the timing and conditionsfor deposition of the megaripples remain partially
undetermined, the fact that they were preserved during waning flow suggestsihat
similar sedimentary structures in geologic
contextsmay be examined and interpreted
in a similar manner.
Sample treatment
Sampleswere taken from the megaripples
by sampling portions of bedforms progressively upstream so that the samples remainedundisturbedand uncontaminatedby
sedimentsagitatedduring the samplingprocess.Thus, troughs downstream from each
megaripplewere sampledfirst, then the lower
slip faces,upper slip faces,the armor at the
crestof the megaripples,and, finally, the interior portion of each megaripple. Not all
megarippleswithin the sequencewere sampled, primarily becausethey were all very
similarin overallcharacteristics.
We were not
convinced that differenceswould be found
between adjacent megaripples, and, therefore, we expectedonly slight differencesbetween the upstream and downstream ends
of the sequence.Where the megarippleswere
dominatedby gravel, about 1 kg sampleswere
taken in an areaof 200-400cm'. The uppermost few cm were scoopedbv hand into a
la,rgeplastic bag. The interiors of megaripples were sampled by clearingaway a[l surface clasts from a broader area and then
scooping the sample from the megaripple by
hand. The dominantly sandy slip faceswere
sampledusing film canistersthat hold about
40 g of sand when full; one to two canisters
of sand were taken from each of the upper
and lower slip faces.
Sedimentsforming the megaripples and
troughswere extremelylooselypacked.\A/hile
walking acrossthe channel, a 55-kg (143lb)
person could sink up to 60 cm into the uncompactedsandy sediments.Although slip
faceswere present on the lee sides of the
megaripples,no internal sedimentarystructures were seen during sampling, probably
becauseslumping occuied immediaietyafter
sampleswere taken, and becauseof the lack
of differentiation of grain sizes into discrete
laminae.Unfortunately, time and expensedid
not allow more sophisticatedtechniquesfor
s a m p l i n g c r o s s s e c t i o n so f s e d i m e n t a r y
structures(such as taking epoxy box cores)
to be used.
In the lab, the samples were dried and
sievedusing sievesizeslisted in Table1. The
entire sample was sieved to a size of 1 mm
in order to have an adequatesampleof large
clasts. Sampleswith abundant portions of
sand less than 1 mm were sDlit to about 50
g and sieved.Then the proportion of the
total was calculated.Duplicatesplits of some
samnleswere sievedto test the adeouacvof
this procedure.
Mass-frequency diagrams were plotted
following Bagnold's (1941)procedures.In the
diagrams that illustrate grain-size distributions (Figs. 5-9), both axes are logarithmic
becauseof the wide rangesin grain size and
masspercentages.The irregularintervalsbetween sieves are standardized by dividing
the mass percent in each size classby the
differencebetween logarithms of the diametersof the largestand smallestgrainsin the
size class.The diagramsshow the mass frequencyof different-sizegrains in such a way
that proportions of one population may remain similar even though other populations
are added or subtractedfrom the total in other
parts of the distribution. In the olots that
iollow, some sievescaughtconsistentlyless
mass than the sieves adiacent to them becauseof deviationsin aperturesize.For example, the 0.5-mm sieve nearly always had
less sand in it than did the 0.43-mm sieve,
indicatingthat someaperturesof the 0.5-mm
sieve are too large and let too many grains
through to the 0.43-mmsieve.Theseinconsistencieshave not been correctedin the plots,
but are clearly apparent as a V-shaped
depressionin the grain-sizecurvesat 0.5 mm.
Somecurvesare discontinuousat the coarse
end becauseno clastswere caught on some
sieves.
TABLE l-Sieve aperture sizes in mm used
in sediment analyies.
38.05
26.67
18.35
IJ.
JJ
9.52
8.00
6.30
5.66
4.76
4.00
3.35
2.83
2.36
2.00
7.68
1.40
1.18
1.00
0.85
0.71,
0.605
0.500
0.430
0.355
0.303
0.250
0.272
0.180
0.150
0.725
0.707
0.090
0-075
0.063
0.04s
from unstream to downstream to illustrate
any lorigitudinal shifts. More subtle trends
are revealedby comparing adjacentparts of
megaripples/troughs for different populations/line slopes and by examining relative
percentagesof different parts of the populations. These differences and trends are
summarizedbelow.
