Introduction

Understanding the basics of fluvial geomorphology and channel processes in planning any project along the rivers will allow the researchers to be able to recognize the relationship between form and process in the landscape. Geomorphic assessments generally consist of data collection, field study, and evaluation of river stability functioning as the basis for analysis and design (The Federal Interagency Stream Restoration Working Group 2001). The morphological data and understanding their inherent dynamics provide a basic tool for river rehabilitation planning strategies and prediction of river recovery potential (Gore 1985; Heede and Rinne 1990; Milner 1994; Brierley and Fryirs 2013). Floodplains are formed by interactions between river processes. Their characteristics and evolution depend on stream power, river sediment behavior, and river channel profile. Floodplains are widely used in human activities (agriculture, housing, and industry) susceptible to morphological changes of channels (Tealdi et al. 2011). A number of human activities including dam construction, water transfer, land-use change, change in water intake, and river sediment can affect morphology of channel and cause financial loss and ecological damages (Gaeuman et al. 2005). Anthropogenic interventions on river systems such as dam construction alter the incoming water and sediment regime and result in varying impacts on the lateral response of alluvial rivers (Williams and Wolman 1984; Johnson 1992; Xu 1996, 1997; Shields et al. 2000; Simon et al. 2002).

A large body of research has evaluated the river’s response to human activities, and according to the results, significant changes of channel generally occur in the form of channel narrowing and change of river plan form (Surian and Rinaldi 2003). Channel changes have been analyzed for several years (Gregory and Park 1974; Petts 1979; Williams and Wolman 1984; Kondolf et al. 2002; Liébault et al. 2002; Uribelarrea et al. 2003; Wishart et al. 2008; Zawiejska and Wyzga 2010; Ma et al. 2012). Observing historical changes of a channel is an important part of recognizing fluvial systems. Understanding the past condition is the strongest way to analysis the recent changes in channel forms and based on, the complicated agents that affects current forms of rivers, was revealed (Winterbottom 2000). Fluvial geomorphology is the study of the interaction between form and river channel processes on a temporal or spatial scale (Charlton 2008). Understanding the processes responsible for channel changes and river stability assessment has been of interest to many geomorphologists and river engineers. Most studies regarding the investigation of lateral changes of river channel have focused on channel migration and erosion of convex side of meander (Richard 2001). Depending on the input conditions and planform geometry of river channel, lateral channel changes can appear in different forms such as canal migration (meander migration), changes in river width, and channel as well as cut-off avulsion (Richard et al. 2005a, b). Lateral channel change rate depends on the resistance of convex bank against erosion, the continuity and magnitude of flow, radius of curvature channel, and the flow capacity for sediment transport. River migration rate (for example, area of floodplain reworked per channel length per year) has been empirically related to combinations of variables describing channel geometry (for example, channel width, meander length, meander wavelength, amplitude, radius of curvature, arc angle, and sinuosity) and discharges of various frequencies (Brice 1982; MacDonald et al. 1991; Garcia et al. 1994). Lateral channel migration rate of big rivers, like some of the meanders of the Mississippi River, reaches 20 m per year. However, the amount of lateral migration in the middle rivers is measured close to 1 m per year (Magdaleno and Fernández-Yuste 2011). The morphological changes in channel include adjusting the width, depth, channel dip, and river planform. Channel adjustment depends on the stream power, operation of river discharge, valley slope, as well as the size and volume of bed material, and riparian (Labbe et al. 2011). River stability and its response to environmental variables strongly depend on channel type, degree of river release, sediments, vegetation density, and human activities. Remote-sensing data and Geographic Information Systems (GIS) techniques are key tools for monitoring geomorphologic variations within the floodplain and river channel which provide good details regarding historical conditions and current river changes (Ramos and Gracia 2012). Planform and the amount of river migration have been studied and measured by remote-sensing data and GIS techniques (Nicoll and Hickin 2010). Many aspects of river morphology have been investigated after World War II. These include channel geometry, mathematical modeling, neo-tectonic effects and mass movements on channel, fluvial systems, experimental river morphology, effects of paleohydrology on channel, and gravel-bed rivers. Scientists working in the U.S. Geological Agency including Langbein and Leopold (1964), Schumm et al. (1987) have studied morphology of river channels investigating fluvial systems and have conducted laboratory experiments to observe river morphology. Allen (1983) has done extensive studies on the characteristics and classification of bed shapes and sedimentary structures by considering deltas, meanders, and floodplains. Ham and Church (2000) and Warburton et al. (2002) have conducted extensive studies to quantify channel and floodplain changes using GIS techniques. Adib et al. (2016) studied the role of construction of large dams on river morphology. Adib et al. (2018) evaluated the erodibility of bends in the Karkheh River using GIS and RS and the CCHE2D model. Fortugno et al. (2017) studied changes in the channel morphology due to land-use changes and check dam. Zema et al. (2018) assessed the effects of check dams on channel geometry, bed sediment size, and riparian vegetation.

