Effects of anthropogenic activities on the Lower Sakarya River Catena

Catena 75 (2008) 172–181
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Catena
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a
Effects of anthropogenic activities on the Lower Sakarya River
Sabahattin Isik a, Emrah Dogan b,⁎, Latif Kalin c, Mustafa Sasal d, Necati Agiralioglu e
a
College of Agriculture and Life Sciences, Texas A&M University, College Station, TX 77840, USA
Department of Civil Engineering, Sakarya University, Sakarya 54187, Turkey
c
School of Forestry and Wildlife Sciences, Auburn University, AL 36849, USA
d
Department of Civil Engineering, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
e
Department of Civil Engineering, Istanbul Technical University, Maslak, Istanbul, 34469, Turkey
b
A R T I C L E
I N F O
Article history:
Received 17 October 2007
Received in revised form 1 June 2008
Accepted 2 June 2008
Keywords:
Effects of dams
Sakarya River
River bed incision
Human activities
River flow
Sediment transport
A B S T R A C T
We investigate the effects of anthropogenic activities on the Lower Sakarya River. The impacts of dam, levee,
and bridge constructions, sand-gravel mining activities and water withdrawals during the industrialization
period of the Sakarya River Basin have been explored by analyzing flow, sediment and channel cross section
data from different periods in time by comparing pre- and post-1975 periods. The year 1975 is roughly
determined to be commencement of heavy human activities. Assessment of data shows that average annual
flow is reduced by almost 20% after 1975. Due to increased regulations after 1975 flow became less variable,
i.e. low-flows are increased and high-flows are reduced. Flow showed less variation with seasons during the
post-1975 period compared to the pre-1975 period. Close inspection of precipitation and temperature
patterns over the course of this period indicates that these changes in the flow regime cannot be attributed to
natural causes and must be instigated by anthropogenic activities. Analyses of sediment data point toward a
consistent reduction in sediment concentration and loadings with years in the Lower Sakarya River. Sediment
rating curves developed for pre- and post-1975 exhibit a similar pattern. The impact of the anthropogenic
activities on the river cross section is also examined by employing data from 1965, 2003 and 2006 at various
points along the river profile. We found as much as 1 m aggradation at the thalweg elevation along the river
profile starting from the river mouth up to the 12th km. Degradation in thalweg elevation is observed
upstream of the 12th km, as much as 7 m at some locations. This research clearly undermines how human
activities can alter the river hydrology and morphology. The adverse impacts of these modifications on the
stream ecology in the Lower Sakarya River unfortunately remain unresolved.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Alluvial river systems change their course and morphology over
time as a result of hydraulic forces acting on the river bed and banks.
These changes could be natural or human induced and may be gradual
or rapid. Any disturbance or modification at a point in a river section
affects both upstream and downstream conditions (Galay, 1983;
Simons and Senturk, 1992). In addition to climate-induced changes
and variations, alluvial rivers counteractively respond to human
activities such as construction of dams, levee and bridges, diversion of
bed material and/or flow, sand mining, water withdrawal for urban,
industrial and agricultural needs, and change of land use (Ye et al.,
2003). These activities can have wide variety of physical, ecological,
and environmental effects in rivers, such as streambed mobilization,
scouring, degradation, and aggradation; changes in hydrologic regime,
etc. (Kondolf, 1994, 1997; Petit et al., 1996; Surian, 1999; Rinaldi, 2003;
⁎ Corresponding author. Tel.: +90 264 295 5749; fax: +90 264 346 0359.
E-mail address: emrahdogan77@gmail.com (E. Dogan).
0341-8162/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.catena.2008.06.001
Choi et al., 2005; Magilligan and Nislowb, 2005; Rinaldi et al., 2005,
Isik et al., 2006b).
Constructions of dams are known to alter hydrology and sediment
transport characteristics in downstream areas. Large impoundments
with dam operations while decrease peak flow discharges, increase
minimum flow discharges as well. Consequently, flow becomes
regulated and less variable. Downstream flooding events become
rare, which may be crucial for a healthy floodplain ecosystem. Thus,
water resources planning and development projects, and river rehabilitation and regulation studies must consider and critically evaluate
climate conditions of the region, geological factors, hydrological factors, geometric characteristics, hydraulic characteristics, ecological
and biological structure, and the political and economical factors
(Simons and Senturk, 1992).
