Effect of dam construction on changes in river's environmental flow (case study: Gorganrood river in the south of the Caspian Sea)

Human activities such as dam construction can lead to significant river flow regime changes and, consequently, river environments and habitats. Different approaches for estimating the environmental flow have developed to improve the river ecosystem's environmental and habitat conditions. The hydrological approach is considered in many studies to estimate environmental flows considering its advantages. Vimba vimba persa is an economically and ecologically valuable species in the Caspian Sea that spawns and reproduces naturally in the Gorganrood River downstream in April–May. After the construction of the Voshmgir dam on the Gorganrood River, the fish population has drastically reduced in recent decades. In this study, to investigate the effect of constructing the Voshmgir dam on the environmental flow of Gorganrood River, a wide range of hydrological methods, including Tennant, Tessman, Arkansas, Aquatic Base Flow, Flow Duration Curve, High Flow-Low Flow, and FDC Shifting have been used. Comparing the correlation between the mean monthly flow time series before and after the Voshmgir dam demonstrates remarkable changes, including Pearson (0.95 to 0.86), Spearman's rho (0.93 to 0.78), and Kendall's τ (0.78 to 0.61) as a consequence of dam construction. The results indicate that estimated environmental flow from Tennant (1.1, 3.4 to 1.3, 3.8 cms) and Aquatic Base Flow (0.5 to 1.8 cms) methods have led to inadequate estimation due to significant changes in flow in different months. The amount of environmental flow during April–May estimated from investigated methods indicates the dam negatively affected water allocation to meet the river downstream's ecological and environmental needs. The flow duration curve analysis confirms that the dam construction has caused the river flow to fluctuate significantly within the year. The dam construction has dramatically reduced the flow in April–May, a critical environmental and ecological season. The April–May period coincides with the river's flood season. The construction of the Voshmgir dam has caused the components of high flow in the environmental flow to face severe shortages.


