River flow provides water that maintains the ecological health of both the river itself and the adjoining riparian zones. However, there is a lack of clear definition and identification method of the riparian zone of inland river basins with narrow river channels and anthropogenic intervention reservoirs. In this study, we developed a new method to delineate semiarid grassland riparian zones by taking the Xilin River (Inner Mongolia, China) basin as a case study for providing an opportunity for a more accurate estimation of the ecological water demand of riparian zone. The extent of the riparian zones with different geomorphological and hydrological characteristics was determined by using unmanned aerial vehicle (UAV) images at five river sections along the reach under the combination of natural variations and the regulation by the dams. After mapping each river section, river length and riparian zone area were shown to have a strong relationship (ρs = 0.9; p < .05), which was extended to calculate the downstream riparian zone area by using its river length. Moreover, we applied the distribution flow method to calculate the river ecological water demand and employed the Penman‐Monteith method to calculate riparian zone ecological water demand. The results showed that the river ecological water demand of the downstream of the reservoir was approximately 6.5×106 m3 per year while riparian zone was 2075.1×106 m3. Only 36, 19, and 15% of the ecological water demand can be met in the hydrological wet, average, and dry years, respectively, because of improper reservoir management and the inability of the discharge of the reservoir to meet the ecological water demand of the downstream riparian zone. Our data illustrate the impact and importance of river flow on the downstream degradation of riparian grasslands and can motivate reservoir management in semiarid areas, which will be helpful to guide water regulations to meet environmental and ecological needs.