The study area

Qara-Sou River is one of the permanent rivers of Ardebil province which originates from the northern slopes of Sabalan Mountain. After joining Ahar Chai River, it is recognized as Darreh Roud River. It joins Aras River near Aslandoz city. The lowest and highest altitudes of the Qara-Sou basin are 760 and 4781 m, respectively, and the average height of the basin is 1765 m (above sea level). The average slope of the basin is 13.68%. The main portion of Qara-Sou basin topography related to Sabalan Mountain is located in the southwest of the basin. The secondary part of topography of Qara-Sou basin related to Baghro Dagh Mountain is located in the southeast of the basin (Table 1). According to a 30-year period report of the corresponding hydrometric station, its mean annual rainfall is 284.3 mm per year and the average annual temperature is 8.9 °C. The minimum and maximum temperature of this basin is 33.8 and 39 °C, respectively. Annual rainfall this basin is from 299 to 402 mm per year. The southern portion of the study area has the highest annual rainfall, while the northern part has the lowest amount. The average annual discharge is 42.3 m3/s. The discharge of the Qara-Sou River is 220 m3/s for return period of 25 years. Among the important rivers that flow into Qara-Sou River before joining Ahar Chai River, we can refer to Balkhli Chai and Khiyav Chai Rivers. In this research, the study area is a reach of the river starting from Qadirlou village and continuing to the vicinity of La’l Ganj village. It is located between 38°32′N and 38°34′N latitude and 47°43′E and 47°52′E longitude. This segment of Qara-Sou River flows into a floodplain and a new quaternary alluvium (Qal). Nowadays, a large part of this floodplain is used for farming purposes. Three major seasonal rivers, namely Chai-Lax, Qotor Chai, and Alni-Chai, form the main confluences of Qara-Sou River in the study area. The approximate length of the river is 31 km in this range. To have a precise study, Qara-Sou channel was divided into three reaches based on anthropogenic effects and floodplain width variations (Fig. 1). There are five types of land use in the study area including river and water body, residential area, barren land, garden and shrub land, and finally agriculture. Most of these land-use types have been recently changed by human activity. River bed encroachments by human activity are the main form of land-use changes in the study area. Therefore, in recent years, the increasing trend in farming and gardening can be obviously seen both in the floodplain and the river bed. The study area is not densely populated. Four villages, namely Qadirlou, Kangarloo, La’l Ganj, and Agh Darq, are the main residential centers located around the Qara-Sou River. The total population of these four villages is about 1012 people. Farming is the main economic activity of this region, while the other economic sectors (industry and services) have not a significant role. In these villages, the major crops are wheat, barley, and various kinds of vegetables. Horticultural activities are also one of the major economic activities in the study area.

Table 1 Physiographic characteristics of the Qara-Sou Basin
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Fig. 1
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Location of the study area in the northwest of Iran

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Methods

The most important data in this research include large-scale topographic maps (scale 1:2000) of Ardabil province regional water company, geological maps of scale 1:100,000 by geological organization, IRS satellite images of geographic organization (with spatial resolution of 2.5 and 5 m), and Sentinel 2A satellite images (with spatial resolution of 10 m) by European Space Agency. Lateral mobility or movement and lateral stability of river channel are the most important indicators that are investigated in river geomorphology studies. Lateral movements are calculated by measuring the changes in the outermost bankline of the channel. Channel Stability Index is also measured based on changed active channel area and also unchanged active channel area within the time frame under investigation (Richard 2001). Lateral movements are divided into two main parts of width change and channel migration. Four indicators of lateral movements are calculated by measuring total outer bankline change. These four indicators are total outer bankline change, normalized lateral movement rates, width change, and true migration. Each of these indicators provides a different view of river channel changes. In the paper by Richard et al. (2005a, b), formulas for determining these indicators were presented. The method of calculating each of these indicators is as follows (Richard 2001, 2005a, b).