Introduction
Rivers are important components of water movement in nature and have a wide relationship with their environment (Ghadim et al. 2020;Ansarifar et al. 2020).Rivers are of great ecological importance and are home to many diverse plants and aquatic habitats (Crook et al. 2020;Jokar et al. 2021).In aquatic ecosystems such as rivers and lagoons, plants and animals adapt to the aquatic environment and depend on the aquatic environment to provide food, shelter, and other needs (Hawke et al. 2021).The river flow pattern affects phenomena such as the migration path of fish for reproduction, Reduction in river nutrients, lack of sufficient oxygen for aquatic organisms, and stress on habitat survival (Nunes et al. 2019).Suppose the flow components, such as the river flow's velocity, depth, and water quality, are appropriate.In that case, the possibility of fish spawning and natural reproduction is provided (Carpenter-Bundhoo et al. 2023;Bradford et al. 2023).If the required conditions are not met, the survival rate of natural reproduction besides fish spawning will be greatly reduced (Schmidt et al. 2020).Maintaining the natural flow regime is the main driving force in 212 Page 2 of 14 managing river habitats (Murry et al. 2019).Numerous human development activities in the impact area and along rivers disrupt existing aquatic habitats, and they can transform a fluvial aquatic ecosystem into an almost sedentary lake aquatic ecosystem (Lomnicky et al. 2019).These landuse changes disrupt river-dependent habitats' environmental and ecological conditions, resulting in the complete extinction of species and some species' migration (Oliveira-Junior and Juen 2019).Also, Reducing available water under the influence of human activities reduces the spawning of aquatic animals and increases food competition (Candolin and Voigt 2020).Humans exploiting rivers will reduce discharges and cross-sectional flow, depth, and even flow velocities, dramatically impacting river-dependent habitats (Jiao et al. 2019).Construction of dams and water abstraction from rivers for development activities in the agricultural sector can significantly affect river flow's natural regime (Yang et al. 2020;Salarijazi and Ghorbani 2019).Various studies have been conducted on the impact of such human actions on the river flow regime (Modabber-Azizi et al. 2023;Kousali et al. 2022;Adib et al. 2016;Adib 2022).Choi et al. (2005) examined the effects of the Hapchon dam on regime change in the Hwang River flow in South Korea.They found that the dam construction caused significant downstream changes in the river path.Magilligan and Nislow (2005) studied changes in the hydrological regime resulting from dam construction on rivers in the United States.This study showed that dams had caused different changes in the monthly flow regimes of rivers in some months of the year.Regulation of the rivers is led to decreasing flow in some months while other months experience increasing flow.Graf (2006) investigated the hydrological effects of large dams on American rivers, compared natural and regulated flows, and estimated the amount of discharge under the influence of large dams between 67 and 90%.In another study, the impact of dam construction on the Yellow River in China was investigated.This study showed a sharp decrease in river flow at different time scales (Yan et al. 2010).Another study showed that the Three Gorges Dam construction caused many seasonal changes in the river flow and significantly impacted the hydrological relationship between Poyang Lake and the Yangtze River (Guo et al. 2012).The study of hydropower operations upstream of the Mekong basin shows an increase in the dry season and a decrease in the wet season in this river.These changes can be seen greatly from the river's hydropower (Räsänen et al. 2017).Another study in this field shows the significant impact of the Akosombo dam on the Volta River in West Africa.This dam has reduced the amount of sediment and river flow to 92% and 23% (Amenuvor et al. 2020).Various researchers have considered this issue due to the different aspects of the impact of dams on river flow, the environment, and river habitats.Wang et al. (2016), considering the middle section of the Yangtze River in China, showed that dam construction on this river hurts the fish community and aquatic biodiversity.It led to the population of fish in the study area has decreased.Arantes et al. (2019) stated that dam construction causes extensive fish functional diversity changes due to river flow changes, pulses, and habitats.The results of Baumgartner et al. (2020) indicate that dams disrupt fish life due to their effect on water level control in downstream areas.In recent years, various methods, approaches, and frameworks for assessing and regulating water flow to protect the environment have been developed (Karimi et al. 2021).These studies are generally defined as general environmental flow estimates (Yarnell et al. 2020).The approaches, methods, and frameworks are fundamentally different in terms of objectives, input information, the accuracy of results, and the required time for study and calculations (Papadaki et al. 2017).Among