The key natural and artificial factors affecting downstream of dams
have been heavily studied during the past three decades (Komura and
Simons, 1967; Stevens et al., 1975; Kondofl, 1997; Graf, 1999; Walling,
2006; Lajoie et al., 2007). More recently, Siakeu et al. (2004) analyzed
suspended sediment concentration data from rivers in central Japan with
special reference to impacts of human activity. Their findings indicate
S. Isik et al. / Catena 75 (2008) 172-181
173
Fig. 1. Layout of the Sakarya River Basin (source: www.eie.gov.tr).
that human activity in industrial countries can result in large variations
in suspended sediment patterns across relatively small areas. These
variations reflect changes in the balance between the effects of
urbanization and associated changes in land use, catchment management, and water pollution and erosion control measures. Jiongxin
(2004) investigated the major anthropogenic seasonal rivers in China
such as the Lower Yellow River and the Haihe River, and some of their
tributaries. He found that, due to strong human activities, some
perennial rivers in north China have been changed into seasonal rivers.
He explained that these rivers can be regarded as a new category of
rivers, and used the term anthropogenic seasonal river for such rivers.
Houben et al. (2006) investigated the effects of land use and climatic
impacts on sediment fluxes in the river Rhine and quantified the
relationships between external controls (land use and climate) and
Holocene sediment fluxes in the Rhine basin. Wang et al. (2006)
explored the relationship between suspended sediment load, channel
geometry and land area increment in the Yellow River Delta. They found
that hydraulic efficiency in the river has increased due to channel
narrowing, and restricted lateral erosion. Lajoie et al. (2007) investigated
the impacts of dam constructions and operations on the river hydrologic
regimes and showed that the extent of such hydrological changes is
highly dependent on the upstream drainage area.
The impacts of the human activities on suspended sediment loads
in rivers have been assessed in many parts of the world, including
Africa (Milliman et al., 1987; Sutherland and Bryan, 1991; Tiffen et al.,
1994), China (Peart, 1997; Chen et al., 2001; Jiongxin, 2004), Europe
(Walling, 1990; Poulos et al., 1996), and North America (Kuhnle et al.,
1996; De Boer, 1997). However, no such study has been carried out in
Turkey despite intense human impacts on the fluvial systems
associated with dam constructions and channel mining activities.
This paper is the first endeavor in Turkey's rivers attempting to
quantify the anthropogenic effects on stream flow hydrology,
sediment loading, and stream morphology.
The focus area of this study is the Lower Sakarya River which is an
alluvial river in Northwest Turkey (Fig. 1). The Lower Sakarya River
cuts through the very fertile Sakarya Plain before draining to Black Sea.
Its watershed inhabits approximately 750,000 people (does not
include Upper and Middle Sakarya Basins). However, the impact of
agricultural production and sand mining activities within the basin
extents to a much larger region, including the Greater Istanbul
Metropolitan Area which resides more than 10 million people.
Therefore, any activities or alterations in the upstream part of the
river have important implications for this area and its people.
Numerous reservoirs at varying sizes have been built and put into
operation for various purposes on the Upper and Middle Sakarya
Rivers during the past 50 years. The same period has seen
construction of levees for flood control, as well as many bridge
constructions over the river and its tributaries. Extensive sand mining
for construction material on the Lower Sakarya River has also occurred
during this period, many of them unregulated or uncontrolled. All
these activities have changed the hydrological and morphological
characteristics of the Lower Sakarya River. So far, these were only
174
S. Isik et al. / Catena 75 (2008) 172–181
Table 1
General characteristics of relatively large dams on Sakarya River and its tributaries
2. Study area: Lower Sakarya River
No Dams
Year
1
1936
The Sakarya River is situated in the northwest Anatolian region of
Turkey and its total length is nearly 810 km (Fig. 1). Its total drainage
area is about 56,000 km2, which constitutes slightly more than 7% of
Turkey. The headwater of the Sakarya River is about 3 km southeast to
the town of Cifteler, which is within the Eskisehir province (Fig. 1). The
river basin is commonly divided into three zones as upper, middle and
lower sections. The Lower Sakarya River has gentle slopes with its
sinuous and meandering shape extending from Black Sea (River
Mouth) up to 110 km upstream, Dogancay gauging station. There are
two flow recording stations on the Lower Sakarya River, namely
Dogancay (station #1221 in Fig. 1) and Botbasi (station #1243 in Fig. 1),
which have about 100 years of total combined flow data. The first dam,
Cubuk I was built and started to operate in 1936 on the Upper Part of
Sakarya River. Numerous dams on the Upper and Middle Sakarya River
ranging in size and capacity followed it in the upcoming years. The
relatively larger dams, with their completed years, purposes and
reservoir capacities are shown in Table 1. There are 20 dams built
between 1936 and 2002 for domestic and industrial water supply,
irrigation, flood control, and energy purposes on the Sakarya River.
