For the past eleven years, the Great Ruaha River which flows through the Ruaha National Park, has ceased flowing during the dry season, with extended periods of zero flow. The drying up has resulted in social conflicts between upstream and downstream users. It has also caused adverse impacts on the ecosystem of the Ruaha National Park, disrupting the lives of many animals and causing changes in their behavior. In this paper we present the findings of a hydrological study conducted to estimate environmental flow requirements. The desktop reserve model was used to determine maintenance high and low flows, and drought low flow requirements within the Ruaha National Park. The results indicate that to maintain the basic ecological functioning of the river requires an average annual allocation of 635.3 Mm3 (equivalent to 21.6% of mean annual runoff). This is the average annual maintenance flow; comprising of maintenance low flows (i.e. 15.9 % MAR; 465.4 Mm3) and maintenance high flows (i.e. 5.8% of MAR; 169.9 Mm3). The absolute minimum water requirement was estimated to be 0.6 m3s-1 with the probability of exceedance of 0.99. The study confirms that in the absence of ecological information hydrological indices can be used to provide a first estimate of environmental water requirements. However, before being applied, greater understanding of the relationships between flow and the ecological condition of the riverine ecosystem is required.