the various approaches to estimating environmental flow, the hydrological approach has been used significantly in research in this field due to its low need for time and cost compared to other approaches and access to more limited data (Acreman and Dunbar 2004).Known methods in this field include Tennant (Tennant 1976;Maavara et al. 2020), Arkansas (Davis 2005), flow duration curve (Verma et al. 2017), high flow-low flow (Smakhtin et al. 2004;Mahmood et al. 2020), Tessman (Goguen et al. 2020), FDC Shifting (Smakhtin and Anputhas 2006) and Aquatic Base Flow (Jacobs 2017).Due to the impact of dams on environmental flow, various studies have been conducted in this field.Hu et al. (2008) investigated the Huaihe River basin in China, facing multiple dams.This study shows dams significantly impact environmental flow throughout the year, which is more noticeable in dry seasons.Studies have reported that examining and modifying the stream's components can be used for habitat management.The study of the impact of the Farakka dam construction on the Ganges River showed that the low flows have decreased at different time scales, which has led to a decrease in fish diversity and population in the river downstream.This study recommended managing environmental flow by establishing minimum flow components, a joint action between the Indian and Bangladesh governments (Gain and Giupponi 2014).The dam's effects on the Shaying River's environmental flows in China have been evaluated.This study indicated that dam construction had reduced the maximum and minimum flow values with different time scales and increased the number of days with zero flows.It has led to significant mortality of aquatic organisms and a serious threat to changing the region's ecological quality (Zuo and Liang 2015).Karakoyun et al. (2016) investigated the effect of the Cambası regulator and Hydropower plant in Turkey on environmental flow using Tennant, Tessman, and flow duration curve methods.They have found that the Tessmann method, compared with other methods in this study, has led to the appropriate calculation of environmental flow.Haghighi and Kløve (2017) studied the environmental flow of the Kor River in Iran.This river is the source of the environmental flow of the Bakhtegan and Tashk lakes, and several dams have been built on it.This study revealed that the withdrawal of flow by dams to meet irrigation in this area impacts changing the flow regime.farmers harvest the environmental flow allocated by the dam on the way to the lakes.Thus, the required environmental flow is not provided.Hydropower plants' impact on the Göksu River's environmental flow in Turkey has been investigated.This river's flow duration curve analysis has shown drastic changes in the flow characteristics before and after the dam construction.The Tennant method results in good conditions to realize environmental flow in this river (Karakoyun et al. 2018).The impact of the construction of several dams on the Bureya and Zeya rivers' environmental flow in the Middle Amur catchment area in Northeast Asia has been considered.This study used long-term recorded data to investigate the river flow before the dam construction.The construction of dams has made it technically impossible to allocate the flow required to supply the floodplain inundation component (Nikitina et al. 2020).Study results from Jo et al. (2021) in South Korea indicate that allocating environmental flows can improve water quality for riverine habitats.Accordingly, managing agricultural reservoirs and large dams can lead to a more appropriate allocation of environmental flows in the study area.
The presented studies review reveals that dams' impact on environmental flow is of great importance due to their significant effect on river water quantity and quality.There are different limitations in most studies in this field.Most related studies have not considered the selection of a fish as an essential representative of the river's ecological and environmental conditions.
Also, the hydrological methods used in these studies have low variability.Besides, in most cases, selecting the critical season in environmental flow allocation is not considered.Hydrological methods estimate the environmental flow in different months of the year.Therefore, not focusing on the critical season can result in misinterpretation and inference.
The Gorganrood basin, located in the north of Iran and downstream of this river, is of great ecological and environmental importance.Gorganrood flows into the Caspian Sea, and downstream of this river, several fish species perform activities related to natural reproduction, including spawning (Patimar et al. 2010).Voshmgir dam, constructed on the Gorganrood River, has affected the river's natural flow and, as a result, the environmental flow of the river downstream.This study develops a framework to investigate the river's environmental flow changes caused by the Voshmgir dam construction.The research novelties include the simultaneous application of long-term data, statistical technics, wide-range hydrological methods, and fish habitat information, beside developing an approach for dam construction analysis.

