Staged Dam Regulation Shifts Snowmelt from Spring to Autumn in the Topalang River, Uzbekistan: Hydrologic Alteration and Environmental Flows

The Topalang, a snow-fed Amu Darya tributary in Uzbekistan, has been regulated since 1986 by a reservoir enlarged in stages. We compared natural discharge (1930–1985) with operator records of release, storage and reconstructed inflow (1990–2024) to separate the reservoir's effect from changes in the water reaching it. As storage reached about a fifth of annual flow, the ratio of release to inflow fell to 0.80 in April–July and rose to 1.90 in August–October. Seasonal ecochange attributable to the reservoir grew about fivefold, and release now lies outside the natural range every year in March and from October to December. August–October inflow fell about a quarter below the natural mean and was lower in warmer years. The reservoir masks that decline downstream while flattening the spring flood, and winter release fell below an environmental-flow benchmark on about 55 days a year. Both pressures call for release rules and an inflow gauge.