IMPACT OF DAMS ON MORPHOMETRIC CHARACTERISTICS
OF STREAMS: A CASE OF SHASHE DAM IN NORTH EASTERN BOTSWANA
B.P. Parida1 , D. B. Moalafhi and Kentse Sebokolodi
Faculty of Science, University of Botswana, Private Bag UB 00704, Gaborone, Botswana
1Corresponding author; paridab@mopipi.ub.bw
ABSTRACT:
Water is a very scarce resource in Botswana, due to semi-arid climate, characterized by recurrence
droughts and needs various methods of harnessing and conservation of water, in order that the growing
demands of this scarce resource could be met adequately. This has necessitated construction of dams
wherever feasible such that water from the ephemeral rivers could be stored and supplied to urban
centers as per requirement. While most of the times the downstream impacts of such constructions are
discussed, the impacts on the river plan form upstream of the structure is often ignored. A case study
on Shashe dam located in the North eastern Botswana shows that such a constructions can highly
impact stream characteristics and hence hydrological processes upstream. It has been seen that there
has been considerable changes in the Horton Order Ratios and also the Stream Frequency as well as the
Drainage Density of the catchment by creation of many first order streams in the headstream regions of
the catchment and reduction in the drainage efficiency.
KEY WORDS: Stream order, Morphometry, Climate change, Stream density, Stream frequency
1
INTRODUCTION
stream, altering water quality and widespread
environmental degradation of rivers (Strahler
and Strahler, 2002). So, the damming of rivers
can not only have impacts on the down stream
It is an established fact that man first attempted
dam construction for the purpose of river
regulation some 5000 years ago in Egypt
(Robert, 2003). Since then, there has been an
increase in the number and capacity of dams
constructed with improved design studies as
the world of science and engineering
developed. Today dams are one of the most
widespread human made features on the
world’s rivers for multi-purpose use such as:
storage of water, flood control, hydropower
generation, fish culture and recreation. While
their importance are enumerable, specially in a
in a semi-arid climate like Botswana where
water is scarce and rivers are ephemeral, they
can have several invisible impacts which are
complex and sometimes difficult to evaluate
(Wellmeyer et al, 2005).
flow regime but also on the physical
characteristics of streams upstream, which feed
the dam-reservoir. Attempts to mitigate these
environmental damages require sound and
scientific understanding on how dams affect
the morphometric characteristics of rivers
(Chorley, 1969).
Despite the rapid increase in geomorphological
researches over the past two decades, and the
awareness of the need to evaluate the
environmental impacts on the landscape on
continental waters and more specifically on the
river channels, the impacts of dam construction
upon stream channel forms have virtually been
neglected (Bousmar & Zech, 2002). However,
in one of the more recent indicative studies, it
has been pointed out that construction of dams
have resulted in modification of stream
characteristics within the drainage basin
(Robert, 2003).
It is well known that the damming of rivers
may change the flow regime and in particular
the seasonal distribution of flows, reduce the
incidence and severity of flooding and
decrease the long-term average flow of streams
including the timing, magnitude and duration
of stream flow (Gregory, 1977). In addition to
these and physical segmenting of streams,
dams disrupt the natural processes by blocking
the transport of sediments in the river or
The goal of river management should then be
to use the baseline conditions of the stream
characteristics as the yard stick against which
1
to study the impacts of the dam with the aim of
optimizing the impacts on the river network
system. With this study, the drainage
characteristics that are commonly used in
geomorphologic studies and to some extent
river engineering will be studied. This study
will help generate information that could be
used in monitoring the dam and in regular dam
operation conditions.
eastward passing through Francistown to
Tonota where the dam is located. The Shashe
is a highly ephemeral river, with flow
restricted to a few days of the year and
contributes about 12.2 % of the mean annual
runoff of the Limpopo Basin It covers a
catchment area of 3600 km2 at the Shashe dam,
with a storage capacity of 85 M m3 at Full
Supply Level (971.5 m above MSL) with an
inflow volume of 90 M m3/a. It is one of the
most imporatnt reservoirs of the country from
which most of the water requirement for the
country is met.