The Sakarya River profile, location of large dams on the Middle
Sakarya River, and the two gauging stations (Botbasi at 42nd km, and
Dogancay at 110th km) are illustrated in Fig. 2.
The Lower Sakarya Basin has a mild climate. Rainfall averages
about 770 mm/year and is fairly well distributed throughout the year
with the heaviest rains occurring in winter and early spring months.
Snowfall is relatively light and average between 20 to 30 cm/year. The
frost-free period ranges from 355 to 360 days. Wind velocities are
generally moderate.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Cubuk I
Purpose
Reservoir Drainage Mean annual
area
area
outflow
(km2)
(km2)
(million m3)
Domestic and
industrial water supply,
Flood control
Sariyar
1956 Energy
Cubuk II
1964 Domestic and
industrial water supply
Bayindir
1965 Domestic and
industrial water supply
Kurtbogazi 1967 Domestic and
industrial water supply,
Irrigation
Musaozu
1969 Irrigation
Porsuk
1972 Domestic and
industrial water supply,
Irrigation, Flood
control
Gokcekaya 1972 Energy
Dodurga
1977 Irrigation, Flood
control
Kaymaz
1977 Irrigation
Enne
1977 Irrigation
Asartepe
1980 Irrigation
Kunduzlar 1983 Irrigation
Camlidere
1985 Irrigation
Catoren
1987 Irrigation
Egrekaya
1992 Domestic and
industrial water supply
Akyar
1999 Domestic and
industrial water supply
Yenice
1999 Energy
Beykoy
2000 Energy
Kizildamlar 2002 Irrigation
1.44
840
Closed
83.83
1.26
41,778
180
2014
20
0.75
70
7
5.8
330
60
0.43
27.70
43
4544
6
169
20.00
2.45
44,650
182
1650
8
0.20
0.58
1.73
2.64
32.2
4.04
2,9
19
262
239
406
753
712
412
0.9
12
27
142
13
79
1.91
253
45
3.64
3.21
0.97
47,859
36
94
1460
1223
12.21
qualitative observations and have not been studied quantitatively.
This study is the first attempt in quantifying the effects of these
activities on the Lower Sakarya River.
The remainder of this paper is structured as follows. The study
area, Lower Sakarya River and its basin, is described first. Following
the section that investigates various aspects of the flow regime of the
Lower Sakarya River, before and after the critical cutoff date of 1975,
we analyze changes in sediment concentrations and loadings after the
1975 period relative to the pre-1975 period, and attempt to elucidate
the association between these modifications and physical activities
and the alterations in the flow regime. We next assess changes in the
river cross sections and stream bed elevations both longitudinally (i.e.
in time) and spatially (in transverse direction and along the river
profile). We wrap up the paper with a summary and conclusions
section.
3. Changes in flow regime
The General Directorate of Electrical Power Resources Survey and
Development Administration (EIE) has been measuring flow at
Dogancay and Botbasi gauging stations (Fig. 2) on the Lower Sakarya
River since 1953 and 1960, respectively (EIE, 2003). Annual flow
volumes in Dogancay between 1953 and 2002 are evaluated and
depicted in Fig. 3a. Despite of the fluctuations in flow with years, the
average annual flow volume has dropped from the pre-1975 average
of 4.3 to 3.5 km3 after 1975, which is about 18% reduction. 1975 is a
critical year in which Gokcekaya Dam started being regulated at full
capacity on the Middle Sakarya River. All the reservoirs listed in
Table 1 are upstream of Gokcekaya, except the Yenice reservoir, which
became fully operational around 2002. The year 1975 at the same time
corresponds roughly to the start of serious water withdrawals and
sediment removals from the river bed for construction purposes.