Distributions from analogousportions of
megaripples
As shown in Fig. 5, troughs show a broad,
skewed peak between 0.25 and 8-10 mm, a
decreasein grains larger than about L0 mm,
a paucity of grains between 0.075and 0.180
mm, and a secondpeak at 0.063mm. Grains
between 0.85 and I mm increasein mass
percent downstream, whereas grains between 0.355and 0.85 mm decreasein mass
percent downstream.
Grains armoring the crest of megaripples
have distributions somewhat similar to
troughs, becausethey have a generalbroad
peak from 0.25to 8 mm and a small peak at
about 0.063 mm (Fig. 5). The broad peak
greaterthan 0.25mm tends to be flatter (more
of a plateau) than the broad peak of the
Results
troughs, and it tends to show slightly lesser
T h e g r a i n - s i z e d i s t r i b u t i o n s f r o m t h e amounts of the coarserclasts (curve has a
megaripplesdepend on where sampleswere negative slope at coarsersizes).Grains betakenwithin eachmegarippleand associated tween 1.4 and 4.76 mm decreasein mass
trough downstream, and on the location of percent downstream, whereas grains bethe megaripple and trough set within the tween 0.25and 1.4mm increasein masspersequence.The differencesin grain size are cent downstream, with the last megaripple
reflectedin the overall distributions at a given having a definite peak between 0.5 and L.18
location within each megaripple, in trends mm (this peak may be due in part to samdownstream along each megaripple/trough pling the underlying megaripple interior). No
sequence,and in proportions of gravel frac- trend is apparentin the amount of fine sand
tions and various sand fractions within in- forming the peak at about 0.063mm.
dividual megaripples and along the entire
Samplesfrom the interiors of megaripples
sequence.The grain-sizedata are organized show a similar broad peak from 0.25 to 6.3
in Figs. 5-9 for each location along mega- mm and a precipitousdeclinein the amount
ripples/troughs,and the data are "stacked"
Nm Mexico Geology May 1985
o
o
o
o
c
c
o
a
N
N
o
o
o
o
o
I
o
o
c
o
c
o
o
o-
o.l
E
o
E
o
J
)
roo
Groin
s i z ei n m m
of troughsloFIGURE
S-Grain-sizedistributions
frommegaripples
A, B, C, and
cateddownstream
J For clarity,curvesB, C, and J havebeenoffset
additionof 0.3, 0.6,
verticallyby the respective
and0.9to theirY-values.
of sand less than 0.25 mm (Fig. 7). In comparison with samples of crest armor, interiors have fewer clastslarger than 6.3 mm.
There appears to be a decreasein fine sand
fraction downstream, but secondary peaks
at 0.107or 0.063 mm or both occur ln all
interior samples.
Upper slip faceshave a narrower range of
grain sizes,commonly exhibiting a peak between 0.25and L mm (Fig. 8). From the upstreammegaripple (J) to the next to the last
megaripple (B) in the sequence/the distribution shifts to a finer peak (from 0.605-0.85
to 0.303-0.43mm), but the last megaripple
(A) is similar to the first. Moreover, samples
from different parts of the upstream megaripple (activeand inactive) are quite different
in proportions of coarseand fine grains (Fig.
8: Ja, active, and Jb, inactive).
Lower slip faces are much better sorted
than either the troughs or the rest of the
megaripple locations. They have a single,
relatively narrow peak, slightly skewed
toward the fine side (Fig. 9). The peak of the
distribution shifts downstream from 1.181,.4to 0.7t-0.85 mm. The amount of grains
between0.063and 0.71mm increasesdownstream,affectingthe slope of the fine side of
the distribution, but no separatepeak of fine
sand occurs. Grains between 1,.4 and 2.83
mm decreasedownstream,but the last lower
slip face (A) has an increasein granulesbetween 3.35 and 4 mm.
