Channel Adjustments in Iranian Rivers : A Review

Channel adjustments in Iranian rivers have been intense over the last decades due to natural and human factors. Iran has six major basins, all with different climates, from very humid to very arid. This work is a review of the available studies and data about channel adjustments in Iranian rivers, and aims to reconstruct a first outline, at a national scale, of types, magnitude, and causes of adjustments. The results show that most of the rivers have undergone incision (1 to 2 m and, in some cases, up to 6 to 7 m) and narrowing (from 19% to 73%), although widening (from 22% to 349%) has occurred in some rivers. Narrowing is due to dams and sediment mining; widening is due to climate change and sediment mining. Incision is due to gravel and sand mining, dams, channelization, with in-channel mining being the main cause of incision. Channel adjustments have occurred in basins with different climates, but it seems that widening has been more intense in arid and semi-arid climates. Such adjustments have several negative effects (e.g., damage to bridges, degradation of river ecosystems, and instability of banks). The comparison between Iran and other countries shows that narrowing and incision have been the dominant processes in most of the rivers, while damming and in-channel mining have been used as the main controlling factors. Data about adjustments in Iranian rivers are neither homogeneous nor complete for all the rivers. This lack of completeness implies that our understanding of channel changes, and their causes, should be improved by further investigation.

Changes in river channels can have social and environmental effects, it can damage structures, increase the potential for flooding, decrease groundwater resources, and cause a reduction in the number of native species.Therefore, a better understanding of channel adjustments is crucial for river management, and to predict and mitigate the negative effects of such adjustments.
During the last few decades, river channels in Iran have been stressed by climate change and several human interventions, such as channelization, the diversion of water for flood mitigation and agriculture, damming, gravel and sand mining, and land-use changes.Although studies have been lacking.By reviewing published articles and data, we addressed the following questions: (1) How much Iranian rivers have changed over the last decades (type and magnitude of change)?(2) What are the main causes of channel adjustments?(3) Are adjustments homogeneous throughout Iran or do some differences exist between the main geographic regions?Finally, channel adjustments of Iranian rivers were considered in a larger context, by comparing the situation in Iran with adjustments in other regions worldwide.

Study Area
The territory of Iran has an area of 1,648,000 km 2 , and is located at 25° to 40° northern latitude and 44° to 64° eastern longitude (Figure 1).Except the narrow plains along the Caspian Sea, the Persian Gulf, and Khouzestan Region, Iran is considered as a highland with an average elevation of 1000 m a.s.l.The lowest point in Iran is located in the northeast, in the salt plain of Shahdad on the margin of Lut Kavir, with an elevation of 350 m.The highest point in Iran is Mount Damavand in the Alborz Mountain range, with a height of 5774 m.From a global tectonic point of view, Iran is part of the Alpine-Himalayan orogenic belt that extends from the Atlantic Ocean to the Western Pacific.Iran features very diverse climatic and geographic characteristics.The highest temperature in the Persian Gulf area can reach up to 53 °C in summer, and the lowest temperature in the northwestern part of Iran can reach up to −40 °C in winter [45].The average annual precipitation is estimated at 250 mm, varying from 50 mm in some areas of the central water basin, to more than 1600 mm in some coastal areas near the Caspian Sea [46].The climate ranges from very humid to very arid, but in the majority of the country it is very arid and arid (Figure 2).C in winter [45].The average annual precipitation is estimated at 250 mm, varying from 50 mm in some areas of the central water basin, to more than 1600 mm in some coastal areas near the Caspian Sea [46].The climate ranges from very humid to very arid, but in the majority of the country it is very arid and arid (Figure 2).
According to the classification of the Ministry of Energy, Iranian rivers can be grouped into six major drainage basins: 1-Caspian Sea Basin; 2-Persian Gulf and Oman Sea Basin; 3-Urmia Lake Basin; 4-Central Basin; 5-Eastern Basin (Hamoon); 6-Sarakhs Basin.All these basins are interior, except for the Persian Gulf and Oman Sea Basin.We found information about channel adjustments for 18 rivers, which are mainly located in the Persian Gulf and Oman Sea Basin (seven rivers) and the Caspian Basin (five rivers) (Figure 1).The main hydrological and physiographic characteristics of the  According to the classification of the Ministry of Energy, Iranian rivers can be grouped into six major drainage basins: 1-Caspian Sea Basin; 2-Persian Gulf and Oman Sea Basin; 3-Urmia Lake Basin; 4-Central Basin; 5-Eastern Basin (Hamoon); 6-Sarakhs Basin.All these basins are interior, except for the Persian Gulf and Oman Sea Basin.We found information about channel adjustments for 18 rivers, which are mainly located in the Persian Gulf and Oman Sea Basin (seven rivers) and the Caspian Basin (five rivers) (Figure 1).The main hydrological and physiographic characteristics of the rivers analyzed in this study are summarized in Table 1.The hydrological regime of all studied rivers is perennial.