Therefore discussions and analysis henceforth will focus on the preand the post-1975 periods.
Fig. 2. Schematic profile of the Sakarya River and location of major dams (not to scale).
S. Isik et al. / Catena 75 (2008) 172-181
175
Fig. 3. Variations of annual flow volumes at (a) Dogancay and (b) Botbasi stations. Horizontal lines show annual average flow volumes before and after 1975.
Annual flow volumes at the Botbasi station between 1961 and
2002 are also evaluated and depicted in Fig. 3b. It is clearly seen that
average annual flow volume is reduced after 1975 at this station from
6.1 to 4.9 km3, which corresponds to about 20% decline. In fact there is
a clear decreasing trend in annual flow volume with years. If we
consider the same periods, i.e. after 1960, the gradual reduction trends
observed in annual flow volumes on the Lower Sakarya River over the
years are consistent at both stations (Fig. 3a and b). This reduction
could be attributed to several factors. Fig. 4 shows the annual
precipitation distribution with years. Although there is a slight decreasing trend in annual precipitation until 1993, and an increasing
trend afterward, these fluctuations are not statistically significant to
cause the reduction in flow observed on Fig. 3a and b. Fig. 5 shows the
variation of unit hydrologic response, R (flow volumes normalized
with precipitation depth and drainage area) with time. This dimensionless quantity is defined to filter out the precipitation effect to
some extent. Even after filtering out precipitation, the decreasing
trend in flow volume is evident at both stations. At Botbasi station
average R is reduced to 0.18 from 0.21 after 1975. In a similar fashion
average R has dropped to 0.14 from 0.21 at Dogancay station. We also
considered potential increase in evaporation due to possible increase
in temperature. Analysis of data did not reveal any appreciable
increase in temperature over the years and consequently in evaporation. The only reasonable explanation for the reduction in flow
volumes over the years then becomes diversion of water for irrigation
and water supply purposes at the Middle and Upper Sakarya basins.
This information is available to us only qualitatively as we know that
1975 is a critical year in which the hydrological characteristics of
Lower Sakarya River started to change considerably. Unfortunately we
have no supporting data to develop a full water budget of the basin.
Average monthly mean flow rates between 1953–1974 and 1975–
2002 in Dogancay gauging station are shown in Fig. 6a. The minimum
and maximum average monthly mean flows are observed during the
months of August and April, respectively. The average of mean August-
Fig. 4. Annual rainfall distribution in the Sakarya River Basin (includes Lower, Middle and Upper River Basins). Solid line shows 5-year-moving averages.
176
S. Isik et al. / Catena 75 (2008) 172–181
Fig. 5. Unit hydrologic response, R (flow volume per unit area per unit amount of rainfall) at Dogancay and Botbasi stations.
flow was 61.9 m3/s during the pre-1975 period and increased to
65.0 m3/s during the post-1975 period. Conversely, the average of
mean April-flow has been reduced to 177.7 m3/s after 1975, which was
247.1 m3/s during the pre-1975 period. Evidently post-1975 period
reduced the flow variations between the driest and wettest months.
The standard deviation of mean monthly flows was 66.3 m3/s
(coefficient of variation, CV = 0.48) during the pre-1975 period and
40.4 m3/s (CV = 0.36) during the post-1975 period, further indicating
the reductions in flow variations. Botbasi station exhibits a similar
behavior as well (Fig. 6b). Before and after 1975 in Botbasi, average
August-flow was 80.5 and 82.0 m3/s, respectively; and average Aprilflow was 329.0 and 256.2 m3/s, respectively. Similar to the Dogancay
station, the standard deviation of average monthly flows dropped
from the pre-1975 value of 92.4 m3/s (CV = 0.48) to 64.1 m3/s
(CV = 0.41), again providing evidence for reduced flow variation during
the post-1975 period.
