A Comparison of Grain Sizes

Suwanee RiverProject
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A Comparison of Grain Textures as Grains Travel Downstream, Suwannee River
D. C. Gilmore
Abstract
For meandering streams it is common knowledge that as grains are transported
downstream they become finer, better sorted and rounder. This study focuses on the Suwannee
River and its characteristic textural maturity and texture with supplements from mean grain sizes.
Because of the geographic location of the Suwannee in the southeastern coastal plains, the river
does not exhibit a very high gradient through its course. A stream with a low gradient usually has
a much lower variability of energy fluctuations so this variable can be treated as a secondary
variable and for this study will not be the focus. This means that velocity of the stream and
volume of water are the primary variable effecting local sedimentation along the Suwannee
River.
By using technical sampling techniques at strategic locations downstream along with
computer data analysis and satellite imagery we can make interpretations about the processes
at work. From the White Springs sampling site to the river mouth/delta we observe a mean grain
size decrease and better sorted sediments upon deposition in the delta, but for the locations
located between White Springs and the river mouth experience fluctuations of grain maturity.
The texture of the grains appear to vary considerably among sample sites and end the process in
the delta with more angular grains.
1 - Introduction

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Florida resident, undoubtedly live on rock and sediment that was in one way or another
transported by or precipitated out of water. The trick to understanding where sediment has come
from and why it exhibits the properties of its current state is that you must understand present
day geologic processes and how they affect the rock and sediment and you also must assume
that the same processes have always happened in the past leading to the present day landscape.
For the state of Florida, water is an especiallyimportant variable in the creation of the landscape.
Because of this we must understand the processes of fluvial morphology and its effects on
sediment deposition.
The Suwanee River is likeany other stream on earth in that it flows from higher elevations
to lower elevations and transport grains downstream to a base level (usually sea level but can
also be lakes or other rivers) where grains are able to settle over time. Its headwaters are located
just off the study area (~ 60 km NE of the White Springs Gauge in Southeastern Georgia) in the
Okefenokee Swamp at an elevation of ~ 42 m (all estimated values for elevation from Google
Earth). The mouth of the Suwanee River is at sea level just north of Cedar Key, Florida. Because
of the geographical location of the Suwanee River it has a low gradient and relatively low
variability in velocity. The Suwanee River is a meandering stream and does the majority of its
erosion into Hawthorne Group in the northern reaches, unconsolidated sediments and
Suwannee Limestone in the middle and finally the Ocala Limestone in the southern reaches until
it reaches the Gulf of Mexico (figure 3.1). All of these geologic formations are primarily limestone
with sands and clays. A tiny sliver of Holocene sediments are at the mouth of the Suwanee River
just landward of the Suwanee delta (figure 3.1). Along the length of the Suwanee throughout the
study area are small settlements that undoubtedly effect the rivers in one way or another. What

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this paper does not take into account is the effects of water use, agricultural organic pollution,
dock dredging, bioturbation etc. Human interaction with the environment can play a very
important role in the current day processes of streamdevelopment. For example, dredging along
the bottom of a streamcan increase streamvolume and slowvelocity, therefore allowing smaller
particles to settleeasier.For this paper it is best to ignore human interference and focus primarily
on fluvial morphology and take direct observations and interpret them as though the Suwanee,
its tributaries, and its springs are an isolated fluvial system.
By using previous geological and fluvial morphological findings and “laws” an
interpretation of sediment maturity at specified sampling sites in relation to its downstream
distance can be made. The further sediment travels downstream, the better sorted and rounded
grains become, which geologically this means that grains become more mature as they travel
downstream, (Knighton, 1980). This article will give a clear indication whether or not that for the
Suwanee River, grains have become more mature as they travel further downstream.
2 - Background
In order to understand the processes at work in North Central Florida we need to
understand a key fundamental law of geology. Nicolas Steno’s law of superposition that states,
Sedimentary layers are deposited in a time sequence, with the oldest layers on the bottom and
younger sediments deposited on top. Whether the sediment is transported as a grain or a
solution does not matter, becauseover time sediment builds over top of sediment below creating
layers that vary in parameters such as grain size, sorting, composition, etc. Distinguishing the
differences between the layers of sediment can give geologists a good idea about what the