It is increasingly becoming necessary to
quantitatively investigate and assess the
morphometric characteristics of the drainage
basins prior to dam constructions and
monitoring for any changes during the life
span of the structure, such that appropriate
catchment management activities could be
undertaken to preserve the initial conditions in
the basin. For this, the Shashe dam catchment
located in North-Eastern Botswana, a major
supplier of water to urban centers of Botswana,
has been chosen as a case study, such that it
would help/contribute towards building sound
river management policies and/ or guidelines.
The vegetation is characterized by tall grass
and short scattered trees, and savanna trees and
savanna vegetation with Mophane prevalent
(Leong, 2001).
2.2
Research Objectives
To study the impact of the Shashe dam on the
the morphometric characteristics of four subcatchments namely, Shekwe-Maeroro, Lonye,
Mooke and Kgonyane, it has been proposed to
address the flowing three research questions:
2.0.
Study Area
Shashe River is located in the northeastern part
of Botswana, about 25 km south of
Francistown and 5 km west of the great
Gaborone/Francistown A1 highway (Figure 1).
It flows from up north down until it passes
through Francistown, to Tonota where the
Shashe dam is. The drainage map for the study
basin at Shashe dam is shown in Figure 2.
(a) How has the drainage patterns in the basin
in terms of drainage density and stream
frequency changed since the dam was
constructed?
(b) What changes in the lengths, numbers and
areas drained by various order streams have
taken place?
(c) How have the Horton order ratios RA, RB
and RL been affected?
Since the Shashe dam catchment is a large
catchment consisting of quite a number of subcatchments, and with the time constraints the
research will only focus on four subcatchments, which are the Shekwe- Maeroro,
Lonye, Kgonyane and Mooke.
3
METHODOLOGY
To address the above research objectives, the
following stream characteristics have been
proposed to be computed using information at
the time of dam construction (1970) and 20
years after (1990).
These are: Drainage
Density (Dd), Stream frequency (F), Horton
Order Ratios namely the Area Ratio (RA),
Bifurcation Ratio (RB) and the Length Ratio
(RL).
2.1.
Physical Environment
The climate is characterized by hot, rainy
season from November to March and cool, dry
season from May to September. The
temperatures mainly range between 24 and 35
degrees Celsius in the rainy season and to as
low as between 13 and 18 degrees in the dry
season i.e. winter. Rainfall is prevalent during
the rainy season in summer and on average the
amount ranges between 350 and 400 mm per
annum.
Aerial photographs and topographic maps have
been used as the main source of data to
establish a baseline survey of plan form change
prior to and even after the dam construction.
These photographs were converted into maps
using arc-view and these maps were then georeferenced and converted to small and
common scales for easy workability.
The environment is typical of the basement
system of Botswana. There are small rivers,
which are ephemeral and flow in the rainy
season from October to March, discharging
their flow into the Shashe River, which flow
3.1
2
Drainage Density (Dd)
This is defined as the length of drainage per
unit area and is expressed in km per km2 as:
Dd = Total length of streams / Basin Area
1970, Tables 5-8 present such characteristics
after the dam construction i.e. for the year
1990. The changes in Drainage density and
Stream frequency have respectively been
presented in Tables 9 and 10 respectively.
(1)
Clearly Dd shows the closeness of channel
spacing. It is an indication of the drainage
efficiency and also of the soil distribution
pattern of the basin.
Average values of the Horton Order Ratios,
Drainage Density and Stream Frequency
however have been summarized in Table 11.
Changes in RA, RB and RLat the chosen subcatchments have been shown in Figures 3 - 5.
Besides, graphical relationship between the
Nw and the stream orders during the year of
dam construction and 20 years after the year of
dam construction has been shown in Figure
6(a) and 6(b) respectively. Similar relations for
Lw have bee shown in Figures 7(a) and 7(b) .