Materials and Methods
A review of studies about morphological changes in Iranian rivers was carried out.It is worth noting that this is the first review about this topic in Iran.We looked for studies, both in English and Persian, dealing with channel changes during the last few decades.The review was focused on: (1) Type and magnitude of morphological changes, (2) causes of changes, (3) effects of changes on structures and environment.
Data were found for 18 rivers and allowed for a description of each river by the following aspects: Channel morphology; morphological changes; location and time of changes; causes of changes.Such aspects were defined after a preliminary screening of available data, taking into account possible limitations (e.g., lack of information) of the studies that we analyzed.As for channel morphology, we opted for just two typologies, single-thread and multi-thread.Morphological changes refer mainly to change in channel width (narrowing and widening) and in bed-level (incision and aggradation).The time of change (the period of time when channel changes have taken place) is crucial information since it allows for an assessment of the magnitude of processes that have occurred.Causes of changes are those mentioned in the studies, therefore we relied on original interpretations for this aspect, as well as for the others.For sediment mining, one of the major causes of changes in Iranian rivers, was if activity was carried out within the channel (in-stream mining).If mining activity was not clearly described, not indicating if activity was carried within the channel or in the fluvial corridor, this was simply reported in our analysis as 'mining'.Climate was defined for each river, as it was an aim of this study to investigate the possible role varying climate has, specifically if relation to channel adjustments (e.g., arid vs. temperate climate).
Finally, to facilitate analysis and interpretation of the dataset, a numerical identifier (ID) was associated to each river or river segment.

Type and Magnitude of Channel Changes
As for planform changes, both narrowing and widening have occurred in cases 13 and 7, respectively (Table 2).Narrowing ranged between 19% and 84% (Figure 3) and, in some cases, was shown to be up to 300 m (e.g., some reaches of the Karun River).Widening ranged between 22% and 349% and has been shown to be more than 100 m in some rivers (e.g., Gamasiyab and Harirroud rivers).Furthermore, changes in channel pattern have occurred in some reaches (Table 2): From braided to meandering and then straight (e.g., Ahar Chai River); from meandering to braided (e.g., Gamasiyab River).As for bed-level changes, incision was observed in eight rivers, while aggradation was seen in three rivers (Table 2).Incision has been commonly seen at 1 to 2 m, but in some reaches, up to 6 to 7 m was seen (e.g., Talar and Khoshkehroud rivers) (Figure 4).An estimate of aggradation is not As for bed-level changes, incision was observed in eight rivers, while aggradation was seen in three rivers (Table 2).Incision has been commonly seen at 1 to 2 m, but in some reaches, up to 6 to 7 m was seen (e.g., Talar and Khoshkehroud rivers) (Figure 4).An estimate of aggradation is not available.As for bed-level changes, incision was observed in eight rivers, while aggradation was seen in three rivers (Table 2).Incision has been commonly seen at 1 to 2 m, but in some reaches, up to 6 to 7 m was seen (e.g., Talar and Khoshkehroud rivers) (Figure 4).An estimate of aggradation is not available.Coupling of processes, both planform and bed-level changes, was available for ten cases out of 21 (Table 2).Narrowing has been associated with incision and aggradation in cases 5 and 2, respectively, while widening with incision and bed-level stability was seen in case 2 and 1, respectively.
Some channel changes started in the 1950s to 1960s, whereas some others begin at the beginning of this century.This suggests that, overall, the morphological changes have been quite intense since they occurred in a time period of 50 to 60 years and, in some cases, of only 15 to 20 years.Coupling of processes, both planform and bed-level changes, was available for ten cases out of 21 (Table 2).Narrowing has been associated with incision and aggradation in cases 5 and 2, respectively, while widening with incision and bed-level stability was seen in case 2 and 1, respectively.Some channel changes started in the 1950s to 1960s, whereas some others begin at the beginning of this century.This suggests that, overall, the morphological changes have been quite intense since they occurred in a time period of 50 to 60 years and, in some cases, of only 15 to 20 years.