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geologic history of the region has been. The geology of the source rock a stream travels through
can alter the weathering pattern of rocks and the grains that are transported downstream. In this
paper, the focus is on fluvial morphology and the processes differentiating grain sorting and
texture relevant to a sediments distance traveled downstream.
The Suwannee River has its headwaters in the Okefenokee Swamp located in south
eastern Georgia. The swamp has a surface geology containing mostly unconsolidated sediments
expected of a swamp (peat). The swamp owes its existence to its subsurface rocks. The area was
below sea level in both the Cretaceous and more recently the Eocene (Gelbart, 2010). During the
Eocene limestone deposition was occurring indicating elevation below sea level. As sea level fell,
the limestone underwent karstification from water erosion and was eventually overlain by an
impermeable clay layer of Pliocene age. The limestone beneath the clay continued to erode as
ground water flowed out of the area due to sea level drop. The Okefenokee Swamp was formed
when the limestone subsided and formed a basin (Gelbart, 2010). The impermeable clay layer
prevented water from draining into the subsurface, essentially trapping the water and forming
the Swamp (Gelbart, 2010). As sea level continued to fall the swamp began draining to the Gulf
of Mexico via the Suwannee River.
From the Okefenokee Swamp and its unconsolidated sediments, the Suwannee then
flows into the Miocene Hawthorne Group which consists of carbonates, sands, clays and
phosphates (fig 3.1). The first sampling site is located along the Suwannee in Hawthorne Group
sediment. From the White Springs gauge, the Suwannee continues meandering northwest and
enters the Oligocene Suwannee Limestone which is the surface geology at Ellaville Gauge, the
second sampling site. The Suwannee Limestone is only a surface rock near the river itself, once

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out of the Suwannee floodplain on either side of the river, you go upsection into
Pleistocene/Holocene unconsolidated sediments. The Suwannee then meanders southward
through Eocene Ocala Limestone, passing out last two gauging locations and empties into the
Gulf of Mexico some 396 km from its headwaters in the Okefenokee Swamp.
3 - Methods and Materials
3.1 Study Area
The study area
encompasses the north central
region of the state of Florida,
primarily at four USGS water
gauges, managed by the
Suwannee River Water
Management District, located
along the Suwanee River.
From furthest upstream working downstream the sediment sampling locations were:
1. White Springs Gauge, on the eastern bank, south of US Highway 41 in White Springs,
Florida.
a. GPS Coordinates (30◦19’29.95” N, 82◦44’18.77” W), Elevation: ~ 17 m.
2. Ellaville Gauge, on the south-eastern bank, northeast of railway tracks that are northeast
of US highway 90 in Ellaville, Florida.
Figure 3.1 – shows the extent of the study area (Northern Central Florida).
The sediment samplingsites areshown alongtheSuwanee River and also the
surface Geology of the region is shown. (Scott et al.)

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3. Branford Gauge, on eastern bank, just south of US Highway 27 in Branford, Florida.
4. Wilcox Gauge, on eastern bank, just south of US Highway 98, Fanning Springs, Florida.
a. GPS Coordinates: (2935’24.15: N, 8256’12.03” W), Elevation: ~ 2 m.
3.2 Methods
At each of these sites, five samples of the sediment were collected in strategic locations
(Figure 3.2). Theses samples follow a specific set of parameters. Each sample was collected on
February 6, 2014 between 12:30 and 5:50 pm. By using a spade to dig ~ 10 cm in depth below
the sediment surface, samples of ~ 500 ml were collected at eachspecifiedlocation at eachwater
gauge.
Descriptions of sample collection at each site are as follows:
BW1: Sample was collected ~ 30-50 cm out into the river below the water line.
BW2: Sample collected ~ 30-50 cm out into the river below the water line at a distance from BW1
along the shoreline of ~ 3 m.
AW1: Sample collected ~ 30 cm above the water line at the top of the erosional scarp as well as
~ 10 cm of the face of the scarp.
Figure 3.2 – shows the sediment sampling technique at each gauge site

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AW2: Sample collected ~ 30 cm above the water line at the top of the erosional scarp as well as
~ 10 cm of the face of the scarp at a distance from AW1 along the shoreline at ~ 3 m.
HAW: Sample collected at ~ 1 m above the water level at a lateral distance between 1 and 2 m
from the shoreline.
The final sediment samples were collected in May 2013 near the mouth of the Suwanee
River on delta deposits. All samples were ~ 1 m below the water level. Samples collected were
transported to the lab where each underwent random selection of grains to a total mass weight
of 2-4 grams. Thesesamples were then sieved(63 µm) to remove claysizedparticles.The samples
were then dried for 1 day and then the grain size distributions were measured in a settling
column. Using the settling column gives a grain size distribution that can be imported to
Microsoft Excel for data analysis and manipulation. I will suggest that for any questions on
processes and techniques used in laboratory analysis beforwarded to Dr. Jaeger of the University
of Florida, who performed the settling velocity analysis on all samples and generated grain size.