Total bankline change (E)

This index can be calculated by dividing the sum of lateral movement of outermost right and left bank of the river during a specified time period by the following equation:

$$E = \frac{\Delta r + \Delta l}{{t_{2} - t_{1} }},$$
(1)

where Δr (dr) is the lateral movement of the outermost right bankline, Δl (dl) is the lateral movement of the outermost left bankline, \(t_{1}\) is the year of the first aerial survey, and \(t_{2}\) is the year of the second aerial survey.

Normalized lateral movement rates (N)

Normalized lateral movement rates are calculated by the average division of changes in the right and left banklines to the rate of changes of active channel width during the respective time period as follows:

$$N = \frac{{\left[ {\frac{{\left( {\Delta r + \Delta l} \right)/2}}{{\left( {w_{1} + w_{2} } \right)/2}}} \right]}}{\Delta t} = \frac{{\left[ {\frac{\Delta r + \Delta l}{{w_{1} + w_{2} }}} \right]}}{\Delta t},\;(\% {\text{ width/year}}),$$
(2)

where, \(t_{1}\) and \(t_{2}\) are the time duration of aerial surveys, Δr (dr) is the lateral movement of the outermost right bankline of the river, Δl (dl) is the lateral movement of the outermost left bankline of the river, \(w_{1}\) is the width of the river active channel for the first time period, and \(w_{2}\) is the width of the river active channel for the second time period. It should be noted that higher changes measured by E and N indices are more than the river migration level due to changes in channel width.

Width change (dw)

There are two measurement methods for river width changes including change in active channel width and change in total channel width. Active channel width includes a channel without vegetation, medial point bar, and medial island. The change index in active channel width is obtained by the following equation:

$$dw_{\text{act}} = \frac{{\Delta w_{\text{act}} }}{\Delta t} = \frac{{w_{\text{act - 2}} - w_{\text{act - 1}} }}{{t_{2} - t_{1} }},\;({\text{m/year}}),$$
(3)

where, \(dw_{\text{act}}\) is the amount of changes in active channel width, \(t_{1}\) and \(t_{2}\) are the time periods of aerial survey, \(\Delta w_{\text{act}}\) is total change in active channel width, \(w_{{{\text{act - }}1}}\) is active channel width in the first time interval, and \(w_{\text{act - 2}}\) is active channel width in the second time interval. Total channel width includes the channels along with bars and medial islands of the river measured by calculating the difference between total channel width changes over the selected time interval based on the following equation:

$$dw_{\text{tot}} = \frac{{\Delta w_{\text{tot}} }}{\Delta t} = \frac{{w_{\text{tot - 2}} - w_{\text{tot - 1}} }}{{t_{2} - t_{1} }},\;({\text{m/year}}),$$
(4)

where \(dw_{\text{tot}}\) is the amount of changes in total channel width, \(t_{1}\) and \(t_{2}\) are the intervals of aerial survey, \(\Delta w_{\text{tot}}\) is change in total channel width, \(w_{\text{tot - 1}}\) is total channel width in the first time duration, and \(w_{\text{tot - 2}}\) is total channel width in the second time duration. Accordingly, the negative value of these two indices indicates the narrowing of channel and their positive value refers to the widening of the channel during the respective time duration (Richard et al. 2005a, b).

