Explicit consideration of in-stream flow requirement (IFR) has now become almost mandatory in many rivers before irrigation withdrawal is made. Thereby, the primary objective was to evaluate the IFR through hydrological approaches and compare the condition with current flow variability and trends. Flow records were collected from five discharge stations for IFR estimation. Performance of the river was also judged with respect to nine hydraulic cross-sectional data and stage data of ten water-level monitoring stations of Bangladesh Water Development Board (BWDB). Results show that since 2000, the upper Karatoa was able to meet IFR, but the lowest part of the river experienced severe deterioration in addressing its dry season functionality. Also, the decreasing trend in off-stream availability is recognized as a threat to the Singra site resulting from severe aggradations of the river beds. Attention to the less off-stream availability at Singra raises concerns over sustaining the river from drying out. It is now evident that a hydrological approach of IFR is more than just an initial rough estimate. Such a precautionary method works well to provide quick technical support and decision reference for a complex system, in particular, to find specific drying out parts of a river of concern.