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4 - Results
4.1 Grain Sorting
The first observations made are on grain
sorting as the grain travels downstream.
Grain Sorting values for all gauge stations
throughout the Suwannee River get
progressively less well sorted as they
travel downstream (figure 4.1). Grain
sorting for the HAW (~ 1 m above the
water line) samples show the least
amount of variation in grain sorting
among all the other samples. Over the course of 225 km
from White Springs gauge to Wilcox gauge the AW
samples (~ 30 cm above water line at erosional
escarpment) showed the greatest amount of variation
from well sorted to moderately sorted. Finally for the
two BW samples (~ 30 cm below water line) the samples
experience variation between the four gauging stations. From White Springs to Ellaville the
sorting gets poorer then gets more wellsorted at Branford gaugewhile seeing the poorest sorting
at the Wilcox location. The only very well sorted samples came from the delta deposits located
at the mouth of the river (average sorting of 0.27Ф).
Sorting
< 0.35Ф very well sorted
0.35 - 0.50Ф well sorted
0.50 - 0.72Ф moderately well sorted
0.71 - 1.00Ф moderately sorted
1.00 - 2.00Ф poorly sorted
2.00 - 4.00Ф very poorly sorted
> 4.00Ф extremely poorly sorted
Table 4.1 – showsterminologyforsorting
standarddeviationvalues.(Folk,1974)
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0255075100125150175200225250275300
GrainSorting(Ф)
DistanceUpstream (km)
HAW AW1 AW2 BW1 BW2 Delta
Figure 4.1 - has been strategically formated to display the from
left to right, the change in grain sorting as you go downstream.
The X-axis displays the river mouth (0 Km upstream) as the right
side of the graph. Grain sorting from bottom to top, very well
sorted to moderately sorted.
White
Springs
Ellaville Branford Wilcox
River
Mouth

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4.2 Texture
The following is an observation of the texture of grains as grains traveled downstream in
the Suwannee River. In some cases it appears that grain roundness becomes more angular. For
example the HAW samples at White Springs Gauge display subangularity while samples 225 km
downstream at Wilcox gauge also display subangular grains. The grains on the delta are even
more angular, and are described as subangular to angular. The AW samples at White Springs
gauge are described as subrounded/subangular and are described as AW1 subrounded and AW2
angular at Branford gauge,~ 150 km downstream. Another ~ 60 km downstream to Wilcox gauge
AW1 is described as subangular and AW2 subrounded. The BW samples are generally described
as subangular to subrounded throughout the 225 km of downstream movement. The
Composition throughout the system remains relatively constant at an average of 97% quartz.

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4.3 Grain Size Analysis
Values of mean and median grain
size are extremely similar,
meaning that there relatively
symmetric unimodal grain size
distributions of each sample so
mean grain size (fig 4.3) will be
used to describe grain size. When
plotting mean and median grain
sizes as they relate to distance
traveled It is clear that mean
grain size between White Springs
(2.05 Ф – 2.28 Ф) and Ellaville
(2.22 Ф – 2.56 Ф) do actually get
smaller before once again getting larger at Branford (2.04 Ф – 2.17 Ф and especially at Wilcox
gauge (1.22 Ф – 1.66 Ф). While once again the delta sediments fall in line with conventional
thinking and display some of the smaller grain sizes (~ 2.5Ф).
5 - Discussion
From the results gathered, I suspect that there must be some change in properties of the rock
that the
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
0255075100125150175200225250275300
MeanGrainSize(Ф)
DistanceUpstream (km)
HAW AW1 AW2 BW1 BW2 Delta
Figure 4.3 - has been strategically formated to display thefrom left to
right, the change in mean grain sizeas you go downstream. The X-axis
displaystheriver mouth (0 Km upstream) as the rightsideof the graph.
Median grain sizeis shown to increasefrombottom to top, Y-axis.
bottom to top. Distanceupstream
White
Springs
Ellaville Branford Wilcox River
Mouth

