Hydropower dams

The Mekong River drains 795,000 km² across six countries before reaching the delta. Two distinct hydrological zones matter here: the Upper Mekong (Lancang) from the Tibetan Plateau through Yunnan, where China's 11-dam cascade now sits; and the Lower Mekong from the Myanmar border through Laos, Thailand, and Cambodia, where a further 11 mainstream dams are operational or under construction, plus hundreds of tributary projects.

Sediment deficit

This is arguably the most severe and least reversible upstream impact. Sediment — silt, sand, and clay carried from the Tibetan Plateau and Yunnan highlands — is not merely a physical byproduct of river flow. It is the building material of the delta itself. Under natural conditions, the Mekong delivered an estimated 160–170 million tonnes of sediment per year to Mekong Delta, continuously replenishing the coastal fringe, river banks, and floodplain surfaces against the erosive forces of tides and waves.

Reservoir trapping has changed this fundamentally. Large dams intercept sediment before it reaches the reservoir outlet: Nuozhadu alone traps an estimated 85–90% of incoming load. Downstream of a dam, the water released is sediment-starved — hungry — and compensates by eroding the channel bed and banks. This "hungry water" effect propagates hundreds of kilometres downstream, deepening channels, lowering adjacent water tables, and depriving the coast of replenishment material.

The sediment load reaching the delta has declined from ~160 Mt/year in the 1990s to roughly 75–85 Mt/year by the late 2010s, and further reductions are projected as additional Laotian mainstream dams fill their reservoirs. Even in a scenario where no further dams are built, the trapping already committed by existing reservoirs will persist for decades — reservoir storage does not refill with sediment at basin timescales relevant to human planning.

The downstream consequences in Mekong Delta are measurable: accelerated channel bed incision of 0.5–1 cm/year in the Tiền and Hậu river branches, lateral bank collapse at rates of 10–40 m/year in the outer distributaries, and the coastal erosion of 40–60 m/year on exposed headlands described in the oceanographic section. Sand mining — primarily in Cambodia and Vietnam — amplifies the deficit further by extracting in-channel sediment reserves that would otherwise migrate to the coast.

Altered flow regime

Pre-dam, the Mekong followed a pronounced seasonal hydrograph: a long, low dry season (December–May) during which the delta received minimal discharge, followed by a rapid monsoon-season rise peaking in September–October. This predictable rhythm shaped the entire ecological and agricultural calendar of Mekong Delta - the timing of rice transplanting, fish spawning runs, and Tonle Sap lake filling all evolved around it.

The Lancang cascade has altered this regime in two quantifiable ways. First, during extreme dry season conditions (when reservoirs are charging or power demand is high), it can withhold water, reducing flows at Chiang Saen (the first cross-boundary gauge) below natural levels. Second, it can release water outside the natural season - during low-demand months when surplus water must be discharged - creating out-of-season high pulses that confuse fish migration cues and damage flood-recession crops.

Water abstraction

Water abstraction from the Mekong basin operates at multiple scales, with cumulative effects that reduce the net discharge arriving at Mekong Delta.

At the interstate level, Thailand's Kong-Loei-Chi-Mun project (KLCM) — a long-planned scheme to divert Mekong tributaries westward into the Chi and Mun basins for irrigation of the Khorat Plateau — would abstract an estimated 1.0–2.5 km³/year if fully built. Similar plans exist in Laos for wet-season water storage for dry-season irrigation. While large-scale mainstream abstraction has been politically constrained by the 1995 Mekong Agreement's consultation procedures, tributary abstraction is largely unregulated.

Within Vietnam itself, the upper delta — An Giang and Đồng Tháp provinces — operates an extensive system of pump-fed irrigation canals drawing directly from the Tiền and Hậu branches. During the dry season, when flow is already reduced by upstream dam operations and ENSO conditions, this in-delta abstraction removes water that would otherwise provide hydraulic head against saltwater intrusion. The interaction is direct: every cubic metre per second extracted for irrigation in An Giang reduces the freshwater head at the salt front downstream.