3.2
Stream Frequency (F)
Horton defined stream frequency as the
number of stream segments per unit area:
F = Total number of segments of all orders
within the given catchment / Area of the
catchment
(2)
3.3
Horton Order Ratios (RA , RB and
RL)
Areas drained by various orders of streams can
be represented by an overall Area ratio for the
basin. Similarly, the average length of streams
and also the number of channels can
respectively be represented by the Channel and
Bifurcation ratios. These can separately be
given by RA , RL and RB as given below:
RA = Area Ratio = Aaw / Aaw-1
It is evident from the aforesaid results, that
changes in all the geo-morphologic
characteristics considered in this study have
taken place over the study period of 20 years.
A closer look at Table 11 and Figures 3-5,
suggest that while average values of RA and
RB generally have decreased by over 40% and
20% respectively, the average value of RL has
increased by about 8%. Even from the same
table it is evident that during the same period
the stream frequency and the drainage density
have gone up by more than 10% and 25%
respectively.
(3)
where Aaw = average area drained by stream
order w in km2
RL = Length Ratio = Law / Law-1
(4)
In addition to these, relations between the
stream orders and number of streams and
stream orders and length of stream have shown
changes in slopes. Stream order and stream
numbers relationship showed that the slope in
1970, which was –0.7512, became steeper in
1990 with a value of –0.5662. Relation of
stream order to stream lengths also showed
similar change in slopes which changed from –
0.3918 in 1970 to –0.27 in 1990. Since these
values are of the average of the entire drainage
basin, some individual sub-catchments have
shown much of the differences especially the
Lonye sub-catchment due to a lot of tillage of
soils because of cultivation and some
plantation activities. However, the overall
changes suggesting a possible change in the
soil and the geomorphologic features may also
be due to impact of some geologic factors and
gradient, slope and relief of the area.
where Law = average length of streams of
order w in km
RB = Bifurcation Ratio = Nw / Nw+1
(5)
where Nw = total length of streams of order w .
For uniform soils, RA, RB and RL usually have
values between 3-6, 3-5 and 1.5-3.5
respectively. In view of this, these parameters
can be used to assess the soil distribution
pattern in the basin and also for runoff
estimation from ungauged basins (Bhaskar et
al., 1997).
4
DISCUSSION OF RESULTS
As said earlier, analyses were undertaken on
the four sub-catchments namely, ShekweMaeroro, Lonye, Mooke and Kgonyane, and
the results have been presented in Tables 1
through 11. In particular, while Tables 1-4
present the Horton Order Characteristics
before the dam construction i.e for the year
5
CONCLUSIONS
Based on the overall results it can be
concluded that ever since the dam was
constructed (even if the time period was much
3
small compared to the life of the structure),
the stream characteristics changes have taken
place in general affecting the Horton Order
Ratios like RA, RB and RL, as well as the
Stream Frequency and Drainage Density.
Chorley R.J (1969); Introduction to physical
hydrology, Methuen & Co. Ltd, USA
1. Decrease in RA by over 40% suggestS that
areas drained by lower order streams
(headstreams) have increased mostly, may be
due to upstream degradation and the watershed
has now more uniform soil pattern, which even
conforms to the observation of Mourier, et al.,
(2008).
Leong G.H (2001); Certificate physical and
human geography, 3rd edition, New Oxford
Progressive Geography, China
Gregory J.K (1977): River Channel Changes,
John Willey and Sons, New York
Mourier, B., Walter, C. and Merot, P., (2008),
Soil Distribution in Valleys According to
Stream Order, CATENA, Vol. 72 (3), p. 395404.
2. Decrease in RB by over 20% suggests that
there has been a reduction in drainage
efficiency even with a general increase of the
number of streams.
Robert A. (2003): River Processes, Oxford
University Press, New York
Straller A. and Straller A. (2002): Physical
Geography; Science and systems of the human
environment, hermitage publishing services
3 Increase in RL by about 8% suggests that
type of soil covering the drainage basin have
changed due to soil erosion or other activities,
which is even supported by about almost
similar percentile increase (10%) in the stream
frequency.