c
o
o
o-
E
o
o
J
2
G r o i ns i z ei n m m
I
G r o i ns i z ei n m m
FIGURE6-Grain-size distributions of crest armor
of megaripples A, B, C, and J. For clarity, curves
B, C, and J have been offset vertically by the respectiveaddition 0.3, 0.6, and 0.9 to their Y-values
FIGURE7-Grain-size distributions of megaripple
interiors. No curves have been offset. Curve A is
the same as that for crest armor A because no
separatesample was obtained.
o
a)
c
c
o
o
N
N
j'Jb
ol
o
o
o
o
o
o
cn
C
o
o
@
3 -r
E
E
o
)
)
-2
oor
o.l
r
"dffi"J.:lt
FIGURE
8-c,",^-,;"
ro
roo
srip
upper
G r o i sn r z et n m m
FIGURE9-Grain-size distributionsof lower slip
A, B, C, andJ CurvesB, C,
facesof megaripples
A, B, C, andJ. CurvesA, B, facesof megaripples
C, and fa arenot offset;Jb is offsetby 0.5Y-units and J are offsetverticallyby the respectiveaddito avoidoverlap Jais from activeslip facein thal- tion of 0.2,0.4,and 0.6to theirY-values.
weg;fb is frominactiveslipface2 m from thalweg.
ferences downstream. The lower slip faces
arenot presentin slip faces.Secondarymodes containprogressivelyless massgreaterthan
at 0.063mm also decreasefrom troughs to 0.605mm downstream, but other megaripslip faces.In two of the progressionsfrom ple portions do not show significanttrends.
lower slip facesto troughs, the troughs have
Discussion
a deficiencyof clastsin the rangeof the peak
comparing grain-size
merely
Rather
than
of the lower slip faces.
distributionsof adjacentsamples,discussion
Other locational indicators
of the above results focuseson the sorting
In comparing curves, it was noted that mechanismsresponsiblefor producing the
points where two curves cross(termed cross- distributions and on interpreting the distriProgressivedownstream changesin
over points) may shift significantly depend- b u t i o n s b a s e d o n t h e p r o p o s e d s o r t i n g
distributions from adiacentportions
i n g o n l o c a t i o n w i t h i n t h e m e g a r i p p l e mechanisms.
of megaripples
sequence.For example,the crossoverpoints
Downstream progressionsalong individ- between crest armor and adjacent trough Sorting mechanisms
Numerous factors determine the grain size
ual megaripplesalso revealtrends in the fol- rangedownstreamfrom 5.66mm atl to 4.76
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, sizeintervalsmay 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 t h a n 2 m m i n t r o u g h s d e c r e a s e sd o w n - 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 ripMay 1985 Nar MexicoGeology
TABLE 2-Examples of longitudinal ranges in weight percent of selectedlarge clast sizes from portions
*armor and interior sampled together
of megaripples along the conveyancechannel of the Rio Grande;
face,
here.
slip
represented
in megarippleA; b, only Jb, inactive upper
megaripple
C
B
percent > 2 mm
in troughs
75.3
75.1,
73.6
72.2
troughs
percent > 0.6 mm in
upper slip
Iower slip
83.3
91.3
91.8
97.3
ples; rather, as flow is increased,such beds
progressfrom being stationary to planar mobile beds to megaripples to upper-flow-regime plane beds, and finally to antidunes (as
summarized in Reineck and Singh, 1980).
Under lower-flow-regime conditions of water
and sedimentdischarge,sedimenttransport
and sorting mechanismsin gravelly megaripples depend primarily on 1) mobile-bed
phenomenasuch as armor developmentand
overpassing,and on 2) flow separition at the
brink of the bedforms (Fig. 1).
As a result of fluid shear stressalong the
bed, mobile (i.e., all grainsmove episodically
and cannot be consideredlag) gravelly beds
develop an armor or pavement at the bedwater interface that results in nearly equal
transport rates for all sizes of clasts in the
bedload (Andrews, 1983;Parkerand Klingeman, 1982);winnowing of finer sizesby differential bedload transport apparently does
not take place. Grains larger than about 4-5
times the median diameter of the bedload
are most exposedto shearstressand tend to
overpass thi bed (Everts, 7973; Allen, 1983;
Andrews, 1983).Fine grains may also overpass, but they are more likely to become
transportedin partial or full suspension.As
grains are remobilized from the pavement,
other grains drop into the "holes" and becomelessmobile.Andrews (1983)and Parker
and Klingeman (1982)imply that grain-size
distributions of pavement should reflect
nearlyequalamountsof a largerangein sizes
of grains.Parkerand Klingeman(1982)noted
that in gravelly streams with plane beds,
bedload and subpavementgrain-sizedistributions tend to be similar, while the pavement at the interface is coarser.However,
with megaripple bedforms other sorting
mechanismsappear to influence the grainsize distributions of the subpavement so that
it is no longer similar to the bedload.
