Effects of anthropogenic activities on the Lower Sakarya River Catena
Transcription
Effects of anthropogenic activities on the Lower Sakarya River Catena
Catena 75 (2008) 172–181 Contents lists available at ScienceDirect 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: [email protected] (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. References Arnaud-Fassetta, G., 2003. River channel changes in the Rhone Delta (France) since the end of the Little Ice Age: geomorphological adjustment to hydroclimatic change and natural resource management. Catena 51, 141–172. Chen, Z., Li, J., Shen, H., Wang, Z., 2001. Yangtze River of China: historical analysis of discharge variability and sediment flux. Geomorphology 41, 77–91. Choi, S.-U., Yoon, B., Woo, H., 2005. Effects of dam-induced flow regime change on downstream river morphology and vegetation cover in the Hwang River, Korea. River Research and Applications 21, 315–325. De Boer, D.H., 1997. Changing contribution of suspended sediment sources in small basins resulting from European settlement on the Canadian prairies. Earth Surface Processes and Landforms 22, 623–639. DSI, 1965. Feasibility Report for Lower Sakarya Project. DSI Matbaasi, Ankara. EIE, 1995. Sediment Transport and Measurement Techniques, Ankara. EIE, 2000. Suspended Sediment Measurements and Transport Rates in Turkey Rivers, Ankara. EIE, 2003. Monthly Mean Flows of Turkey rivers, (1935–2000), Ankara. Galay, V.J., 1983. Causes of river bed degradation. Water Resources Research 19 (5), 1057–1090. Graf, W.L., 1999. Dam nation: a geographic census of American Dam and their largescale hydrologic impacts. Water Resources Research 35 (4), 1305–1311. Houben, P., Hoffmann, T., Zimmermann, A., Dikau, R., 2006. Land use and climatic impacts on the Rhine system (RheinLUCIFS): quantifying sediment fluxes and human impact with available data. Catena 66, 42–52. Isık, S., Sasal, M., Dogan, E., 2003. Investigation on Changes of Sediment Transport and Flow in the Sakarya River. Report, Sakarya University. 109 pp. (in Turkish). Isik, S., Sasal, M., ve Dogan, E., 2006a. Investigation of Riverbed Changes at the Lower Sakarya River. Report, Sakarya University, (unpublished report). Isik, S., Sasal, M., Dogan, E., 2006b. Investigation on downstream effects of dams in the Sakarya River. Journal of the Faculty of Engineering and Architecture of Gazi University Ankara 21 (3), 401–408. Jiongxin, X., 2004. A study of anthropogenic seasonal rivers in China. Catena 55, 17–32. Komura, S., Simons, D.B., 1967. River-bed degradation below dams. Journal of the Hydraulics Division, ASCE 93 (4), 1–14. Kondolf, G.M., 1994. Geomorphic and environmental effects of instream gravel mining. Landscape and Urban Planning 28, 225–243. Kondolf, G.M., 1997. Hungry Water: Effects of Dams and Gravel Mining on River Channels, Environmental Management, vol. 21(4). Springer-Verlag, New York Inc, pp. 533–551. Kuhnle, R.A., Bingner, R.L., Foster, G.R., Grissinger, E.H., 1996. Effect of land use on sediment transport in Goodwin Creek. Water Resources Research 32, 3189–3196. Lajoie, F., Assani, A.A., Roy, A.G., Mesfioui, M., 2007. Impacts of dams on monthly flow characteristics: the influence of watershed size and seasons. Journal of Hydrology 334, 423–439. Magilligan, F.J., Nislowb, K.H., 2005. Changes in hydrologic regime by dams. Geomorphology 71, 61–78. Milliman, J.D., Qin, Y.S., Ren, M.E., Saito, Y., 1987. Man's influence on the erosion and transport of sediment by Asian rivers: the Yellow River (Huanghe) example. Journal of Geology 95, 751–762. Ozbek, T. and Ozcan, C., 2001. River sediment transport. TMMOB Chamber of Civil Engineers, Ankara. (in Turkish). Peart, M.R., 1997. Human impact upon sediment in rivers: some examples from Hong Kong. In: Walling, D.E., Probst, J.L. (Eds.), Human Impact on Erosion and Sedimentation. . IAHS Publ, vol. 245. IAHS Press, Wallingford, pp. 111–118. Petit, F., Poinsart, D., Bravard, J.P., 1996. Channel incision, gravel mining and bedload transport in the Rhone river upstream of Lyon, France (“Canal de Miribel”). Catena 26, 209–226. Poulos, S.E., Collins, M., Evans, G., 1996. Water-sediment fluxes of Greek rivers, southeastern Alpine Europe: annual yields, seasonal variability, delta formation and human impact. Zeitschrift fur Geomorphologie 40, 243–261. Rinaldi, M., Simon, A., 1998. Bed-level adjustments in the Arno River, Central Italy. Geomorphology, 22, 57–71. Rinaldi, M., 2003. Recent channel adjusments in alluvial rivers of Tuscany, Central Italy. Earth Surface Process and Landforms 28 (6), 587–608. Rinaldi, M., Wyzga, B., Surian, N., 2005. Sediment mining in alluvial channels: physical effects and management perspectives. River Research Application. 21, 805–828. Siakeu, J., Oguchi, T., Aoki, T., Esaki, Y., Jarvie, P.H., 2004. Change in riverine suspended sediment concentration in central Japan in response to late 20th century human activities. Catena 55, 231–254. Simons, D.B., Senturk, F., 1992. Sediment transport technology. Water Resources Publications, Littleton, Colorado. Stevens, M.A., Simons, D.B., Schumm, S.A., 1975. Man-induced changes of Middle Mississippi River. Journal of the Waterways Harbors and Coastal Engineering, ASCE 101 (2), 119–133. Surian, N., 1999. Channel changes due to river regulation: the case of the Piave River, Italy. Earth Surf. Process. Landforms 24, 1135–1151. Sutherland, R.A., Bryan, R.B., 1991. Sediment budgeting: a case study in the Katiorin Drainage Basin, Kenya. Earth Surface Processes and Landforms 16, 383–398. Tiffen, M., Mortimore, M., Gichuki, F., 1994. More People, Less Erosion: Environmental Recovery in Kenya. Wiley, New York. Walling, D.E., 2006. Human impact on land-ocean sediment transfer by the world's rivers. Geomorphology 79, 192–216. Walling, D.E., 1990. Linking the field to the river: sediment delivery from agricultural land. In: Boardman, J., Foster, I.D.L., Dearing, J.A. (Eds.), Soil Erosion on Agricultural Land. Wiley, New York, pp. 129–151. Wang, S., Hassan, A.M., Xie, X., 2006. Relationship between suspended sediment load, channel geometry and land area increment in the Yellow River Delta. Catena 65, 302–314. Williams, G.P., Wolman, M.G., 1984. Downstream effects of dams on alluvial rivers. U.S. G.S. Geological Survey Professional Paper 1286. Ye, B., Yang, D., Kane, D.L., 2003. Changes in Lena River streamflow hydrology: human impacts versus natural variations. Water Resources Research 39 (7), 1200 8–1:8–14.