A Comparison of Grain Sizes

Suwanee River Project

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 especially important 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 like any 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

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undoubtedly effect the rivers in one way or another. What 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 stream development. For example, dredging along the bottom of a stream can

increase stream volume and slow velocity, therefore allowing smaller particles to settle easier.

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, because over time sediment builds over top of sediment below

creating layers that vary in parameters such as grain size, sorting, composition, etc.

3


Distinguishing the differences between the layers of sediment can give geologists a good idea

about what the 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

4


second sampling site. The Suwannee Limestone is only a surface rock near the river itself, once

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.

5

Figure 3.1 – shows the extent of the study area (Northern Central Florida). The sediment sampling sites are shown along the Suwanee River and also the surface Geology of the region is shown. (Scott et al.)



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 each specified location at each

water 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.

6

Figure 3.2 – shows the sediment sampling technique at each gauge siteFigure 3.2 – shows the sediment sampling technique at each gauge siteFigure 3.2 – shows the sediment sampling technique at each gauge siteFigure 3.2 – shows the sediment sampling technique at each gauge siteFigure 3.2 – shows the sediment sampling technique at each gauge siteFigure 3.2 – shows the sediment sampling technique at each gauge site


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.

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. These samples were then sieved (63 µm) to remove clay sized particles. 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 be forwarded to Dr. Jaeger of the

University of Florida, who performed the settling velocity analysis on all samples and generated

grain size.

7


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 well sorted at

Branford gauge while seeing the poorest sorting at the Wilcox location. The only very well

8

Table 4.1 – shows terminology for sorting standard deviation values. (Folk, 1974)

02550751001251501752002252502753000.200

0.300

0.400

0.500

0.600

0.700

0.800HAW AW1 AW2BW1 BW2 Delta

Distance Upstream (km)

Grai

n So

rting

(Ф)

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.

WhiteSprings Ellaville Branford Wilcox

RiverMouth

Sorting< 0.35Ф very well sorted

0.35 - 0.50Ф well sorted0.50 - 0.72Ф moderately well sorted0.71 - 1.00Ф moderately sorted1.00 - 2.00Ф poorly sorted2.00 - 4.00Ф very poorly sorted

> 4.00Ф extremely poorly sorted


sorted samples came from the delta deposits located at the mouth of the river (average sorting

of 0.27Ф).

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

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02550751001251501752002252502753001.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

HAW AW1 AW2 BW1 BW2

Distance Upstream (km)

Mea

n Gr

ain

Size

(Ф)

Figure 4.3 - has been strategically formated to display the from left to right, the change in mean grain size as you go downstream. The X-axis displays the river mouth (0 Km upstream) as the right side of the graph. Median grain size is shown to increase from bottom to top, Y-axis.bottom to top. Distance upstream

WhiteSprings Ellaville Branford Wilcox River

Mouth


From the results gathered, I suspect that there must be some change in properties of the rock

that the

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.

11

Table 5.1 – Distance of gauge locations upstream from mouth of Suwannee River in km. (USGS, 2014)

Location Distance Upstream from River Mouth

Headwaters 396 kmWhite Springs 275 km

Ellaville 201 kmBranford 114 kmWilcox 53 kmDelta 0 km


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 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

12

0 25 50 75 100 12502468

101214161820

Distance from arbitrary location beyond left bank (m)

Elev

ation

abo

ve S

ea Le

vel (

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.

Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year. Figure 5.1 – shows the discharge rate in the four gauging stations for the 2012 – 2013 water year.

Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.Figure 5.2 (a) – shows the mean river height (blue line) against the river profile for the Wilcox Gauge site.


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 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.

13


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 again becomes well sorted

but mean grain size coarsens for all samples. 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 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.

14

0 20 40 60 80 100 120-2

0

2

4

6

8

10


Elev

ation

abo

ve S

ea Le

vel (

m)

Figure 5.2 (c) – shows the mean river height (blue line) against the river profile for the Branford Gauge site.


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 smallest mean

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 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 specific pattern

throughout the system. In the end texture remained subangular to angular at the last place of

possible deposition for the Suwannee River, the delta.

6 - Conclusions

15

0 20 40 60 80 100 120 140 160-8

-6

-4

-2

0

2

4

6

8


Elev

ation

abo

ve S

ea Le

vel (

m)

Figure 5.2 (d) – shows the mean river height (blue line) against the river profile for the Branford Gauge site.


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.

Bibliography

District, S. R. W. M., SRWMD River Stations: http://www.mysuwanneeriver.org/rivers.htm, SRWMD, USGS.

16


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., Geologic Map of the State of Florida: Florida Geological SocietyFlorida Department of Environmental Protection.

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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A Comparison of Grain Sizes

Documents

suwannee river

river mouthdelta

suwanee riverproject

suwannee limestone

transport grains

angular grains

low gradient

sediment deposition