FIow separationat the brink of megaripples takes some grains beyond the brink to
settle through zones of differential turbulence to the trough or stossside of the next
m e g a r i p p l ed o w n s t r e a m ( j o p l i n g , 1 9 6 4 ) .
Grains settling beyond the brink are sorted
according to forward momentum, settling
velocity, and strength of turbulence. Small
grains are carried away in suspension.Grains
near the bed in the zone of differential turbulencemay be caught in a zone of reverse
flow, being transported toward the slip face
of the megaripple upstream. Other grains
aredepositedat the brink and avalanchedown
the slip face.Grainscaughtin avalanchesare
sorted by differential shearing and relative
dispersivepressure/as well as by turbulent
suspensionof smaller grains.
99.8
97.7
86.4
81.9
a/.5'
72.8
28.7
52.3
armor
interior
83.7
75.4
77.4
65.6
/6.J
/ /.2
83.0*
Interpretationsof distributions based on
sorting mechanisms
These mechanismsof grain movement in
megaripplessuggestthat the interiors of the
bedforms should be made up of upper and
lower slip faces,and that pavement on crests
and stosssidesof megaripplesshould be derived in part from reworking megaripple interiorsalong with addition of grains that passed
over previous bedforms. These implied relations appearto be substantiatedby the grainsize distributions illushated in Figs. 5-9.
The interiors of megaripplesappear to be
made up of sedimentsfrom the slip faces,
although some clastscoarserthan those of
the slip facesalso occur.Thesecoarseclasts
in the interior of the megaripplesmay be due
to flow conditions at an earlier time of deposition; the sampled slip facesformed later
during waning flow. Megaripple interiors C
andJ aresimilar to upper slip faces,whereas
A and B appear to contain components of
both upper and lower slip faces.In part, this
may be a problem of sampling at equal depths
or at equivalentpositions in adjacentmegaripples.
The excellentdegreeof sorting of the lower
slip facesis unexpectedconsideringthe proposed mechanismof intermittent avalanching of grainsdown the slip face.In bedforms
studiedin other streams,the lower slip faces
are the least well sorted locations. If avalanching was the chief sorting mechanism,
coarsergrainsshould be expectedat the base
of the slip face. Instead, the shape of the
grain-size distributions suggests sorting
during unidirectionalflow similar to that of
sand in ripples and plane beds. Therefore,
the sand of the lower slip face mav be resorted and accumulatedby reverse'flowin
the wake of the megaripples. Under this
mechanism,avalanchingcoarsegrains either
may be trapped occasionallyat the base of
the slip face or may roll to the trough and
continue downstream. The medium to coarse
sand,on the other hand, may accumulateat
the baseof the slope where ihe reversecurrents begin to rise. Finer sand would continue in suspension or perhaps would be
depositedon the upper slip face.
Armor could have been derived either by
1) reworking megarippleinteriors, removing
large amounts of finer clasts, and accumuIating coarserclastsas lag, or by 2) additions
of coarserclastsin a mobile bed that moved
over the troughs and up stoss sides of the
megaripples.The nearly flat "plateaus" between 0.25 and 8 mm of the grain-sizedistributions of the armor (Fig. 6) could indicate
eitherthat clastsin that sizerangewere equally
mobilebecauseof equal shearstressto move
all sizes(cf.Andrews, 1983)or that the grains
were differentially exposed to shear stress
depending on size and placement of surr o u n d i n g g r a i n s ( P a r k e ra n d K l i n g e m a n ,
1982).The plateausof nearly equal amounts
of grains are much wider (8 mm is 32 times
as lig as 0.25 mm) than p,redictedby Andrewi (roughly 14 times from 0.3xd50 to
4.2xd50).This discrepancymay indicatethat
the larger grains are moving under a nearly
constaXtliw value of criticll dimensionless
shear stress.Lower shear stressin troughs
would force accumulationof otherwise mobile large grains, accounting for the slight
"hump" in the massof coarsegrains (Fig. 5).