Causes and Effects of Channel Adjustments
All the analyzed studies made an attempt to identify the causes of channel adjustments.Both anthropic (e.g., sediment mining, dams, land-use changes) and natural (climate change) causes have been recognized.Channel adjustments have been related to a combination of different causes in 11 cases, whereas to a single cause in ten cases (Table 2).As for multiple causes, a combination of anthropic and natural factors were identified in five cases, whereas a combination of different anthropic factors were identified in six cases.As for single causes, both sediment mining and dams have been identified in four cases, whereas land-use change was identified in two cases.
The next step in our analysis of the dataset was to identify the main causes of adjustments.Being aware of some limitations, namely that we had to select a main factor for those rivers characterized by multiple causes, and the relatively small size of the dataset, this analysis could be useful to explore possible relationships between causes (e.g., controlling factors) and channel adjustments.Three causes turned out to be the dominant causes in 16 out of 18 rivers (Figures 5 and 6): Gravel and sand mining (river ID: 2, 3, 4, 8, 10, 16, 17); dams (ID: 5, 6, 12, 14, 18); climate change (ID: 7, 9, 11, 15).Channelization and land-use change (coupled with climate change) were identified for the other two rivers of the dataset.If channel response to gravel and sand mining is straightforward in terms of bed-level change (all the available case studies show mining associated with channel incision; Figure 6), it is not in terms of planform change.In fact, the response to sediment mining was both narrowing and widening (Figure 5).On the contrary, the response to dams seems more direct, since narrowing and incision have been observed in all the rivers with such human disturbance.Finally, climate change seems to mainly induce channel widening (Figure 5).
Water 2019, 11, x FOR PEER REVIEW 9 of 16

Causes and Effects of Channel Adjustments
All the analyzed studies made an attempt to identify the causes of channel adjustments.Both anthropic (e.g., sediment mining, dams, land-use changes) and natural (climate change) causes have been recognized.Channel adjustments have been related to a combination of different causes in 11 cases, whereas to a single cause in ten cases (Table 2).As for multiple causes, a combination of anthropic and natural factors were identified in five cases, whereas a combination of different anthropic factors were identified in six cases.As for single causes, both sediment mining and dams have been identified in four cases, whereas land-use change was identified in two cases.
The next step in our analysis of the dataset was to identify the main causes of adjustments.Being aware of some limitations, namely that we had to select a main factor for those rivers characterized by multiple causes, and the relatively small size of the dataset, this analysis could be useful to explore possible relationships between causes (e.g., controlling factors) and channel adjustments.Three causes turned out to be the dominant causes in 16 out of 18 rivers (Figures 5 and 6): Gravel and sand mining (river ID: 2, 3, 4, 8, 10, 16, 17); dams (ID: 5, 6, 12, 14, 18); climate change (ID: 7, 9, 11, 15).Channelization and land-use change (coupled with climate change) were identified for the other two rivers of the dataset.If channel response to gravel and sand mining is straightforward in terms of bed-level change (all the available case studies show mining associated with channel incision; Figure 6), it is not in terms of planform change.In fact, the response to sediment mining was both narrowing and widening (Figure 5).On the contrary, the response to dams seems more direct, since narrowing and incision have been observed in all the rivers with such human disturbance.Finally, climate change seems to mainly induce channel widening (Figure 5).Morphological changes in river channels may have negative effects on hydraulic structures, infrastructure, agricultural lands, and the environment.Gravel and sand mining has led to the instability of banks, damage to structures, increasing flood hazard and potential damages to bridges (Figure 7).Widening of the channel has led to a loss of agricultural land (e.g., Harirroud River).Finally, it is likely that channel changes have had effects on environment and ecosystems, but such effects were not evaluated in the available studies.Morphological changes in river channels may have negative effects on hydraulic structures, infrastructure, agricultural lands, and the environment.Gravel and sand mining has led to the instability of banks, damage to structures, increasing flood hazard and potential damages to bridges (Figure 7).Widening of the channel has led to a loss of agricultural land (e.g., Harirroud River).Finally, it is likely that channel changes have had effects on environment and ecosystems, but such effects were not evaluated in the available studies.