3.1. Annual peak flows
Flow duration curves (FDC) for AMDF are generated using data at
Botbasi and Dogancay stations for pre- and post-1975 periods (Fig. 7)
(AMDF data were obtained from EIE upon request). As expected post1975 periods (lines with triangular markers) have smaller peak flows
(flows that have low exceedance probabilities) than their pre-1975
period counterparts. On the high probability of exceedance end, post1975 period flows have larger annual peaks. Also note that post-1975
FDCs are flatter than the pre-1975 FDCs. All these reaffirm our
previous observation that construction of the Gokcekaya Dam reduced
the flow variation downstream of the dam.
One may notice from the figure that Dogancay station has higher
annual peak flow rates at low exceedance probabilities than the
Botbasi station during the pre-1975 period. This may seem counterintuitive from the figure as Botbasi station is downstream of Dogancay
Fig. 6. Monthly mean flows at (a) Dogancay and (b) Botbasi stations.
S. Isik et al. / Catena 75 (2008) 172-181
177
Fig. 7. Flow duration curves (FDC) for annual maximum daily flows at Botbasi and Dogancay stations for pre- and post-1975 periods.
and supposedly have higher flows. In both Fig. 3 for annual flow
volume and Fig. 4 for mean monthly flows, Botbasi has higher flows at
all times. The FDC for Botbasi is generated using data from 1961 to
2002, whereas FDC for Dogancay is generated using data from 1953 to
2002. Incidentally the highest annual peak flow at Dogancay was in
1953, causing this anomaly in the figure. Using data from same periods
could have eliminated this incongruity. Nevertheless, our purpose is
comparison of pre- and post-1975 period flow conditions, rather than
comparing the two stations.
Expected probable flood discharges for 5, 10, 25, 50, and 100 years
were also determined by using annual maximum discharges observed
at Dogancay and Botbasi. Probable flood discharges were calculated by
using the Gumbel method and are shown in Table 2 for both stations.
For instance, at Dogancay station estimated 5, 10 and 100-year probable flood discharges are 760, 926 and 1447 m3/s for the pre-1975
period and 492, 569 and 810 for the post-1975 period, respectively.
Post-1975 probable flood discharges are lower in each case. Similarly,
at Botbasi predicted 5, 10 and 100-year probable flood discharges
are 770, 900 and 1308 m3/s for pre-1975 period and 636, 720 and
983 m3/s for the post-1975 period. In other words the 5, 10, and 100year probable floods are estimated to be reduced by 17, 20 and 25%,
respectively. Similar to Dogancay, construction of Gokcekaya dam
resulted in smaller predicted probable flood discharges at Botbasi.
Again note that if longer data were available for the Botbasi station as
well, the % reduction in expected maximum floods would be higher,
comparable to % reductions observed at Dogancay station.
4. Changes in river morphology
4.1. Trends in sediment concentrations
Due to high agricultural activities and the relatively poor
vegetation cover in the Upper Sakarya River Basin, sediment load is
Table 2
Expected maximum floods in Dogancay and Botbasi gauging stations (G.S.)
Dogancay G.S.
Botbasi G.S.
Return
period
(year)
Pre-1975
(m3/s)
Post-1975
(m3/s)
Reduction Pre-1975
(%)
(m3/s)
Post-1975
(m3/s)
Reduction
(%)
5
10
25
50
100
760
926
1136
1292
1447
492
569
667
739
810
35
39
41
43
44
636
720
826
905
983
17
20
22
24
25
770
900
1065
1187
1308
very high in the Lower Sakarya River. After the construction of dams,
sediment loads started decreasing in the Lower Sakarya River, which is
literally downstream of all the dams, due to sediment trapping. As
most of the sediment is deposited behind the dams, reservoir
spillways typically release clear water to downstream, which has
excessive energy to scour streambed and carry more sediment. This is
essentially an outcome of the dynamic equilibrium between stream
power and the sediment transport capacity of the stream.
Systematic data collection, compatible with international standards on sediment transport characteristics of surface waters in
Turkey was initiated by EIE in 1962 within the framework of the
hydrometric observations from a basic data station network (EIE,
1995). Suspended sediment load is measured monthly by EIE using
Dept-Integration method with USDH-48 and USD-49 sampling
equipment. Sediment concentrations of the samples are measured
by filtration technique and analyses on size gradation of these samples
are performed to determine sand, silt and clay contents of the samples. Suspended sediment concentration of the samples are expressed
as part per million (ppm) by dry weight of the samples.