If megaripplesand troughs migrate, stoss
sides of megaripplesmust be progressively
erodedaway as the form shifts downstream'
The differences between crest armor and
trough armor may be explained either by
progressive
accumulation(lag)of coarsegrains
br 5v additions of coarsegrains in transit
froni upstream. Basedon tfie results of Andrews (1983) and Parker and Klingeman
(1982),armor is not due to lag. Therefore,
the coarsegrainsmust come from uPstream,
probably from erosion and overpassingof
s t o s s s i d e s o f m e g a r i p p l e s .T h e r e l a t i v e
abundanceof coarsegrains in troughs compared
'weak to crestsmay be due to the relatively
overall currents in the zone of reaitachment (Fig. 1). Large grains may not be
moved out of this area as readily as smaller
grains.
The abrupt decreasein mass percent of
grains less than 0.25 mm in the armor of
irests and troughs suggestsan efficientsorting processsuch as suspensionof finer grains,
but secondary peaks commonly occur at
0.090-0.107and 0.063mm in the armor of
the troughs and crests.This sorting of secondary modes takes place despite the fact
that no ripples form along beds with average
grain sizelargerthan about 0.6 mm. Bridge
(t98t; explored the production of grain-size
distributions with abrupt decreasesin the
amountof fine grainsdue to suspension.Most
of his theoreticaldistributions show an abrupt decreasein amounts of grainsfiner than
u fiiu"tr size determined by-the strength of
bed shear stress.For example,Bridge's calculated grain-size distribution for a mean
shearstressof 4 dynes/cm'yields a curve that
abruptly declinesin massof grains less than
0.21mm in diameter.
Bridge (1981)proposedtwo alternativesto
explainthe presenceof fine grainsin gravelly
deposits.He suggestedthat some fines may
be presentdue to entrapmentand infiltration
between larger grains, particularly as flow
wanes, and other fines may be present due
to mechanicalcrushing of some clasts between larger clasts.Mechanicalcrushing to
producefines in the range of the grains seen
in the megaripplesappearsto be extremely
unlikely. Some trapping of fines may occur
becausethe bedforms are loosely packed.
However, if one assumesthat the 0.25 mm
grains (the smallest grains in great abundance)contTolthe sizeof intersticesbetween
NezoMexicoGeology May'1985
grains,the largestspacebetweengrainswould
be about 0.100mm with cubic packing and
0.039 mm for close rhombohedral packing
(derived from formulae in Pettijohn, 7957).
Under rhombohedral packing, void fillers
could be between0.104and 0.056mm in size,
but the void-filling grains could not filter between grains. In any case, smaller grains
should be favored over larger grains in an
entrapmentprocess,and such definitepeaks
at about 0.063mm should not be expected.
Therefore,somesorting processof fine sand
along the bed must be taking place. The
mechanismappearsto takeplaceduring mobile conditions rather than after movement
of all coarsegrains has ceasedbecausesome
subpavement samples exhibit secondary
peaks. Fine grains may be sorted into secondary peaks by reverse flow up the slip
face,but this mechanismwould not explain
secondarypeaksin armor. The small modes
of fine sandin armor suggestthat secondary
currentssort fine sand moving between larger
grains, but the nature of the sorting processesproducing secondary peaks of sand
remains to be examinedfurther.
The downstream trends in qrain-sizedistributions from analogonsporlio.ts of megaripples along such a relatively short reach
are difficult to explain, but may be considered in terms of 1) changesin flow conditions, 2) pulses of sediment, and 3) relative
mobility of grains.The actualexplanationmay
be a combinationof thesefactors.Deoosition
(anderosion)at eachpoint alongeachmegaripple/trough must be controlled by local flow
conditions and availability of grains. The
sampledreach is only a small portion of the
conveyance channel containing megaripples. No obvious changes in channel geometry or size of bedforms occur along the
reach,so changesin averageflow conditions
along the reachappear to be unlikely causes
of changesin sediment sorting. However,
the reach is in a gently curving portion of
the channel (Fig. 2) and flow could change
along the curve. Possiblythe waning stages
of deposition changed progressivelydownstreamso that deposition, particularly along
slip faces,continueddownstreamafterit had
ceasedupstream.
