River morphological change, prescribed environmental flow regimes, and the water supply ability of rivers are correlated. In water-scarce regions, the ability to ensure planned water supply reliability is a major criterion for deciding reasonable environmental flow and the morphology prescription for rivers. In this research, a method is proposed to determine reasonable environmental flow and morphology prescription. Using this method, three major morphology options are considered: 1) allowing the erosion of rivers 2) preserving the existing river morphology, and 3) constructing new, smaller rivers. Different environmental flow regimes are designed that can support the three different morphology scenarios. The effect of river morphological and environmental flow settings on water supply ability is also explored. To demonstrate the applicability of the method, we employ China's Tanghe River as a case study. The results show that the new method is useful for directing water supply and reservoir operations.