Lateral migration rates (M)

This index is measured by subtracting total channel width (\(dw_{\text{tot}}\)) from total bankline change (E) through the following equation:

$$M = \frac{\Delta r + \Delta l}{{t_{2} - t_{1} }} - \left| {\frac{{w_{\text{tot - 2}} - w_{\text{tot - 1}} }}{{t_{2} - t_{1} }}} \right| = E - \left| {dw_{\text{tot}} } \right|,\;({\text{m/year}}),$$
(5)

where M is the lateral migration or true migration rates, \(t_{1}\) and \(t_{2}\) are the intervals of aerial survey, \(w_{\text{tot - 1}}\) is total channel width within the first time period, and \(w_{\text{tot - 2}}\) is the total channel width in the second time period (Richard et al. 2005a, b). Another method for calculating lateral migration rates was offered by Shields et al. (Fig. 2). It uses the following equation (Giardino and Lee 2011; Shields et al. 2000):

$$Rm = (A/L)/y,$$
(6)

where Rm refers to lateral migration rate, A is the area between two centerlines of the channel, L is the length of centerline at time \(t_{1}\), and y is the number of years. In this context, Planform Statistics add-in was used to increase the accuracy of calculations in centerline of the channel and other calculations.

Fig. 2
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a Methods for measuring channel geometry, and b determination of channel centerline location and channel migration rate (Magdaleno and Fernández-Yuste 2011)

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Lateral stability index

The lateral stability of channel is calculated based on the changes in river active channel without vegetation in the respective time period of the study. In other words, lateral stability index is measured by dividing unchanged active channel area to the previous active channel area:

$${\text{Lateral stability index}} = \frac{\text{unchanged active channel area}}{\text{previous active channel area}}.$$
(7)

When the output number of this index is close to one, it means that the channel has not moved, and when it is closer to zero, it indicates a change in the time period of the study (Richard et al. 2005a, b). Each of the above-mentioned indicators provides a lot of information about changes and lateral stability of river channel which can easily be measured by GIS techniques of high accuracy.

Results and discussion

GIS techniques were used to measure dynamic lateral channel of Qara-Sou River by quantitative geomorphometric indices. In this regard, 271 cross sections were plotted on 31 km of Qara-Sou River channel and each geomorphometric index was calculated for each separate cross section. Regarding lateral channel change and the rate of lateral channel migration in particular, there are different methods and indices with different computational bases. Lateral movement rates of river channel were calculated by measuring the movement of left and right banklines of the channel in aggradation/degradation lines during the time period of the study. Each scale or measure of lateral motion reveals various characteristics of channel movement. Total bankline change index (E) is a combination of width changes and lateral channel migration. The index (E) is not average, but actually represents the total changes of bankline, and therefore, the value of this index is twice the rate of migration in other studies. Its value was 0.8 for the first reach; it was 1 for the second reach, and for the third one, it was 0.89. The high value of E is justified by the narrowing of the channel in Qara-Sou River. Therefore, during channel narrowing, there would be a lot of changes in banklines of the river and this increases the index value (E) in all three reaches, especially in the second reach (Fig. 3).

Fig. 3
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Methods for calculating total bankline change (E) in Arc GIS

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Normalized lateral movement rates (N) incorporate width change and lateral migration and normalizes it for the river size. This index can be calculated for any river. The average of the obtained results for this index based on Eq. (2) was calculated 0.048% for the first reach, 0.2% for the second reach, and 0.057% for the third one (Fig. 4).

Fig. 4
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Methods for calculating normalized lateral movement rates (N) in Arc GIS

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River channel width includes active channel width and total channel width. The active channel width does not include bars or islands in the channel and is considered as the modifier of the formed channel by flow and sediment dominant regime. Change in active channel width refers to the movement of channel toward lateral stability. Total channel width includes bars and islands in the channel. For a single-channel river with meander landform, as most parts of Qara-Sou River, active channel width and total channel width are equal. Therefore, in this study, the obtained results from \(dw_{\text{act}}\) index are completely identical to the obtained results by \(dw_{\text{tot}}\) index. The average of active channel width change \((dw_{\text{act}} )\) was − 0.5 for the first reach, − 0.67 for the second, and − 0.51 for the third one. Given that the negative value of this index indicates narrowing of the channel and its positive value indicates widening of the channel, Qara-Sou River has undergone a process of aggradation, narrowing, and widening of the channel during the last 12 years (2005–2017). Farming activities around Qara-Sou River, along with intervention in bed and riparian of the river channel and river channelization as a result, have led to the narrowing of the river channel and acceleration of inundation during the occurrence of flood in adjacent lands. It should be noted that despite the construction of Sabalan dam and due to the confluence of important rivers like Khiyav Chai, Chai-Lax, Qotor Chai, and Alni-Chai Rivers down the Sabalan dam, often associated with flood regimes, especially in the spring, a large part of Qara-Sou River is still affected by flood (Fig. 5).