Wellmeyer J.L., Slattery M.C. and Phillips
J.D, (2005): Quantifying Downstream Impacts
of Impoundment on Flow Regimeand Channel
Planform, Lower Trinity River, Texas, Jr.
GEOMORHPHOLY, Vol. 69 (1-4), p. 1-13.
4 Increase in drainage density by more than
26% suggest that more area of the watershed
are now required to maintain unit length (1
km) of the drainage channel in the catchment.
5. The changes in the slopes in the relations
between stream orders and numbers as well as
lengths of streams suggest that creation of the
reservoir has particularly impacted on
increases in lengths and numbers of lower
order streams i.e creations of such streams
which did not exist earlier.
Because of the above noticed changes in the
morphometric characteristics in dammed
catchments, unit hydrograph parameters
derived for ungauged sub-catchments within
such a catchment using the geomorphologic
features, should be used with caution.
6
REFRENCES
Bhaskar, N.R., B.P. Parida and Atul K. Nayak,
(1997). Flood Estimation for Ungauged
Catchments
Using
Geomorphological
Instantaneous Unit Hydrograph (GIUH), Jr. of
Water Resources Planning and Management,
American Soc. of Civil Engrs. (ASCE), Vol.
123 (4), p. 228 – 238.
Bousmar D. & Zech Y (2002): River flow
2002, Volume 2, A.A Balkema Publishers,
Tokyo
4
Figure 1. Index map showing the location of the study area
Shashe Dam Drainage Basin Map
505000
510000
515000
520000
525000
530000
535000
540000
N
545000
550000
7660000
7655000
ok
Mo
7655000
Rive r
7660000
7665000
7665000
7670000
7670000
Shashe
Mp
han
e
7650000
7650000
e
ane
e
Lo ny
Kg
oro
ny
7640000
7640000
7645000
7645000
Maeroro
Shweke
Legend
Rivers.shp
Dam.shp
Catchment.shp
7635000
7635000
505000
510000
515000
520000
525000
530000
535000
540000
545000
550000
Scale 1: 500 000
Figure 2. Drainage Basin map of the Shashe River and its tributaries at the Shashe dam
5
10
9
8
RA values
7
6
1970
5
1990
4
3
2
1
0
mooke
shekwe
kgonyane
lonye
Sub-catchments
Figure 3. Comparison of RA values for various sub-catchments during (1970) and 20 years after
(1990).
8
7
RB values
6
5
1970
4
1990
3
2
1
0
mooke
shekwe
kgonyane
lonye
Sub-catchment
Figure 4. Comparison of RB values for various sub-catchments during (1970) and 20 years after
(1990).
4.5
4
RL values
3.5
3
2.5
1970
2
1990
1.5
1
0.5
0
mooke
shekwe
kgonyane
lonye
Sub-catchment
Figure 5. Comparison of RL values for various sub-catchments during (1970) and 20 years after
(1990).
6
Relation between stream order and Log Nw(1970)
Relation between stream order and Nw(1990)
1.8
2
1.6
1.4
1.5
Log Nw
log Nw
1.2
1
0.8
0.6
0.4
1
0.5
0
0.2
1
0
1
2
y = -0.7512x + 2.3495
2
3
4
-0.5
3
y = -0.5622x + 2.141
stream order
(a)
stream order
(b)
Figure 6 Relation between stream order and Log Nw; (a) before the dam construction (b) after the
dam consruction
Relation between stream order and Lw(1990)
2
1.8
1.6
1.4
Log Lw
log Lw
Relation between steam order and log Lw(1970)
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1.2
1
0.8
0.6
0.4
0.2
0
1
1
y = -0.3918x + 2.0555
stream2 order
3
2
3
y = -0.27x + 1.9576
(a)
4
stream order
(b)
Figure 7 Relation between stream order and Log Nw; (a) before the dam construction (b) after the
dam consruction
Table 1: RA, RB and RL values for the Shekwe-Maeroro sub-catchment during 1970.
