The Old Brahmaputra River plays a key role in supporting agriculture in north-central Bangladesh. In recent years, the river’s water flow has decreased, especially during the dry season. This poses a serious problem for farmers who depend on it for irrigation. This research investigated the reasons behind the river’s declining flow, whether it is caused more by human activities such as farming and water extraction, or by climate change, including shifting rainfall patterns. This study analyzed river flow data from two locations i.e., Jamalpur (upstream) and Mymensingh (downstream), for two periods: G1 (1998–2007) and G2 (2008–2017). Rainfall, groundwater levels, rice cultivation area, and irrigation coverage were also examined. The results showed that river flow declined at both stations, with the downstream area being more severely affected. While the river maintained sufficient flow during the rainy season, it often fell short during the dry season. The findings suggest that the main reason for this decline is not climate-related changes in rainfall, but the expansion of irrigation and dry-season rice farming. This indicates that land use change plays a greater role than climate change in reducing river flow, highlighting the urgent need for improved water use planning and sustainable resource management.