Fig. 5
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Narrowing of Qara-Sou River channel due to the development of farming activities in the bed and river riparian (the second and third reaches)

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Different methods have been presented by various researchers to calculate river migration rate. One of the major measurements in river channel migration is related to the comparison of changes in the centerline of channel which is based on measuring the changes of channel centerline (Hooke 1997; MacDonald et al. 1991; Shields et al. 2000). In this study, the amount of channel migration from Qara-Sou River was calculated using geomorphometric methods for each cross section. The amount of channel width migration of the river depends on resistance of banklines to erosion, the duration and magnitude of flow, radius of curvature, and the ability of flow for sediment transport. If we subtract the amount of total movement of channel bankline index (E) from active channel width change index \(( dw_{\text{act}} )\), another index known as true migration rate index (M) can be calculated. The average of true migration index was measured 0.36 m/year, by Eq. (5) for the first reach, 0.33 m/year for the second, and 0.49 m/year for the third reach. In addition, the results of migration rate of Qara-Sou River by channel centerline change method by Eq. (6) showed that during the last 12 years, the amount of river migration rate in the first reach was 0.47 m/year, 0.54 in the second, and 0.46 m/year in the third one. Comparing the amount of both methods of calculating river migration rate showed that the results of both methods were very close to each other and both methods showed a lower amount of channel migration for Qara-Sou River. Reduction of river discharge due to the construction of Sabalan dam in the upstream of the river and consequently human intervention in the bed and riparian of Qara-Sou channel over the last 12 years have led to a decrease in the amount of river channel migration in each of the three study time reaches.

Lateral stability index

The lateral stability index is calculated by measuring the amount of unchanged active channel area and the previous active channel area. A value close to 1 in lateral stability index indicates that channels were not moving and were relatively stable, and a smaller amount of this index refers to the existence of change in the position of active channel. According to stability index formula, the amount of lateral stability for the selected reaches of Qara-Sou River was also calculated. The results of calculations showed that the amount of lateral stability index for the first reach was 0.3. It was 0.21 for the second reach, and 0.22 for the third one. The low value of stability index in various reaches of Qara-Sou River due to its low migration rate indicates the low level of unchanged active channel area during the respective time period and its reason is narrowing of Qara-Sou River channel (Fig. 6).

Fig. 6
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Methods for calculating lateral stability index of Qara-Sou River in a part of the fifth reach

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Channel planform geometry

The quantitative indicators such as sinuosity coefficient, central angle, and radius of bend were used for investigation of Gara-Sou River channel planform (Fig. 7). The results showed low changes of each indicator during study period. The sinuosity coefficient has slight increasing trend during 2005–2017 in each river reaches that related to river width decrease during last years. The sinuosity coefficient indicated that the Gara-Sou River channel planform has been meandering form in both periods (2005 and 2017). The central angel also has a slight increasing trend in all river reaches. The maximum value of this factor was related to developed meanders. The calculated central angle indicated that meanders of Gara-Sou River were belonging to developed class, according to the Kornise (1980) classification, in both periods. (Table 2). Considering the relationship between radius of bend and development level of channel bends, the lower radius of Gara-Sou River bends in 2017 in compared with 2005 indicates more developing of river bends in recent years. Therefore, the results in Table 2 show that Gara-Sou River was tended to dynamic stability.

Fig. 7
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Meanders and their circle tangent in 2005 and 2017

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Table 2 Changes of sinuosity coefficient, central angle, and radius of meanders in 2005 and 2017
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Floodplain and land-use changes

The morphology of floodplains is intimately linked with the form and behavior of the river channels shaping them (Charlton 2008). The Qara-Sou River flowed to floodplain in the study area and the major marginal sediment was composed of silt and clay. Qara-Sou River follows single-channel patterns in the floodplain. The historical change analysis of river channel during a 12-year interval revealed a total narrowing of the river channel (Fig. 8). Base flow controlling by Sabalan dam has led to growth some plants and also stabilizing point bar deposits in bed of the Qara-Sou River. The major part of the floodplain is occupied by farming, and in some areas, farmers encroach on river bed and cause land-use changes. The land-use classes in this study include river and water body, residential area, barren land, garden and shrub land, and agriculture. Temporal analysis of land-use changes from 2005 to 2017 indicated that the area of each land-use classes (agriculture, residential area, garden and woodland, river, and water body) has increased, while the barren land class has decreased in area.