Channel Adjustment in Iranian Rivers and Comparison with Other Rivers Worldwide
Channel adjustment has been remarkable in Iranian rivers (a narrowing up to 73% and incision up 6 to 7 m).These channel adjustments bring to mind the question whether such adjustments in Iran are similar to those that have occurred in other rivers worldwide.Studies about rivers in Europe (e.g., Italy, France, Spain, UK, Poland, and Hungary), and elsewhere (China and USA) show that narrowing and incision have been the dominant processes in these rivers, and that sediment mining,  Morphological changes in river channels may have negative effects on hydraulic structures, infrastructure, agricultural lands, and the environment.Gravel and sand mining has led to the instability of banks, damage to structures, increasing flood hazard and potential damages to bridges (Figure 7).Widening of the channel has led to a loss of agricultural land (e.g., Harirroud River).Finally, it is likely that channel changes have had effects on environment and ecosystems, but such effects were not evaluated in the available studies.

Channel Adjustment in Iranian Rivers and Comparison with Other Rivers Worldwide
Channel adjustment has been remarkable in Iranian rivers (a narrowing up to 73% and incision up 6 to 7 m).These channel adjustments bring to mind the question whether such adjustments in Iran are similar to those that have occurred in other rivers worldwide.Studies about rivers in Europe (e.g., Italy, France, Spain, UK, Poland, and Hungary), and elsewhere (China and USA) show that narrowing and incision have been the dominant processes in these rivers, and that sediment mining,

Channel Adjustment in Iranian Rivers and Comparison with Other Rivers Worldwide
Channel adjustment has been remarkable in Iranian rivers (a narrowing up to 73% and incision up 6 to 7 m).These channel adjustments bring to mind the question whether such adjustments in Iran are similar to those that have occurred in other rivers worldwide.Studies about rivers in Europe (e.g., Italy, France, Spain, UK, Poland, and Hungary), and elsewhere (China and USA) show that narrowing and incision have been the dominant processes in these rivers, and that sediment mining, damming, channelization, over grazing, and land-use changes are the main controlling factors (Table 3).As for planform adjustments, narrowing was the dominant process in Iranian rivers, but it is worth noting that widening took place in 7 out of 20 cases (Table 2 and Figure 3).Similarly, in regard to bed-level changes, incision was dominant, but aggradation occurred in 3 out of 11 cases (Table 2 and Figure 4).As for controlling factors, sediment mining and damming are the most important in Iranian rivers, as well as in several rivers worldwide, although climate change turned out to be the key factor in 4 out of 18 rivers (Figures 5 and 6).This comparison suggests that Iranian rivers share several aspects with other rivers worldwide (e.g., type of adjustments, controlling factors).At the same time, some aspects are worthy of further detailed exploration, specifically the role of climate change on Iranian rivers.
Observations unequivocally show that Iran has been rapidly warming over recent decades, triggering a wide range of climatic changes.Iran has warmed by nearly 1.3 • C during the period 1951 to 2013 (+0.2 • C per decade), with an increase of the minimum temperature at a rate two times that of the maximum.Consequently, an increase in the frequency of heat extremes and a decrease in the frequency of cold extremes have been observed.The annual precipitation has decreased by 8 mm per decade, causing an expansion of Iran's dry zones.Previous studies have pointed out that warming is generally associated with more frequent heavy precipitation because warmer air can hold more moisture [78].The percentages of areas with extremely dry and dry climate have increased by nearly 8% and 6%, respectively, while a decreased percentage of Iran is now characterized by relatively wet climate [78,79].Dry zones have been pushed into the southwest and northeast of Iran, which previously had a mostly semi-dry climate.Some areas in south Iran with a dry climate in the past have recently been characterized by an extremely dry climate.West Iran, which was previously characterized by a semi-wet climate, has recently become mostly semi dry [78].It seems that the Central Basin and Persian Gulf and Oman Sea Basin and Sarakhs Basin have been affected by climate change, with as dryer climate being more common than other basins.That is why climate change is likely the major controlling factor of channel adjustments in these basins (e.g., rivers 7, 9, 11, and 15).

Dataset Limitations and Research Perspectives
It is worth noting that this work represents the first review about channel adjustments in Iranian rivers, and data turned out to be neither homogeneous nor complete for all the rivers.Some major limitations of the available dataset are: (i) Few data about bed-level changes, especially about channel aggradation, (ii) lack of reconstruction of evolutionary trajectory, and (iii) few case studies in some basins (e.g., Central, Eastern and Sarakhs basins).This implies that understanding channel adjustments, and their causes, could be improved notably by further studies.Reconstruction of evolutionary trajectory will be crucial for a sound understanding of factors controlling adjustments and for river management [80].Moreover, the role of climate change in the recent evolution of Iranian rivers is a very challenging issue that future studies should address.