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Suwannee River meanders through to create such
unconventional grain size, grain sorting and textural
properties as grains travel downstream. There must
be a reason why grain sorting gets poorer between
the White Springs and Ellaville gauge then seems to
progress towards a more better sorted sediment at
Branford gauge. Between Branford and Wilcox the
sorting once again become poorer before becoming very well sorted in the delta deposits.
Research into the coarsening of grain size and worsening of grain sorting led to a Google
Earth investigation
of the region.
Following the river
looking for changes
in Suwannee River
discharge or some
other reason why
grain sorting would
get poorer over the
74 km stretch
between White
Springs gauge and Ellaville gauge. The Ellaville gauge is located just downstream and across on
the other side of the Suwannee from the confluence of the Suwannee River and the
Location
Distance Upstream from
River Mouth
Headwaters 396 km
White Springs 275 km
Ellaville 201 km
Branford 114 km
Wilcox 53 km
Delta 0 km
Table 5.1 – Distance of gauge locations
upstreamfrommouthof Suwannee River
in km. (USGS, 2014)
0.00
100.00
200.00
300.00
400.00
500.00
600.00
700.00
800.00
900.00
10/1/2012 11/30/2012 1/29/2013 3/30/2013 5/29/2013 7/28/2013 9/26/2013
Discharge(m3/s
Time (days)
DischargeRate Over Time
White Springs Ellaville Bradford Wilcox
Figure 5.1 – showsthe discharge rate inthe fourgaugingstationsfor
the 2012 – 2013 wateryear.

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Withlacoochee River. The Withlacoochee River originates just northwest of Valdosta, Ga, 185 km
upstream from its confluence with the Suwannee. That is an additional 64 km of the potential for
grains to transport which would explain why grains are extensively smaller here than anywhere
else in the Suwannee system. The increase in volume of water also increases the discharge rate
(area of cross section * mean velocity) which is an increase in width or depth of the river, the
velocity, or both. By comparing river profiles and graphical representations of mean discharge
rates between the White Springs and Ellaville gauge it is obvious that just downstream from the
confluence of the Withlacoochee River, the Suwannee has a much larger river profile and much
higher stream discharge rates, especially during times of higher rainfall values. Ribeiro et al.
(2012) suggests that at river confluences the tributary enters the main channel and only
penetrates the upper portions of the water column while the main river water column is hardly
hindered. This would be true here assuming that the Withlacoochee River has a higher bed
discordance (channel elevation) than the Suwannee River. Ribeiro et al, further explains that
sediment transport capacity is increased due to an increase in stream velocity. This conforms to
the data and observations made at the Ellaville gauge just south of the confluence. As the higher
0
2
4
6
8
10
12
14
16
18
20
0 25 50 75 100 125
ElevationaboveSeaLevel(m)
Distancefrom arbitrary location beyond left
bank (m)
10
12
14
16
18
20
22
24
26
28
30
0 20 40 60 80 100
Distancefrom arbitrary location beyond left
bank (m)
Figure 5.2 (a) – shows the mean river height (blue line)
against the river profile for the Wilcox Gauge site.
Figure 5.2 (b) – shows the mean river height (blue line)
against the river profile for the Ellaville Gauge site.

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portion of the water column flows across the top of the Suwannee, it is possible that its effect on
the opposite bank (where Ellaville gauge is located) creates a deposition bar that reduces the
flow area and causes flow acceleration that contributes to an increase in sediment transport
capacity.” (Ribeiro et al, 2012). This explains why there is a decrease of sorting at Ellaville and a
decrease in mean grain size. The sorting gets worse because the finer material from the
Withlacoochee is building up along the bank of the Ellaville gauge essentially becoming more of
the sediment percentage along the bank while river velocity increases because of the thinning of
the river profile compared to just before the confluence.
Between the Ellaville gauge and Branford gauge exhibited no visible evidence water
injection or any other
additional change to water
volume and velocity other than
the obvious accumulation of
rain water. By using samples
that were below water line for
both Ellaville and Branford
locations and looking at their
graphs, grain sorting once
againbecomes wellsorted but mean grain sizecoarsens for allsamples.According to convention,
an increase in stream velocity conforms to an increase in grain size. Therefore the Suwannee
River must be accumulating large amounts of water between these two gauge sites. A look at the
river profile reveals that the stream cross section is even larger than the Ellaville gauge which
-2
0
2
4
6
8
10
0 20 40 60 80 100 120
Distancefrom arbitrary location beyond left bank (m)
Figure 5.2 (c) – shows the mean river height (blue line) against
the river profile for the Branford Gauge site.

