The Brahmaputra River, known as Yarlung Tsangpo in China, is a transboundary river vital for both India and China, as well as Bangladesh. Water sharing between India and China has emerged as a significant geopolitical issue, primarily due to China's upstream activities and a lack of a comprehensive bilateral water-sharing agreement.

Key Issues:

1. Chinese Dam Construction: China has been actively constructing large-scale hydropower projects and dams on the Yarlung Tsangpo, including the Zangmu, Dagu, and Jiacha dams. While China maintains these are 'run-of-the-river' projects designed for power generation with minimal impact on downstream flow, India harbors significant concerns.
2. Reduced Water Flow: India fears that these upstream projects could divert water or significantly reduce the lean season flow of the Brahmaputra, impacting agriculture, livelihoods, and ecology in its northeastern states, particularly Assam and Arunachal Pradesh, which are heavily dependent on the river.
3. Flash Floods and Environmental Impact: There are concerns about the potential for sudden release of water from Chinese dams, which could lead to flash floods in downstream areas. Additionally, large-scale construction in a seismically active region raises environmental and geological stability concerns.
4. Lack of Transparency and Data Sharing: A major point of contention is China's perceived lack of transparency regarding its hydrological data and dam operations. While an Expert-Level Mechanism (ELM) exists for data sharing during flood seasons, India often finds the data to be insufficient, delayed, or incomplete, hindering its ability to manage potential flood situations effectively.
5. Absence of a Water-Sharing Treaty: Unlike with other riparian neighbors, India and China do not have a formal, comprehensive water-sharing treaty. This absence creates a trust deficit and makes it difficult to establish a mutually agreeable framework for equitable water resource management.
6. Geopolitical Implications: The issue is intertwined with broader India-China relations, which are often characterized by competition and mistrust. Water, a critical resource, has the potential to become a source of conflict rather than cooperation, especially given China's 'upper riparian' advantage.

In conclusion, the current issues in Brahmaputra water sharing stem from China's unilateral development of upstream projects, India's concerns about downstream impacts and environmental security, and the persistent lack of a robust, transparent, and legally binding bilateral mechanism for water resource management. This situation underscores the need for greater dialogue, data sharing, and a cooperative framework to ensure sustainable water use for all riparian states.