Conclusions
(1) Remarkable channel adjustments took place in Iranian rivers during the last few decades.
Adjustments seem comparable with other rivers worldwide, although lack of evolutionary trajectories hinders a detailed analysis of timing and magnitude of morphological changes.(2) Human interventions, specifically sediment mining and dams, have been the main causes of channel adjustment in several rivers, though climate change has played a major role in some rivers.(3) Besides analyzing type, magnitude, and causes of channel adjustments, this review turned out to be useful for the identification of key issues that should be addressed in the future.More detailed studies will allow a better understanding of controlling factors, specifically to what extent climate change is driving the evolutionary trajectory of Iranian rivers.

Figure 1 .
Figure 1.Location map showing Iranian river network, main drainage basins, and rivers analyzed in this study.

Figure 1 .
Figure 1.Location map showing Iranian river network, main drainage basins, and rivers analyzed in this study.Iran features very diverse climatic and geographic characteristics.The highest temperature in the Persian Gulf area can reach up to 53 • C in summer, and the lowest temperature in the northwestern part of Iran can reach up to −40 • C in winter[45].The average annual precipitation is estimated at 250 mm, varying from 50 mm in some areas of the central water basin, to more than 1600 mm in some coastal areas near the Caspian Sea[46].The climate ranges from very humid to very arid, but in the majority of the country it is very arid and arid (Figure2).According to the classification of the Ministry of Energy, Iranian rivers can be grouped into six major drainage basins: 1-Caspian Sea Basin; 2-Persian Gulf and Oman Sea Basin; 3-Urmia Lake Basin; 4-Central Basin; 5-Eastern Basin (Hamoon); 6-Sarakhs Basin.All these basins are interior, except for the Persian Gulf and Oman Sea Basin.We found information about channel adjustments for 18 rivers, which are mainly located in the Persian Gulf and Oman Sea Basin (seven rivers) and the Caspian Basin (five rivers) (Figure1).The main hydrological and physiographic characteristics of the

Figure 3 .
Figure 3. Channel narrowing in the Karun River due to human intervention, especially dam construction [66]: (A) aerial photograph of 1954; (B) IRS imagery of 2006.

Figure 3 .
Figure 3. Channel narrowing in the Karun River due to human intervention, especially dam construction [66]: (A) aerial photograph of 1954; (B) IRS imagery of 2006.

Figure 5 .
Figure 5. Changes in channel width and relative causes in Iranian rivers.GSM: Gravel and sand mining, NF: Natural factors, D: Dam, LUC&NF: Land-use change and natural factors, C: Channelization.

Figure 5 .
Figure 5. Changes in channel width and relative causes in Iranian rivers.GSM: Gravel and sand mining, NF: Natural factors, D: Dam, LUC&NF: Land-use change and natural factors, C: Channelization.

Figure 6 .
Figure 6.Changes in bed elevation and relative causes in Iranian rivers.GSM: Gravel and sand mining, D: Dam, C: Channelization.

Figure 7 .
Figure 7. Channel incision due to in-stream gravel and sand mining (A); Effects of channel incision along the Talar River and effects on bridge stability (B) [57].

Figure 6 .
Figure 6.Changes in bed elevation and relative causes in Iranian rivers.GSM: Gravel and sand mining, D: Dam, C: Channelization.

Figure 6 .
Figure 6.Changes in bed elevation and relative causes in Iranian rivers.GSM: Gravel and sand mining, D: Dam, C: Channelization.

Figure 7 .
Figure 7. Channel incision due to in-stream gravel and sand mining (A); Effects of channel incision along the Talar River and effects on bridge stability (B) [57].

Figure 7 .
Figure 7. Channel incision due to in-stream gravel and sand mining (A); Effects of channel incision along the Talar River and effects on bridge stability (B) [57].
Water 2019, 11, 672 3 of 16 rivers analyzed in this study are summarized in Table 1.The hydrological regime of all studied rivers is perennial.Water 2019, 11, x FOR PEER REVIEW

ID River Drainage Basin Area (km 2 ) Length (km) Basin Relief * (m) Mean Annual Precipitation (mm yr −1 ) Mean Annual Discharge (m 3 s −1 )
* Basin relief is a difference between maximum elevation in the basin and elevation at basin outlet.

Table 2 .
Channel adjustments in Iranian rivers and relative causes.
* a, b, and c are the different reaches of the same river.

Table 3 .
Channel changes due to human activities: Case studies from different countries and environments.