Gorganrood river and its basin
Gorganrood is one of the most important rivers in the southern part of the Caspian Sea.The minimum height in this region is the Caspian Sea coast − 26 m, and the maximum is 3600 m (Sarmoo mountains).The Gorganrood basin slope is 1.27%, and the slope of the main river is estimated to be 0.1%.Gorganrood basin is bounded on the north by the Atrak-Sofla river basin, south by the Central Iran basin (Semnan province), and west by the Caspian Sea.Gorganrood River originates from Khorasan province and has several tributaries in Golestan province and is located between 54° 00′ to 56° 29′ east longitude and 36° 36′ to 36° 47′ north latitude.The main tributary of the Gorganrood River, 200 km long, flows into the Caspian Sea in an east-west direction around Khajeh-Nafas. Figure 1 demonstrates the location of the studied basin.The Gorganrood basin area is important in agriculture, economy, environment, and ecology.In this study, the recorded data from two hydrometric stations of Gonbade-Kavus and Aq-Qala have been used.Table 1 shows the specifications of these stations.

Voshmgir dam
Voshmgir dam was constructed in Golestan province in northern Iran, near Aq-Qala city, 62 km northeast of Gorgan, near Abbas-Abad village Gorganrood river (Jansouz et al. 2017).Its distance to the Caspian Sea coast is 70 km, and 24 km to the border between Iran and Turkmenistan.Studies of the dam began in 1944, and in December 1946, the dam construction began.February 1950 was the time of the dam operation.The Voshmgir Dam irrigation network area is 25,000 hectares (Azari et al. 2017).This dam, with a crest length of 430 m, is made of homogeneous soil type with silt and clay materials and now has a main reservoir and three sub-reservoirs.In the first operation in 1950, the volume of the main reservoir was 60 million cubic meters.The volume of the first, second, and third sub-reservoirs were 18, 12, and 6 million cubic meters, respectively, and a total of 96 million cubic meters (Zare et al. 2019).The volume of regulated water at the time of operation was 175 million cubic meters, which has been changed.Figures 2 and  3. show the dam and the Gorganrood's downstream connection to the Caspian Sea.

Environmental conditions in Gorganrood river downstream
Fish is a good indicator of environmental change's longterm (multi-year) and widespread effects.It is due to their relatively long life, as they have an extended biological capacity.How fish are in an environment indicates the environment's health and can be the reason for choosing the fish species to assess the river's environmental flow (Roberts et al. 2000).Due to the long life and sensitivity to fish conditions, if the conditions for their survival and habitat are maintained, conditions for other organisms' survival in this habitat are provided (Arthington et al. 2006).The Caspian Sea's southern shores have different economic fish species (Zakeri et al. 2020).However, most fish migrate to the rivers south of the Caspian Sea, including the Gorganrood River, for spawning.Caspian Vimba vimba persa (Fig. 4) belongs to the Cyprinidae family and is native to the Caspian Sea (Rahmani et al. 2011).This fish is anadromous species and enters the Caspian Sea basin's freshwater rivers for spawning.It is considered one of the valuable species of the Caspian Sea.According to the International Union for Conservation of Nature (IUCN) classification, a species needs protection (Kiabi et al. 1999).
The spawning of Vimba vimba persa is at its peak for Gorganrood downstream in April and May.The parents return to their original habitats after natural reproduction.In recent decades, due to the construction of dams, uncontrolled abstraction from rivers, and the entry of pollutants, the natural reproduction of Vimba vimba persa in Gorganrood downstream has decreased (Jolodar and Abdoli 2004).  is more than 40% of the mean annual flow, the environmental flow is equal to 40% of the mean annual flow.Otherwise, the environmental flow equals the mean monthly flow (Goguen et al. 2020).Figure 5 shows the flowchart for estimating environmental flow by the Tessman method.

Arkansas method
Filipek et al. ( 1987) developed a method using the mean monthly flow instead of the mean annual flow to estimate the environmental flow of Arkansas rivers more accurately.They also use the relationship between flow and river habitat in three Divided periods, and environmental flow for each period calculate as a percentage of the mean monthly flow (Table 2).

Flow duration curve (FDC) analysis method
The flow duration curve is essential for showing river flow from low to flood flows (Chouaib et al. 2019).This curve is the relationship between flow rate and frequency.In other words, it represents the number of days of the year greater than the flow rate for that day and covers the entire range of flows (Fouad and Loáiciga 2020).The flow duration curve parameters such as low, high, and medium flow can obtain the river's characteristics.First, to plot the flow duration curve, arrange the river discharge data in series in descending order.Then use the Weibull formula for each data to calculate the probability of occurrence.Finally, the flow change curve relative to the probability calculated with that discharge draws to obtain a flow duration curve.
m , flow rank in sorted (descending) time series; n , total num- ber of data frequencies; and p probability of occurrence.Most studies use Q90 and Q95 to estimate environmental flow (Młyński et al. 2020).

