Evaluating the Environmental Flow of Beshar River, Using Tennant's Hydrological Method Based on the Biological Requirements of Indicator Fishes

Environmental flow of the Beshar River was evaluated using Tennant's hydrological method, the comparison of the depth and speed of the water in the transverse sections of the river, and the needs of indicator fish species in different stages of the life cycle. Field samplings were conducted in October and January 2021 to monitor the status of fishes in the Beshar River and to collect cross-sections of the river. Indicator species were selected based on the expert judgement survey method. Among 23 fish species of the Beshar River, five species from the Cyprinidae family recieved the highest scores. The biological requirements of these species were extracted and used as the basis of comparison with the data of cross sections of the river. The obtained results showed that in the upper reaches of the river (between the two hydrometric stations of Qalat and Shah Mokhtar) the good class of the Tennant method, that includes 40% of the average annual discharge for the period of high flow (April to May) and 20% of the average annual discharge for the period of low flow (June to November), is suitable to provide the biological requirements of adult and larva of cyprinid species of the Beshar River, respectively.