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means that in order for velocity to increase, an injection of water must occur between the Ellaville
and Branford gauges. The only viable explanation is of aquifer spring injection into the river and
rain accumulation.
By examining the length of the Suwannee between Branford and Wilcox it is clear that
another tributary flows into the Suwannee. Only this time it is ~ 47 km upstream from the Wilcox
gauge (~ 15 km downstream from the Branford gauge). This obviously further increases the
stream discharge. By looking at the river profile, we see that the stream has become significantly
lower gradient throughout its travel from the Okefenokee to the Wilcox location. The stream bed
has also become smoother across the section. Between Branford and Wilcox gauges the grains
of all samples get more poorly sorted and progressively coarser. This suggests that the mass of
water at Wilcox is moving at an even quicker velocity than anywhere else on the stream, which
conforms to discharge
rates throughout most of
the year (fig 5.1).
Finally at the delta
we have the smallestmean
grain sizes (2.5 Ф) and its
average sorting is very well
sorted (0.27 Ф). From
common knowledge it makes sense that the smallest grains would accumulate here where the
Suwannee fans out and loses the majority of its velocity as it empties into the Gulf of Mexico. So
this is where all the small grains that were missing from the majority of the Suwannee River
-8
-6
-4
-2
0
2
4
6
8
0 20 40 60 80 100 120 140 160
Distancefrom arbitrary location beyond left bank (m)
Figure 5.2 (d) – shows the mean river height (blue line) against
the river profile for the Branford Gauge site.

15
ended up. In the end grain size fines downstream while sorting follows suit and becomes more
well sorted downstream. The last parameter, angularity, is the only outlier that showed no
specificpattern throughout the system. In the end texture remained subangular to angularat the
last place of possible deposition for the Suwannee River, the delta.
6 - Conclusions
It is concluded that a variety of factors (tributary injection and rainfall runoff) led to
increases in stream velocity effectively increasing the grain entrainment potential to carry larger
and larger grains as they travel downstream the Suwannee River. Sediment grain sizes did get
smaller from White Springs gauge to the river mouth but at locations in between the grain sizes
were dependent on the presence of an increase in water, either from a tributary confluence or
possible spring injection from the aquifer. Grain sorting also became more well sorted overall
from headwaters to mouth, but once again the local sorting depended on a variety of variables,
even which side of the bank samples were taken from can affect the results of the study. Finally
grain texture failed to become rounder as they moved downstream. I have no hypothesis why
other than because of their extremely small sizes they have become more tabular in shape like
clays and silts tend to be.

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Bibliography
District, S. R. W. M., SRWMD River Stations: http://www.mysuwanneeriver.org/rivers.htm,
SRWMD, USGS.
Folk, R. L., 1974, Petrology of Sedimentary Rocks, Austin, Tx, Hemphill, . 182
Gelbart, M., 2010, The Geological and Ecological History of the Okefenokee Swamp, Volume
2014: http://markgelbart.wordpress.com/2010/11/19/the-geological-and-ecological-
history-of-the-okenfenokee-swamp-part-one/, p. Provides a brief Geologic History of
the Okefenokee Swamp.
Knighton, A. D., 1980, Longitudinal changes in size and sorting of stream-bed material in four
English rivers.: Geological Society of America Bulletin, v. 91, no. 1, p. 55-62.
Ribeiro, M. L., Blanckaert, K., Roy, A. G., and Schleiss, A. J., 2012, Flow and sediment dynamics
in channel confluences: Journal of Geophysical Research-Earth Surface, v. 117.
Scott,T. M., Campbell,K.M.,Rupert,F.R., Arthur,J. D.,Missimer,T.M., Lloyd,J.M., Yon, J.W., and
Duncan,J. G., GeologicMap of the State of Florida:FloridaGeological SocietyFlorida
Departmentof EnvironmentalProtection.
Snelder, T. H., Lamouroux, N., and Pella, H., 2011, Empirical modelling of large scale patterns in
river bed surface grain size: Geomorphology, v. 127, no. 3-4, p. 189-197.
USGS, National Water Information System: Web Interface, Volume 2014:
http://waterdata.usgs.gov/nwis, USGS, p. Provides location and water data for a variety
of water gauges operated by the USGS.

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