Tenant method
The studies of Tenant ( 1976) have led to the presentation of a widely used method of estimating environmental flow.This study used multiple cross sections in different rivers to achieve reliable results.Detailed information on various aspects of fish habitat was collected and analyzed, such as depth, velocity, channel width, bed sediments, lateral streams, temperature, and Benthic invertebrates' movement.
The required flow for fish survival was estimated using the correlation between geometric, physical, and ecological p = m∕(n + 1)  parameters with the flow.Different classification methods are defined to establish the Fair class's environmental flow is most often used by researchers.In this category, for two periods, including autumn-winter and spring-summer, 10 and 30% of the mean annual flow are considered environmental flow, respectively (Kumar and Jayakumar 2021).

High flow-low flow method
Smakhtin et al. ( 2004) used a combination of low flow requirement (LFR) and high flow requirement (HFR) to estimate the environmental flow of 128 basins around the world.In this method, LFR is the minimum ecological water requirement of the irrigation system per year.Also, HFR is effective during floods and affects the river's morphology and surrounding plants in the river's floodplain.In this method, if the river's condition is in "fairly good" condition, the low flow requirement should equal Q90.Q90 is the amount of flow that 90% of the year's days, the river flow is equal to or more.Also, the calculation method for rivers with variable flow differs from rivers with constant flow.Therefore, basins are classified into four classes.For rivers with variable flow, if Q90 is less than 10% of the mean annual flow, the high flow requirement is estimated at 20% of the mean annual flow.However, if Q90 is more than 30% of the mean annual flow in rivers with constant flow, the high flow requirement is considered zero.If Q90 is between 10 and 20% and 20-30% of the mean annual flow, the need for high flow requirements is 15% and 7%.This method's environmental flow demand equals the sum of low flow and high flow requirements.Smakhtin and Anputhas (2006) developed a new method to calculate the river system's environmental flow.This method provides a hydrological regime to protect the river in the desired Environmental condition called "Flow Duration Curve (FDC) Shifting."There are four main steps in the FDC Shifting method, which are as follows: 1-Simulation of existing hydrological conditions, 2-Definition of environmental management classes, 3-Establishing of environmental flow duration curves, and 4-Simulating continuous monthly time series of environmental flows.This method uses the concept of the natural flow duration curve (FDC) in the desired river range (Smakhtin and Eriyagama 2008).Six management classes estimate the global environmental flow in the FDC Shifting method.

FDC shifting method
Rivers in Classes A and B indicate unmodified and largely natural conditions in which no or limited modification is present or should be allowed from the management perspective.Habitat integrity is generally not affected and damaged considering a management perspective for moderately modified river habitats (Class C rivers).Largely modified habitats have undergone significant modifications concerning their natural state in which sensitive biota experienced a reduction in extent and size.Classes E and F are critically and seriously modified habitats normally in poor conditions where habitats' services and functions are lost.
Some studies use transitional management classes (e.g., A/B, B/C) to be more flexible in management decisionmaking (Hughes and Munster 2000).The schematic form of the calculations in the FDC Shifting method is presented in Fig. 6.

Aquatic base flow method
In this method, the median monthly time series of river flow creates and assumes that the flow in the driest month of the year is sufficient to maintain the river habitat's environmental conditions (Hardy 2015;Jacobs 2017).In a situation with no data and information limitation, this issue is investigated whether more river flow is needed to estimate the needs of spawning and natural reproduction.If more flow is required based on ecological studies, the corrected information will replace the previous calculations (World Bank Group 2018).

Upstream-downstream connection
It is necessary to study the relationship between the upstream and downstream of the dam structure to investigate the dam's impact on the river flow.The dam's most significant impact is the river flow change.Therefore it is appropriate to investigate the relationship between upstream and downstream two hydrometric stations on the main river.Correlation coefficients are used to check the relationship between the river flow in these two hydrometric stations.Various correlation coefficients can lead to relatively different results, so three correlation coefficients, Pearson, Spearman, and Kendall, were used in this study.Below are the equations related to these correlation coefficients.
Rx i , rank of x ; Ry i , rank of y , M , number of disconcord- ant pairs; n , number of data; P , number of concordant pairs, SRRC , Spearman's rank correlation coefficient, KRRC ,

