A Comparative Study of Grain Size Distribution and Mineralogy of Sandy
Sediments between Kasekera Stream (Forested) and Mtanga Stream (Deforested) as
Related to On Shore and Near Shore Environments: Lake Tanganyika, East Africa
Students:
Morgan Helfrich, University of Arizona
Mwasiti Rashid, University Dar es Saalam
Mentor:
Kiram Lezzar
Introduction
Lake Tanganyika is an ancient, tropical, fresh water lake associated with the Great African Rift.
Due to active deforestation in many areas surrounding Lake Tanganyika, there is a measurable increase in
sediment discharge entering the lake. Certain protected areas (protected from fire and deforestation) allow
for comparison of sediment discharge, including grain size distribution and lithological characteristics,
between deforested and forested regions.
Lake Tanganyika’s main water source is surface rainfall and direct runoff. The annual rainfall in
the Tanzanian part of the Lake Tanganyika watershed is approximately 1200mm per year. Only a small
percent of the surface rainfall percolates into groundwater, whereas a much higher percent of rainwater
flows freely over the land. Most of the rain is trapped by the high escarpments and fed into streams and
river valleys that feed into the lake (Nkotagu, 2007). These catchment systems transport sediments from
very fine grain to pebble size into river valleys and the lake. The following study investigates recent
patterns in grain size distribution between 0.063mm and 2.0mm, and clastic mineralogical components of
two streams feeding into the lake. The deforested Mtanga Stream watershed, and the forested Kasekera
Stream watershed are extremely similar, except that one is protected against fire and deforestation while the
other is populated, commonly deforested and subject to frequent fires. By collecting surface grab samples
from along the stream channel and into the deltaic regions, one can estimate recent sedimentation patterns
resulting from erosional differences of the surrounding land and bedrock. The amount of each size and
lithology of sediment deposited on shore in the river valleys and in near shore environment depends on the
running rivers and stream energy, and the physical properties of the streams (steep or gentle slope, forested
or deforested catchments). This is important because it potentially helps us understand the effects of
deforestation on watershed dynamics and impacts as related to onshore erosion and offshore platform
sedimentation patterns. By understanding present day sedimentation processes and how they affect
variability in grain sizes and mineralogy within protected and unprotected stream channels, we may be
better able to reconstruct the past history of such streams from sediment cores (e.g. Hartwell, 2005).
The two watersheds that were sampled from are in close proximity. Deforested Mtanga (S: 46.503,
E: 36.227, EV: 774m), and forested Kasakera (S: 40.191, E: 37.374, EV: 770) are approximately 30km
apart from each other at the shore mouth (Ramos, 2007). As a result they share the same general climatic
conditions. There is a wet season from October through April, which is followed by a dry season from
May to September. All of the sampling was done in the middle of the dry season during the month of July.
Both streams feed into the eastern shore of Lake Tanganyika, Tanzania, East Africa. Kasekera and Mtanga
are both ~3.5km2 (Lezzar, 2007). Kasekera is located in the middle of Gombe National Park (16km North
of Kigoma Town) and is not populated. Mtanga is located at the Mtanga Village 5km south of Gombe
National Park and has a population of approximately 800 people (field observation). The population of
Mtanga has contributed to the deforestation of the region due to local farming and frequent uncontrolled
fires. Both streams have similar gradients upstream. Also, they have the same bedrock lithology of
Kigoma Quartzite. Because of these similarities, Kasekera and Mtanga are ideal localities for a
comparative deforestation study.
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Methods and Materials
Sample Collection
Samples were collected from each stream, from the north and south banks and also from the center
of the stream. From 0m to 500m from the shoreline, the samples were collected at 50m intervals and from
500m to 1000m, the samples were collected at every 100m. At each of these locations, the width and depth
was measured and recorded as well as gravelometry and sediment depth by use of a penetration rod. In the
offshore deltaic region, the geo-boat team collected grab samples on an offshore delta transect at various
depths. The geo-dive team collected grab sample 30m north and south of the transect at various depths.