Fig. 8
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Floodplain boundary delamination in a reach of the Qara-Sou River

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Characterization of channel morphology

The Rosgen method was used to morphological analysis of Gara-Sou River channel. Rosgen method was widely used in river management and restoration projects. The Rosgen (1994) classification system emphasizes the use of hydraulic parameters, such as width/depth ratio, sinuosity, slope, and channel form, to define river systems. Based on performed calculations, all three reaches that investigated in this study was belonged to class E and C of Rosgen model. Gara-Sou River in class C has wider and shallower channel. As well as the floodplain of the river in this class is developed and its valleys are widespread. The Gara-Sou River in class E has narrow and deep channel, but its flood-prone areas is wide and developed. The width of floodplain in first and second reaches significantly increases. In these river reaches, the river bed mostly composed of gravel and the slope of river bed in this area is about 0.02–0.039. Silt and clay materials are the main ingredients of the river banks that characterized by high cohesive and gentle slope of river bank, which lead to relatively stable banks. Cohesive sediment and vegetation cover increased the stability of river banks. Developing the meandering planform in these river reaches is closely related to low stream power and cohesive sediment of Gara-Sou river banks. Considering these variables, the majority area of first and second reaches are belonging to class C4b, but the end part of this reaches of Gara-Sou River is belonging to E4b class. In third reach, the floodplain width has been decreased. The field survey was revealed that the river bed in this reach is mostly composed of gravel and sand. The river slope range in this reach is about 0.02–0.039. Accordingly, the major length of river in this reach was belonging to C4b class, but small area in this reach was belonging to E4b class (Fig. 9). The results indicated that the sections of Gara-Sou River that belong to class C is very sensitive to disturbance, which have good recovery potential, high sediment supply, and very high vegetation controlling. While in the sections of Gara-Sou River that belong to class E, the river is very high sensitive to disturbance, and has good recovery potential, moderate sediment supply, and very high vegetation controlling.

Fig. 9
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Classification of Gara-Sou River cross sections based on Rosgen method

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The effect of Sabalan dam construction on downstream morphology of Qara-Sou River

The effect of dam construction on the pattern and downstream channel width of the river has been examined by several researchers including Surian (1999), Williams and Wolman (1984), Kellerhals and Church (1989), and Shields et al. (2000) (Richard 2001). Reservoir dams are among the great hydraulic structures built to save river water for drinking, agricultural, and industrial uses. Reservoir dams affect the hydraulic geometry of river by changes in the width of channel, depth of flow, size of bed material, and the vegetation of sides. In general, the construction of Sabalan dam on Qara-Sou River has directly and indirectly affected the morphology of the river channel. Considering the direct role of Sabalan dam on the morphology of downstream dam, it can be said that with its construction, along with the reduction of bankful flows, and hence effective flows, as discharges forming the channel, lateral channel dynamic of the river has decreased.

By reviewing 1RS satellite images with resolution of 2.5 m from 2005 to 2017 (before and after the construction of the dam), the extracted vector pattern showed that the channel of Qara-Sou River has encountered narrowing in the majority of time periods of the study. Furthermore, indirectly, the existence of Sabalan dam and the size of its reservoir have increased the encroachment on bed and riparian (even the main bed of the river). In fact, with the construction of Sabalan dam, a large part of bed and riparian of Qara-Sou River (especially the lateral bars of the river) have been converted into farming land by farmers in the region which is one of the main factors in channel narrowing of Qara-Sou River (Fig. 10). Although the Sabalan dam construction leads to significant land-use changes in Gara-Sou River floodplain, but these changes were not so strong that lead to changes in Gara-Sou River channel planform. In study area, the river banks mostly composed of silt and clay materials that covered with farming and gardening land use. In all sections of Gara-Sou River, where river bank composed of clay, due to clay cohesiveness properties, lowing stream power of Gara-Sou River (after Sabalan dam construction) and gentle slope of river bank, developing of Gara-Sou River meanders has been limited and significant changes in meanders has not been occurred. The interaction of mentioned factors has been lead to stability of Gara-Sou River banks and also decreased the lateral channel change of river, so that, in study area, majority of changes were demonstrated as narrowing of river channel.