The uniformity or periodicity of sediment
transport cannot be assessedwithout measurementsof flow and sediment discharge
during formation of the megaripples. Differentpulsesof grainsmay have reacheddifferent portions of megaripples at different
times during deposition, providing more
coarsegrains to upstream megaripplesthan
to downstream megaripples, or providing
more grains in the range of 0.85-8 mm to
downstreamtroughs after passingupstream
bedforms.
The trough farthest downstream contains
all but the coarsestclasts, so most sizes of
grains have been transported through the
reach.The largestgrainsmust be lessmobile
than smallergrains so that movement of large
grains from trough to trough is slowed.
Therefore, upstream troughs should have
more large claststhan downstreamtroughs,
until large clastshave migrated throughout
30
May 1985
New Mexico Geology
the reachto achieveequalratesof movement
from bedform to bedform. The possibility of
Iarger-scaleperiodicity in deposition along
the conveyancechannel cannot be evaluated.
Conclusions
In gravelly megaripplesalong the conveyance channel of the Rio Grande, grain-size
distributions and some of their d'erivative
parameterscan be used to locate samples
within a longitudinal sequenceof bedforms.
Each part of a megaripple (crest, interior,
upper slip face, lower slip face, or trough)
has a diagnosticgrain-sizedistribution. Distributions from eachportion of megaripples
changeslightly downstream so that locating
a samplewithin a train of megaripplesis possible.Along a reach of only 10 megaripples,
the crests,lower slip faces, and troughs of
upstreammegaripplesare coarserthan their
downstream counterparts. Similarly, the
crossoverpoints between grain-sizecurves
of crests and troughs shilt to finer sizes
downstream.Although the mass percent of
all clastslarger than 0.6 mm in troughs shows
no significanttrends downstream, the mass
percentof clastslarger than 2 mm does decreasedownstream. In lower slip faces,the
massof clastslarger than 0.6 mm decreases
downstream.
Sorting mechanismsthat produce different grain-sizedistributions along megaripples apparently are related to mobile-bed
formation of armor along troughs, stosssides,
and crestsand to flow separation,avalanching, and reverseflow along slip faces.Suspension of fines less than 0.25 mm appears
to take placefrom troughs to crestsof megaripples. Fine sand is also sorted into secondary modes that may indicatethe presenceof
secondary currents between larger gravel
clasts.The reasonsfor trendsin distributions
downstream over a distance of only 30 m
may include changes in flow conditions,
pulses of sediment, and/or relative mobility
of the largestgrains.
The grain-size distributions illustrated
aboveshow the importance of knowing the
location of samples with respect to bedforms. Only the grain-sizedistributions of
the lower slip facesappearto have relatively
simple probability distributions. The variation in grain-size distributions along bedforms demonstratesthat many fundamentallv
different distributions occur in fluvial deposits.
The techniqueof relating grain-sizedistributions to locationwithin bedformssuggests
severalpotential directionsfor further studv.
Where grain-size data are gathered from different bedforms formed under known flow
conditions, it may be possibleto relate the
distributions to flow conditions more quantitatively.Alternatively,more data may demonstratethat severalsets of flow conditions
can produce similar distributions or that one
set of flow conditionscan produce dissimilar
distributions. If quantification of flow conditions is possibleusing grain-sizedistributions from similar bedforms. it also mav be
possibleto determine what entire bedforms
were like from the portions that are preserved in geologic sectionsas well as their
relative longitudinal position within a progressionof bedforms.
ACKNOWLEDGMENTS-We
are grateful to the
New Mexico Bureau of Mines and Mineral
Resourcesfor supporting this study. We thank
Ron Broadhead,John MacMillan, and Steve
Cather for their helpful comments and criticisms.We thank Cherie Pelletierfor drafting
the figures. We are especiallythankful to Deb
Shaw for editing and reducing the length of
the manuscript, and for helping to produce
the figures.
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