Results and discussion
Comparing the relationship between the two upstream and downstream hydrometric stations' river flow is necessary to investigate the Voshmgir Dam construction impact on the Gorganrood River hydrological regime.The Gonbade-Kavus hydrometric station is located upstream of Voshmgir Dam.The recorded data at Gonbad-e-Kavus hydrometric station are from 1955.The data recorded at Aq-Qala hydrometric station are from 1950, so the recorded data joint period from 1955 to 2017 was considered for analysis.Voshmgir dam is the most critical factor on the main river between Gonbad-e-Kavus and Aq-Qala hydrometric stations.Therefore, the changes in the relationship between river flow in these two hydrometric stations can be considered the dam construction effect.Correlation coefficients were used as numerical criteria investigate the relationship between river flows at Gonbad-e-Kavus and Aq-Qala hydrometric stations.A scatter plot was applied as a visual criterion.Comparing the relationship between the mean monthly river flow at Aq-Qala hydrometric station as a function of the Gonbad-e-Kavus hydrometric station is implemented in two cases.In the first case, without considering the effect of the Dam, the relationship between the mean river flow in these two hydrometric stations was evaluated.In the second case, according to the Voshmgir Dam construction date, the relationship between the mean monthly flow in these two stations in the two periods before and after the Dam construction was done separately.The values of the correlation coefficients and the scatter plot related to the cases are described in Table 3 and Fig. 7.
Comparing the results indicates that the correlation between the values recorded in these two hydrometric stations before the Dam construction was significantly higher than in the second period.In other words, the dam construction has significantly reduced the relationship between the mean monthly flow recorded at the Aq-Qala and Gonbad-e-Kavus hydrometric stations.Also, the lack of data separation into the two periods before and after the dam construction has caused a significant reduction in the relationship between the river mean monthly flow at the two Aq-Qala and Gonbad-e-Kavus hydrometric stations.Based on the explanations given, it can be concluded that the Voshmgir dam construction has caused a significant impact on the flow regime in the Gorganrood River.
Due to the Voshmgir dam construction's impact on changing the river flow regime, the environmental flow was estimated in the two periods before and after the dam construction.Table 4 presents the mentioned results.A comparison of the mean monthly flow of the river at the Aq-Qala hydrometric station in the period before and after the construction of the Voshmgir dam shows that in June-March, the mean monthly flow in the period after the dam construction was higher than corresponding values in the period before the dam construction.The mean monthly flow after the dam construction has decreased compared to before during the critical April-May season.The decrease in river flow in April and May after the dam construction is due to storage and diversion of flow by the dam to meet agricultural water needs.Comparing the two periods before and after the dam construction shows that the mean annual flow in the second period has increased compared to the first period.It may lead to the misconception that the dam construction has not affected ecological and environmental conditions downstream of the river.A crucial point to note is that April and May for the natural reproduction of Vimba vimba persa in the Gorganrood River downstream are extremely important, while the river flow in these months has decreased after the dam operation.
Tennant method results before and after the Voshmgir dam construction periods show that the estimated environmental flow is deficient and does not correspond to the river flow pattern.Therefore, the Tennant method results without modification cannot estimate the environmental flow in the downstream area of the Gorganrood River.The Arkansas method results agree with the Tessman method's results in estimating the environmental flow and confirming the effect of exploiting the vast dam on reducing the environmental flow in the critical season of April-May.The estimated value for the environmental flow in both periods before and after the dam construction is deficient in the aquatic base flow method.The very low estimation of environmental flow by the aquatic base flow method is because the amount of flow in different months of the year has many changes in this river.Therefore, the aquatic base flow method is unsuitable for estimating this river's environmental flow.The results indicate that the environmental flow in April-May after the dam construction has decreased compared to the previous period considering Tessman, Arkansas, flow duration curve, High flow-low flow, and FDC Shifting methods.In other words, using various hydrological methods to estimate the environmental flow shows the harmful effects of dam construction on the river's environmental conditions.The  Aq-Qala MMF (cms) Gonbad-e Kavus MMF (cms)