Stream
Order
No. of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
49
51.75
1.06
80.25
1.64
-
8.17
-
2
6
10.75
1.79
26.25
17.25
10.82
6.0
1.69
3
1
8.30
8.30
18.0
124.50
7.0
-
4.64
8.91
7.09
3.17
Average
7
Table 2: RA, RB and RL values for the : Lonye sub-catchment during 1970..
Stream
Order
No. of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
21
37.65
1.79
33.75
1.61
-
4.2
-
2
5
13.9
2.78
14.25
9.60
5.96
5.0
1.55
3
1
8.75
8.75
12.0
60.0
6.25
-
3.15
6.11
4.60
2.35
Average
Table 3: RA, RB and RL values for the Mooke sub-catchment during 1970
Stream
Order
No. of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
51
61.95
1.21
64.75
1.27
-
6.38
-
2
8
19.1
2.39
16.75
10.19
8.02
8.0
1.98
3
1
8.6
8.69
21.25
1.2.75
10.1
-
3.60
9.06
7.19
2.79
Average
Table 4: RA, RB and RL values for the : Kgonyane sub-catchment during 1970
Stream
Order
No. of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
38
49.44
1.30
68.5
1.80
-
4.22
-
2
9
19.37
2.15
42.25
12.3
6.83
4.5
1.65
3
2
7.4
3.70
12.75
61.75
5.02
-
1.72
5.93
4.36
1.69
Average
Table 5 RA, RB and RL values for the Shekwe-Maeroro sub-catchment during 1990
Stream
Order
No.
of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
51
71.55
1.40
99.0
1.94
-
5.67
-
2
9
15.25
1.69
9.0
12.0
6.19
4.5
1.21
3
2
22.0
11.0
16.50
62.25
5.19
-
6.50
5.69
5.09
3.86
Average
8
Table 6 RA, RB and RL values for the : Lonye sub-catchment during 1990.
Stream
Order
No.
of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
25
44.25
1.77
32.75
1.31
-
3.57
-
2
7
18.0
2.57
21.0
7.68
5.86
7
1.45
3
1
13.3
13.3
6.25
60.0
7.8
-
5.18
6.83
5.29
3.32
Average
Table 7 RA, RB and RL values for the Mooke sub-catchment during 1990
Stream
Order
No.
of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
58
70.0
1.21
87.25
1.50
-
5.27
-
2
11
22.45
2.04
7.5
8.61
5.74
5.5
1.69
3
2
11.75
5.88
4.5
49.63
5.76
2.0
2.88
4
1
8.35
8.35
3.5
102.75
2.07
-
1.42
4.52
4.26
2.00
Average
Table 8 RA, RB and RL values for the : Kgonyane sub-catchment during 1990
Stream
Order
No.
of
streams
Total
length
(km)
Av.
Length
(km)
Drainage
Area
(km2)
Av.
area
(km2)
RA
RB
RL
1
41
52.96
1.29
74.25
1.81
-
3.15
-
2
13
21.42
1.65
33.75
8.30
4.58
4.33
1.28
3
3
9.2
3.07
15.5
41.17
4.96
-
1.86
4.76
3.74
1.57
Average
9
Table 9 Drainage Density values at various sub-catchments for the years 1970 and 1990.
Catchment
Drainage
Density (1970)
Shekwe-Maeroro
0.57
Drainage
Density
(1990)
0.87
Lonye
1.01
1.23
Mooke
0.87
1.10
Kgonyane
0.62
0.68
Table 10 Stream Frequency values at various sub-catchments for the years 1970 and 1990.
Catchment
Shekwe-Maeroro
Stream
Frequency
(1970)
0.45
Stream
Frequency
(1990)
0.50
Lonye
0.45
0.55
Mooke
0.58
0.70
Kgonyane
0.46
0.40
Table 11 Average values of stream characteristics for the years 1970 and 1990.
Channel parameter
1970
1990
Change
RA
7.5025
4.45
- 41%
RB
5.81
4.595
- 21%
RL
2.5
2.6875
+7.5%
Stream frequency
0.485
0.5375
+10.7%
Drainage density
0.7675
0.97
+26.5%
10