Grain Size Analysis
From each wet stream and delta sample, ~ 50g was weighed out. The samples were wet sieved
through a 0.063mm sieve. The sample < 0.063mm was separated for fine grains size analysis. Each
sample was then dried in a drying oven at 110º C overnight. When the sample was dried, the dry weight
was recorded. Each sample was dry sieved through a stack of sieves for ten minutes. The fractions
separated were: > 2.0mm (separated for large grain size analysis), 2.0 – 1.0mm, 1.0 – 0.5mm, 0.5 –
0.425mm, 0.425 – 0.25mm, 0.25 – 0.125mm, 0.125 – 0.063mm, and < 0.063mm. Each fraction was
weighed on a balance to +/-0.01gm accuracy and the percent of each fraction was calculated.
Mineralogical Analysis
Each wet stream and delta sample was stirred thoroughly to mix all minerals of various densities
and sizes. Approximately 5g of each sample were weighed out on a balance to 0.01gm accuracy. Each
sample was placed in a 6” test tube and 20 drops of 50% H2 O2 were added and mixed in. The samples
were placed in a boiling water bath to increase the reaction rate. Every 20 minutes 10 more drops of H2O2
were added and mixed in. This process continued for 2 hours until the organic material was burned off and
only clastic material was left. The samples were dried in the drying oven at 110º C until completely dry
(~5hrs). Each sample was analyzed under binocular microscope. For the coarser grain sediment (onshore
sediment), all of the lithic minerals were picked out with tweezers and analyzed. Percent lithics to quartz
was calculated by weight percent. For the finer grain sediment (offshore sediment), 1cm of mixed
sediment was analyzed in a scaled Petri dish. The number of lithic grains and the number of quartz grains
were counted and a percentage was calculated from counted grains. For each sample, the lithics were
categorized into five groups: magnetite, muscovite, an unknown blue mineral, iron, and all other minerals.
These were counted and a percent of each mineral was calculated from counted grains.
Results
Grain Size Results
For analysis of the grain size data, data was first compared in the simplest way by showing the
comparison of the modal fraction of grain size from the center transect of each stream sampled from 1000m
up shore to the shore mouth (figure 1). Also illustrated is the comparison of the modal fraction of grain
size from the shore mouth to furthest depth sampled in each deltaic region (figure 2). The data illustrates a
difference between forested and deforested region of a linear path of sediment distribution, not including
the banks of the streams.
For a more in-depth representation of grain size distribution, lateral graphs were made of both the
deforested Mtanga region (figure 3) and forested Kasekera region (figure 5) illustrating the fluctuation of
the modal grain size as it is distributed up stream from 0m (shore mouth) to 1000m. This representation
was repeated for the second greatest fraction of grain size as it is distributed up steam. Comparing both the
greatest abundance fractional grain size and the second greatest abundance fractional grain size gives a
perspective of multiple variations in sandy sediment distribution. A similar representation was made for
the offshore deltaic region comparing both the modal fractional grain size and the second greatest
18
abundance fractional grain size as is distributed from 3m depth to 95m depth. Data was analyzed for both
deforested Mtanga (figure 4) and forested Kasekera (figure 6).
Mineralogy Results
Mineralogical data was compared between the deforested and the forested regions. Illustrated in
graphical format is the fluctuation of lithologic percentages compared to quartz. There is a linear trend
showing an apparent increase in lithologic composition flowing down stream and into the deltaic regions.
Although there is a very high percentage of quartz in both the deforested and forested systems, there are
noticeably more lithic minerals in the forested region.
Conclusion
This study compared multiple stream analyses from both field observations and laboratory
experimentations. The significant differences between these two streams are correlated with patterns of
deforestation. Deforestation was common up the entire coast of Lake Tanganyika. This occurs in all
uncontrolled areas surrounding the lake. Increased farming and other anthropologic reasons combined with
the occurrences of frequent uncontrolled fires are impacting the environment dramatically. The following
statements conclude the research illustrated in this paper:
1. In the forested watershed, there was an apparent fining trend down to the shore, which continued into
the deltaic region.
2. In the deforested region, there was no systematic trend of grain sizes. The grain sizes were distributed
randomly throughout both the onshore and offshore regions.
3. In the forested region, there was a higher percentage of non-quartz minerals to quartz. This proportion
increased downstream and into the deltaic region.
