Spatially resolved glacial meltwater retainment in glacial lakes exerts increasing impacts in High Mountain Asia

•8.18 ± 0.64 Gt of glacial meltwater was impounded in glacial lakes in 1990–2020 generally ignored in glacio-hydrological modeling.•The lakes retaining glacial meltwater and having hazard potential threatens ∼ 51 % of the hydropower projects in HMA.•Annual contribution of meltwater to sea-level rise is overestimated by ∼ 1.5% for HMA because of retainment of meltwater. Most of glacial lakes hold glacial meltwater in summer, and have shown a trend of expansion in recent decades. However, few studies have distinguished the amount of meltwater from the total water nourishment of these lakes. We quantified the retained glacial meltwater in glacial lakes by comparing the water balance of glacier-fed and non-glacier-fed lakes in High Mountain Asia (HMA). Results showed that 8.18 ± 0.64 Gt of meltwater was retained by 8115 (∼44 %) of the glacier-fed lakes in HMA from 1990 to 2020 and the annual contribution of glacier melt in HMA to sea-level rise may have therefore been overestimated by ∼1.5 %. In the glaciation zones, the retained meltwater from glaciers was 6.94 × 103 m3/km2 with a spatial variability over the past 30 years from 0.40 × 103 to 25.29 × 103 m3/km2 across different subregions. Retained glacial meltwater substantially contributed to the increased volume in glacier-fed lakes with an average contribution rate of 92 % across different subregions of HMA. Moreover, retained glacial meltwater was a substantial portion of the regional water resource and represented ∼5.6 % of the annual eco-water demand in the glaciation zones in HMA. A total of 21.66 ± 0.43 Gt of water was stored in 4209 (∼23 %) glacial meltwater impounded lakes with an outburst potential, threatening ∼51 % of the hydropower projects in HMA.