In this section we assessed the suspended sediment dynamics by
employing measured suspended sediment data collected between
1964 and 1999 at the Botbasi gauging station (EIE, 2000). Unfortunately, there have been no other sediment data collection efforts along
the Lower Sakarya River. Once again, suspended sediment records
were analyzed by partitioning data into pre- and post-1975 periods.
There were 85 data samples from the period before 1975, and 295
samples after 1975. First, the trend in suspended sediment load with
time is investigated by making use of the collected samples. Later,
sediment rating curves, which provide a relationship between flow
discharge and suspended sediment load, are developed and
compared.
Fig. 8 shows average suspended sediment concentrations over the
years. Both pre-1975 and post-1975 periods exhibit a declining trend
in sediment concentration, although it is more stable after 1980s. The
rate of decline in sediment concentration before 1975 is higher than
the rate of decline in the post-1975 period. This is more likely due to
initially very high concentrations in the river. Once the sediment
concentrations fell below more moderate levels, the rate of decline
diminished. Evidently, the declining flow over the years has an
apparent impact on this reduction as well.
4.2. Intraannual sediment concentration distribution
While floods and flow rates are reduced with the advent of
dam regulations on the river, reduction in sediment load
178
S. Isik et al. / Catena 75 (2008) 172–181
Fig. 8. Average suspended sediment concentrations measured at Botbasi.
transportation has also been observed. The intraannual variation in
suspended sediment concentrations at the Botbasi station is depicted
in Fig. 9 for the pre- and post-1975 periods, i.e. before and after the
regulations. While average concentration was 1029 ppm before
the 1975 period, after 1975 this value dropped substantially to
467 ppm, which roughly corresponds to a 55% reduction in sediment
concentrations.
Fig. 9 reveals some very interesting results. Before 1975 the
distribution of sediment concentration shows somewhat erratic
behavior. The highest average monthly sediment concentration during
this period is observed in December. Average flow in December during
the same period is moderate at most. February and July also have high
sediment concentrations. Comparison to Fig. 6 reveals no correlation
between average monthly sediment concentrations and the average
monthly mean flows for the pre-1975 period. On the other hand,
effects of reservoirs clearly show up during the post-1975 period,
where the monthly variations in sediment concentrations follow a
very similar pattern with the variations in average monthly mean
flows as depicted in Fig. 6.
The effects of the anthropogenic activities on the sediment rating
curves are also explored. Sediment rating curves typically relate
suspended sediment loads to flow rates through a power function
Qs ¼ aQ b
ð1Þ
where Q = flow rate (m3/s); Qs = suspended sediment load (ton/day)
and a and b are site specific constants. The relationship between
monthly suspended sediment load and water discharge for the pre1975 and post-1975 periods are obtained as
Qs ¼ 2:5866Q 1:6247 ; R2 ¼ 0:71
ðpre 1975Þ
ð2Þ
Qs ¼ 0:5309Q 1:7882 ; R2 ¼ 0:67
ðpost 1975Þ
ð3Þ
If the entire sampling period, i.e. 1964 to 1999, are considered the
rating curve is
Qs ¼ 0:5674Q 1:8094 ; R2 ¼ 0:68
ð1964–1999Þ
ð4Þ
Fig. 10 plots the sediment rating curves for each period for comparison purposes, which shows the apparent impact of Gokcekaya
Dam on sediment loading. It is clear from the figure that the same
flow discharge, pre-1975 conditions would have resulted in much
higher sediment loads. For instance at Q = 300 m3/s, pre-1975
condition is estimated to produce twice as much sediment load than
post-1975 conditions.
4.3. Changes in cross sections and river profiles
In this section, river bed changes are investigated at the Lower
Sakarya River. Cross section data from a planning study conducted by
the Turkish State Hydraulic Works (DSI) in 1965 and two research
projects carried out in 2003 and 2006 (DSI,1965; Isik et. al., 2003, 2006a,
Fig. 9. Average monthly sediment concentration variations before and after 1975.
