Environmental flow (EF) is the quantity, quality and timing of water needed for ensuring the sustainability of aquatic ecosystems. The Karaj River is one of the five protected rivers in Iran. It provides drinking water for the cities of Tehran and Alborz, water needed for agriculture, and is also an important power supply source for the country. While the river has fulfilled for a long time environmental requirements of downstream areas, this has been threatened in recent years by increasing demands of the rapidly growing population in Tehran and Karaj. In the present study, we tried to find an acceptable environmental flow range by using Flow Duration Curve (FDC) and Indicators of Hydrological Alteration and compared the results with the Tennant method which has been officially used by the Energy Ministry of Iran. Results are presented in monthly resolution and at the scale of sub-watersheds to provide a spatio-temporal EF analysis that can be used in watershed management planning. Based on the results, highest and lowest amounts of EF were calculated by FDC-Q95 and Tennant methods, respectively. For instance, the monthly mean Q95 in last gauge (Sira-Karaj) equals 5.75 m3/s, while the mean value estimated by Tennant is just 2.35 m3/s. Eventually, this study suggests a range of the EF values obtained by the FDC method as the upper monthly threshold and the Indicators of Hydrological Alteration as lower monthly threshold for Environmental Flow in Karaj River.