Environmental flow is an essential factor in the survival of river ecosystems. Growing populations cause increased demand for water and a decrease in water available for habitat. The objective of this study was to estimate the environmental flow of the Kashkan River in Iran, where a dam is going to be built. Due to a lack of ecological data in the area, and to allocate a reasonable value of flow to environmental needs in the Kashkan River Basin, we applied several hydrological methods. We used Tennant, aquatic base flow (ABF), ABF of Maine, Hoppe, Arkansas and flow duration curve methods. We compared the results, and considered the estimated flow reported by the Ministry of Energy of Iran. We estimated the best value of environmental flow was a minimum amount equal to or exceeding 70% of the time (Q70) and a flow equal to or exceeded 95% of the time (Q95). Results indicated that Tennant is a suitable method for estimating the lowest value of flow, and is higher than the minimum values that should be allocated to the environmental flow. The amount of environmental flow that needs to be allocated to the river in the mainstream and a tributary are 6–14.3 m3/s and 0.7–1.14 m3/s, respectively.