S. Isik et al. / Catena 75 (2008) 172-181
179
Fig. 10. Suspended sediment rating curves for pre- and post-1975 periods. Also shown is the rating curve based on entire sampling period for completeness.
respectively) are utilized for this purpose. Among many, only cross
sections at the 4th, 16th, 44th and 80th km of the river are shown in
Fig. 11 for years 1965 and 2003 for illustrative purposes. Comparison of
the cross sections from 1965 and 2003 reveals that while aggradation
occurs near the river mouth, degradation continues in an accelerated
fashion from the river mouth to upstream. Starting from the river
mouth, at 4th km nearly 2 m aggradation, at 16th km 1 m degradation, at
44th km 7 m degradation which is the most dramatic change in the river,
and at 80th km 5 m degradation took place at the thalweg elevation. The
river bed has been broadening since 1965. Note that the cross section
drawings in Fig. 11 for 1965 are only approximate. However, the left
bank, right bank and the thalweg locations and elevations are exact. The
overlaying of the two cross sections at each measurement points is
performed using the actual coordinates provided in the 1965 planning
study and the 2003 research project.
The 1965 study had additional cross section data at other points
upstream of the sites where data was previously collected during the
2003 research project. Three of these sites are approximately located
at 90th, 100th, and 110th km from the river mouth. Cross section data
at these three sites are collected in 2006 by Isik et al. (2006a). Fig. 12
compares these cross sections from 1965 and 2006. The same channel
scouring and widening effects are clearly visible.
The main reason for these observed changes in channel profiles
could be explained as follows. Incoming sediment trapped in the reservoir cannot reach downstream of the dam, therefore sediment transport capacity of the river at downstream increases resulting in scouring
to supplement its sediment deficit (Komura and Simons, 1967; Stevens
et al., 1975; Ozbek and Ozcan, 2001). In other words, reservoirs hold
most of the incoming sediment and relatively clear water is released
downstream. The river tries to compensate the loss of sediment by
scouring near and far downstream bed of the river. In addition to the
sediment trapping effects of the reservoirs as explained above, there has
been immense sediment withdrawal activities along the Lower Sakarya
River. Sediment withdrawals not only lead to scours in river bed but also
make sediment at the river bed erodible. However, the sediment withdrawal activities along the Lower Sakarya River have mostly been either
unlicensed or without much control. Even with the licensed withdrawals, in many occasions the amounts of sediment withdrawn from
the river have far exceeded the permitted levels (personal communications). Hence, quantification of the sediment withdrawn from the
river for the purpose of developing a sediment budget was close to
impossible and therefore our discussions are limited to qualitative
judgments.
The variation of the river bed profile from 1965 to 2003 and 2006,
with thalweg elevation as the reference, is shown in Fig. 13. Aggra-
dation occurs up to 12th km from the river mouth. This is apparently
due to settling of sediment particles picked up from upstream locations under the effect of slower flow (compare the bed slope in
between 0–12 km with the rest of the profile) and reduced stream
power. After 12th km degradation begins.
The largest degradation during the 41-year period (1965–2006)
was 7 m at the thalweg of the 44th km in Lower Sakarya River. The
channel incisions at the measured sites were much faster and deeper
than the documented cases of relatively large rivers. For example, Po
River in Italy incised 1–6 m along all alluvial reaches during the period
1880–1930 (Rinaldi and Simon, 1998), Rhone River in south France
incised up to 4 m from 1952 to 1990 resulting in considerable channel
deepening entrenchment of low flow channel (Petit et al., 1996), and
the Rhone Delta incised ranging from 1.1 m to 6.8 m during 1907–1991
(Arnaud-Fassetta, 2003).
Williams and Wolman (1984) provided a detailed analysis of 21
dams in semi-arid, western USA. Data from 287 measured cross
sections, each resurveyed periodically since about the time of dam
closure, clearly showed that in predominantly sand-bed channels,
changes with time can be described by simple hyperbolic relationships. Channel degradation is fastest immediately after erosion begins
and gradually slows with time. Bed erosion ranged from virtually zero
to 7.5 m, and most cross sections were eroded to one-half of their
predicted eventual maximum depths within 10 years of the start of
erosion. Rates of degradation during the initial period were about 0.1
to 1 m per year.
The continued sand mining from the Lower Sakarya River is believed
to be the main reason for degradation and channel incision at higher
rates than the ones reported in the literature. With the continued
sediment withdrawal from the river bed, it is unrealistic to expect the
Lower Sakarya River System reach a morphologic equilibrium. We
foresee that this trend of change in river profile will continue in the near
future.