1950-2016
Page 11 of 14 212 Arkansas method has led to the highest estimate of environmental flow among the studied methods.In contrast, the flow duration curve method has resulted in the lowest estimation.
The estimated values for the environmental flow in the flow duration curve method after the dam construction were deficient due to the river flow's very high variability.The flow duration curve method for the period after the dam construction shows that dam construction has caused the river flow's natural pattern to experience fundamental changes.
The study of the river flow regime at hydrometric stations shows that the critical April-May season corresponds to the flood and high flow season in the Gorganrood River.The dam construction has caused the amount of environmental flow to be significantly reduced in this period.These results confirm that although the Voshmgir dam could not affect the mean annual flow in the Gorganrood River, it significantly reduced the environmental flow in the critical season of April-May.Also, due to the dam characteristics, the supply of high flow and flood components in the river habitat has been severely disrupted.Literature reviews show the estimation of environmental flow is implemented by Esmaili et al. (2018) and Daechini et al. (2021).The first study did not consider the dam impact and river habitat, while the second did not use the ecological information on the Gorganrood River's downstream.The findings of this study emphasize that dam impact and ecological information can change the estimated environmental flow using conventional unrevised models.

Conclusion
The construction of dams on rivers affects the river flow regime and, consequently, river habitats' water supply conditions.In this study, the effect of the construction of the Voshmgir dam on river flow changes.Consequently, the environmental flow rel downstream of the Gorganrood River has been investigated.In April-May, the downstream of Gorganrood River as the habitat of Vimba vimba persa is of great importance for spawning and natural reproduction.
To investigate the dam impact on river flows, the time series of two hydrometric stations, including Gonbad-e-Kavus and Aq-Qala, upstream and downstream of the dam, were used, and data correlations were calculated using Kendall's Tau, Rho-Spearman, and Pearson.Based on this, it was found that the construction of the Voshmgir dam has significantly reduced the monthly flow correlation between these two stations.Analyzing the mean monthly flow at the Aq-Qala hydrometric station (dam downstream) revealed its significant decrease in April-May due to the dam construction.
On the other hand, the April-May period also coincides with the river's flood season.Therefore, it revealed that dam construction has significantly impacted the caused by flood high flow components in the environment flow.Floods impact pawning and natural reproduction of Vimba vimba persa during the April-May critical season.Therefore, it can be concluded that flood reduction and elimination are important factors related to the dam construction's negative impact on the river habitat's environmental conditions.The study of environmental flow estimation methods shows that due to severe river flow changes in different months of the year, the estimates obtained from the Tennant and aquatic base flow methods are not reliable.
Moreover, the analysis of the flow duration curve results confirmed that the dam construction had caused the flow increase in some months of the year, which is also a negative point for river habitats.Comparing environmental flow rates in different methods indicates that the dam construction has caused a significant reduction in environmental flow in the April-May critical season for this river habitat.On an annual scale, the amount of environmental flow after the dam construction has not changed significantly.However, the results are entirely different in the critical season.It confirms the critical season's attention in interpreting the results of hydrological methods for estimating the river's environmental flow.

Fig
Fig. 5 Flowchart to determine the environmental flow by the Tessman method

212 Page 8 of 14 Fig. 6
Fig.6Schematic diagram of environmental flow estimation using the FDC Shifting method(Smakhtin and Anputhas 2006)

Fig. 7
Fig. 7 Visual relationship of the mean monthly flow of two hydrometric stations of AqQala and Gonbad-e-Kavus

Table 3
Correlation coefficient between Aq-Qala and Gonbad-e-Kavus for two studied cases Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post 212 Page 12 of 14