Fig. 10
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Qara-Sou channel narrowing due to farming activities within the boundaries of river bed and riparian

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Decreasing the sediment load of river, due to constructing the Sabalan dam, resulted in increasing sediment transport capability of river downstream. This process has eroded the bed and banks of river. Some evidence of this erosion is observed as incision in downstream of the Sabalan dam. With the passage of time, followed by erosion and slope reduction, the stream power reduces and consequently erosion and sedimentation of Gara-Sou River will almost equal, and finally, river will tended into dynamic equilibrium.

Conclusion

In this study, stability and lateral movement of Qara-Sou River channel with 31 km long were investigated using quantitative geomorphometric indices and field studies. The geomorphometric indices used in this study identified an important insight regarding the type and manner of calculating the degree of stability and lateral movement. All these quantitative indices were calculated by the creation of a geo-database and the combination of various GIS functions with high accuracy for river channel from 2005 to 2017. For our purposes, lateral movements of the channel were divided into two main parts of the river width change and channel migration. Four indicators of lateral movement were calculated by measuring total outer bankline change. These four indicators are total outer bankline change, normalized lateral movement rates, width change, and true migration. High mobility of (E) and (N) indices is not related to true migration, but it has a strong connection with large changes in the width of the channel. The results of analysis in lateral channel change of Qara-Sou River indicate the high value of (E) and low value of (M), and this is due to the occurrence of change in channel width (narrowing of the channel) of Qara-Sou River during the last 12 years. In fact, due to the construction of Sabalan dam and encroachment on bed and riparian of the river, the amount of width migration has decreased in Qara-Sou River, and instead, the index value of (E) shows a high number for all three study reaches. Lateral migration rates of Qara-Sou River were measured by two different methods. The first method was true migration rate index (M) which was calculated by the difference between total bankline change (E) and total channel width change \((dw_{\text{tot}} )\) index. Another used index by Shields was measured based on the amount of the area between two centerlines of the channel along the length of channel centerline. The results from both methods by Eqs. (5 and 6) showed very little amount of migration rate for all three reaches. Based on the results, the low value of lateral stability index in the channel of Qara-Sou River in all three reaches refers to the low level of unchanged channel area due to a decrease in the area of Qara-Sou River channel (channel narrowing) in 2017. In addition, the results of comparison of lateral movement in the three selected reaches showed that the second reach has the most lateral movement in the channel, and the first and third reaches come next. In other words, in the second reach, due to maximum effect of anthropogenic factors (encroachment on bed and riparian of the river), lateral movement index was in the highest level and lateral stability index was in the lowest level. Based on the results, with decreasing the volume of peak discharges of Qara Sou River due to the construction of Sabalan dam and the stabilization of point bars inside the banks of the river channel with vegetation, it seems that the river has moved towards dynamic equilibrium over the last 12 years. The results of land-use changes from 2005 to 2017 revealed a significant increase in farming and gardening land-use both in the floodplain of Qara-Sou River. Followed by construction of Sabalan dam, the river discharge decreased leading to notable land-use changes in downstream of the basin. Encroachment on Qara-Sou River bed not only led to social problems, but also resulted in decreasing flow capacity for return period of 10 years by narrowing the river channel. According to field survey, farming encroachment, also has been reaches to channel bed and lead to significant narrowing of river channel. Regarding the main confluences of Qara-Sou River as Chai-Lax, Qotor Chai, and Alni-Chai Rivers in downstream of Sabalan dam especially in spring (with flood regime), there is still a risk of flood in the study area. Quantitative geomorphometric indices can provide a comprehensive and accurate picture of the lateral movement in the channel of the river and it can be used in floodplain management studies. Therefore, the present study attempted to introduce new methods for fluvial studies with further quantification of river channel changes.