4. In the deforested region, the trend in quartz/non-quartz minerals downstream and onto the delta was
similar to that of the forested region. However, the percentage of quartz to non-quartz was consistently
higher than for the forested stream.
5. Due to greater sediment discharge in the deforested region, the morphology of the stream is wider, flatter
and straighter than that of the forested region.
6. A smaller number of roots and amount of vegetation holding the earth together in the deforested region,
probably results in decreased stability of the ground surface on the steep escarpments, making these areas
prone to mass wasting.
References
Hartwell, R.J. and Daudi, F. (2005) Sediment distribution and analysis of TAFIRI Bay and Luiche River
platform, Lake Tanganyika, East Africa: a prediction of hydrodynamic transport and wave-influenced
deposition on clastic sedimentation. The Nyanza Project Annual Report. The University of Arizona, Dpt.
Geosciences.
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Figure 1 (right): Distribution
of modal fraction of
grain size from the center
of each stream: Kasekera
(forested), and Mtanga
(deforested). The relation
of each stream as it fluctuates
from 0m (shore mouth) to
1000m upstream. Grain size
was much more consistent in
the forested region and
considerably more variable
in the deforested environment with
the exception of one anomaly.
Figure 2 (right): Distribution of
modal fraction of grain
size from the offshore deltaic
transect of each stream: Kasekera
(forested), and Mtanga (deforested)
between 0-95m water depth (deepest
sample collected).
Grain size was much less variable
in the forested region relative to the
deforested environment. The two
shallowest data points in the Kasekera
graph are questionable and should be
disregarded.
Largest
Percentage
of
Sand
Fraction
On
S hore
0 5-
Mtanga
0 425-
Kasekera
0.25- 0.125
0 125 0 063
0
50
100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 100
0
Distance
upstream
Distribution
of
in
Modal
meters
Grain
Size
in
Sand
Fraction
Off
Shore
0.50-0.425
Mtanga
0 425 0 25
Kasekera
95
90
85
80
75
70
65
60
55
Water
Figure 3 (right):
Distribution of modal and 2nd most
abundant fraction of grain
size distribution from the
north and south banks, and
the center of Mtanga
(deforested) during
dry season.
20
50
depth
45
40
in
meters
35
30
25
20
15
10
5
0
Figure 4 (right):
Distribution of modal and
2nd most abundant fraction
of grain size distribution
from the offshore delta
transect of Mtanga
(deforested) during
the dry season.
Figure 5 (right):
Distribution of modal and
2nd most abundant fraction
of grain size distribution from the
north and south banks, and
the center of Kasekera
(forested) Stream during the
dry season.
Figure 6 (right):
Distribution of modal and
2nd most abundant fraction
of grain size distribution
from the offshore delta
transect of Kasekera
(forested).
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Fluxuation of Lithologic Minerals in Mtanga Stream
Figure 7 (right): Proportion of non-quartz
to quartz minerals distributed through
the center of the entire Mtanga water
column from 1000m up shore to 28m
depth. Onshore, samples were analyzed
at 1000m, 500m, and 0m from river mouth).
Offshore, samples were analyzed at
13.6m depth and 28m depth. There is
a linear trend of increasing non-quartz
minerals feeding down stream and into
the deltaic region.
30
25
20
15
10
5
0
R l ti
Site 1
Site 2
Site 3
Site 4
Site 5
P
Site Where Sample was Collected (1000m Upshore to 28m Deep in the Delta)
t f Lith
Mtanga X vs Mtanga X
Fluxuation of Lithologic Minerals in Kasekera Stream
Figure 8 (right): Proportion of non-quartz to
quartz minerals distributed through
the center of the entire Kasekera water
column from 1000m upstream to 16m water
depth. Onshore, samples were analyzed
at 1000m, 500m, and 0m from the river
mouth. Offshore, sample was analyzed at 16m
depth. There is a linear increase in the
proportion of non-quartz mineral feeding down
stream and into the deltaic region. There
is also a higher percentage of lithic grains
in the Kasekera region.
30
25
20
15
10
5
R
l ti
0
Site 1
Site 2
Site 3
Site 4
Site Where Sample was Colleted (1000m Upshore to 16m Deep in the Delta)
P
t f Lith
Kas vs Kas
22