5. Summary and conclusions
There are many factors, natural and artificial including human
activities, affecting the hydrological and morphological characteristics
of rivers. In this study, the effect of human activities such as dams,
levees and bridge constructions, and extensive sand mining on the
hydrologic and morphologic characteristics of the Lower Sakarya River
is explored. Flow changes before and after dam constructions are
scrutinized using Dogancay and Botbasi gauge stations data along the
Lower Sakarya River. It was shown that monthly mean flow rates
decreased about 20% at both stations, which is mainly attributed to
180
S. Isik et al. / Catena 75 (2008) 172–181
Apart from our own data collection efforts, we gathered all the
available published data for this study and our analyses are based on
these rather limited data. We were able to assess how the river morphology has changed from 1965 to 2003 and 2006. However, no other
data was available from the period 1965 to 2003 to reach any concrete
conclusion regarding how the river profile and the river cross sections
have changed over the course of this time period. On the contrary, the
diminishing rate of decline in suspended sediment concentrations over
the years, and the relative stabilization of suspended sediment concentrations after mid 80s points towards an equilibrium stage for suspended sediment concentrations. However, it is hard to say the same
thing for the river profile as continuing sand mining is expected to keep
destabilizing the river bed. Hence it is highly unlikely to see a morphologic equilibrium in the river profile in the near future.
Results from this study and other studies clearly accentuate
how human activities can alter the river hydrology and morphology.
Any future management and development strategies should involve
critical consideration of these effects and effective mitigation strategies should be developed accordingly. For the Lower Sakarya River, first
of all sand mining activities along the river must be fully regulated and
be under strict control. Secondly, a study on the sustainable sand
withdrawal rate from the river bed needs to be conducted.
The indirect effect of these changes in river hydrology and morphology on the river habitat, riparian zone and in general river health
is known to a lesser extent. One known problem in the region related
to sand mining and stream aggradation is the increased number of
fatal drowning incidents. Uncontrolled sand mining and/or over withdrawals of sediments sometime result in loss of private lands adjacent
Fig. 11. Comparison of river cross sections from years 1965 and 2003 at the 4th, 16th,
44th, and 80th km of the Sakarya River.
water withdrawals and diversions from the river and reservoirs.
Changes in expected probable floods are estimated by using annual
maximum discharges at both stations. As a result of regulated river
flows, probable flood discharges were estimated to decrease in both
Dogancay and Botbasi stations with larger reductions associated with
higher return periods.
Changes in sediment transport regime owing to dam constructions,
and their effects on the river cross sections and bed profile were also
investigated. Sediment rating curves were developed and compared by
using suspended load measurements from the pre- and post-dam
construction periods. It was observed that the sediment transport rate
was decreased at a rate of 40 to 65% after 1975. Comparison of river
cross sections at several locations measured in 1965, 2003 and 2006
revealed that, aggradation occurred from the river mouth up to the
12th km. Nearly 1 m of aggradation is observed at the thalweg elevation of the 8th km. Degradation is observed upstream from the 12th
km with almost 7 m degradation at the thalweg of the 44th km.
Fig. 12. Comparison of cross sections from years 1965 and 2006 at the 90th, 100th, and
110th km of the Sakarya River.
S. Isik et al. / Catena 75 (2008) 172-181
181
Fig. 13. Profile of Lower Sakarya River in 1965, 2003 and 2006.
to river banks, which often end in the landowner suing the sand
mining company. Further, changes on river morphology at such a fast
pace will certainly have adverse impacts on aquatic life, such as fish
spawning.
Although floods are usually known for their negative impacts as
they cause monetary losses and sometime lives, they also have
benefits for the ecosystem. For instance, they help fish migration and
provide new food sources. Some species in floodplains are adapted to
flooding and their sustainability depends on floodwaters. With dam
regulations flood risks are reduced and floodplain ecology suffers from
lack of this valuable resource. Studies on anthropogenic effects often
ignore this facet and only focus on the engineering and economic
aspects. In that sense, future studies should be integrated in nature
involving different disciplines.
Acknowledgements
Authors are grateful to Sakarya University in supporting this study.
This paper is based on research projects supported by Sakarya
University Research Fund; Project No:. 2002/25 and 2005/11.
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