Evaluate the role of storage reservoirs in flood moderation in the Brahmaputra basin, and the ecological costs that go with them.

Assam floods almost every monsoon, so storage reservoirs upstream in Arunachal are often offered as the answer. The Siang Upper Multipurpose Project (SUMP) is one example. It is planned as a 280–300 m dam with about 9.2 BCM of live storage, and the government says it will reduce flood risk [1]. Reservoirs can help moderate floods, but they are only one part of a solution and they carry heavy ecological costs.

Role in flood moderation: merits

• Peak absorption: empty live storage can hold back monsoon peaks and release the water later, which lowers flood crests in Assam [1].
• Strategic buffer: a reservoir could absorb sudden releases from China's planned mega-project at Medog, further up the same river.
• Multipurpose gains: 11,000+ MW of hydropower and a regulated flow in the dry season [1].

Limits

• Two jobs, one reservoir: to catch floods, the reservoir must stay partly empty in the monsoon. To earn the most from power, it must stay full. Operating rules have to say which comes first.
• Partial coverage: much of Assam's flooding comes from heavy rain over tributaries that join the river below the dam sites, and from embankments that breach. An upstream dam cannot control these.
• Unproven claims: SUMP is still at a preliminary stage of investigation, and its Detailed Project Report (DPR) has not progressed. Its flood benefit has not been measured [2].

Ecological costs

• Sediment trapping: the dam would hold back silt, starving Assam's floodplains and river islands of the soil that renews them.
• Habitat fragmentation: the Siang basin has 114 fish species, and 44 hydro projects are proposed in it. Each dam breaks the river's continuity a little more [3].
• Seismic hazard: the eastern Himalaya has had very large earthquakes, including the 1950 Assam–Tibet quake. A dam failure here would turn flood control into a flood risk.
• Submergence: Adi groups say forests and 27 villages would go under water [1].

Verdict and way forward

Reservoirs moderate floods only if they are run with flood control as the first priority, built safely and checked basin-wide. The Central Water Commission's cumulative impact study of the Siang basin should guide appraisal [3]. Other steps should go with it: early independent safety review under the Dam Safety Act, 2021; real-time river-data sharing with China; and restoring wetlands and zoning floodplains. Together these would advance SDG-6 (water) and SDG-13 (climate action) without sacrificing the river's ecology.